Ivor on sun 12 sep 04
The discussion on water content of silica sources exposes a
fundamental issue for glaze analysis.
It is a given that the same glaze can be made up from different
mixtures of raw materials.
Since our materials each contain a range of elements, it follows
that the same Seger formula can be formulated from different
source compounds. Even the same chemical glaze formula can be
derived from different mixtures of ingredients.
The recent discussion of the use of Wollastonite (Calcium Silicate)
not only as a source of Calcium but also to replace some of the
Quartz or Flint in a recipe is a good example. Zircon, Zircopax
(Zirconium silicate) for Zirconium and Silica is another.
We now learn that our sources of silica can contain different
amounts of water. Up to 12% according to Steve Mills. My
impression is that Wollastonite and Zircon and often china or ball
clay is dry whilst flint is wet.
This means that the amount of silica getting into our glazes is
determined by the particular source of silica used.
Different individuals using different raw materials could conceivably
arrive at different proportions of silica in the actual glaze made up
despite using the same recipe!
However, unless they allowed for the water in their analysis they
would all believe they had the same formula!
Indeed one individual could arrive at different glazes by using
different chemicals to arrive at a common formula. This would
occur despite balancing his recipe using a glaze analysis
programme unless he allowed for differing proportions of water in
his ingredients.
This is a separate source of variability in our calculations although I
doubt that it is new.
Thank you David Hewitt for starting off this discussion.
Ivor
Ivor J Townshend
Macclesfield UK
John Britt on sun 12 sep 04
Ivor,
What you say is correct that =93Different individuals using different raw
materials could conceivably arrive at different proportions of silica in
the actual glaze made up despite using the same recipe!=94 But it is not
just the silica that is a potential problem, it is almost every materials
we use! And the silica is probably the least of the problems.
The question is how to correct the problem. How to standardize
ingredients! You cannot. There are too many producers, too many countries,
too many materials, too much variation in each material. Even if you did
manage to standardize all the materials, potters in arid climates would
have drier materials than those in the tropics, just because of the
humidly levels. Also I guarantee you that potters will not dry out all
their materials every time that they make up a glaze, and frankly, it is
not necessary.
If potters use the same silica supplier (in the US or in a region of the
US) they are getting a very consistent product. So if they share recipes
their will be a relatively high level of success. If they share recipes
from other countries or regions with different suppliers then they will
have to test. This has always been the case and will continue to be the
case. The water in silica is the least of the problems. Just take a look
at the analysis of gerstley borate and how it changes over time. But
people are still using the same recipes they use 20 years ago and putting
in gerstley borate at the same percentage. Also people buy materials and
keep them stored, sometimes for 20 years without using them up. If they
give you a recipe based on their materials it will certainly differ from
ones produced today. (Just based on advances in grinding technology alone).
The UMF only gives the illusion of accuracy. It gives us a ball park range
which is helpful, but it is only that, a ballpark. Using glaze programs
and analysis is not really accurate we just want to think it is.
To keep this short, I will list some of the variables: water in the
materials(including silica, clays (some are air-floated) zinc oxide,
gerstley borate, borax, etc) , accuracy of analysis from the mine and if
you don=92t get the most recent analysis you many be using a 2-year old
analysis, (bear in mind that this is an average, not the exact composition
of each bag or each scoop and there are often errors in that analysis, we
are dealing with humans here), UMF measures materials above the line which
does not include opacifiers, suspenders (bentonite) or colorants, it
doesn=92t account for volatile materials or soluble materials or
crystallization (so slow cooling may create concentrations of oxides on
the surface not reflected it UMF as with crystalline glazes), and it
doesn=92t account for variations in atmosphere, like reduction or wood, salt=
or soda firing. (Even though people want to say that they count for
reduction in the programs you merely click a button reduction or
oxidation. But there is no possible way to account for the incredible
variability of atmosphere conditions in a kiln-- at what rate of
reduction, for how long, at what time did the reduction begin, what type
of fuel, etc. None of these questions are asked nor could they be pinned
down. Merely the wind blowing across a chimney will cause a variation in
the reduction/oxidation of a kiln. And I won=92t even get into the variation=
in scales and ability to use the scales (this is probably the largest area
for variation), or the need for reading glasses and their lack of use, or
the sieving (which removes weight out materials), mixing, and application
part of glazing.
I could go on but this is probably more than was necessary already in
order to say that the water content of silica is of little consequence
when considering the overall variability of ceramic glaze materials and
analysis. And UMF analysis is not as precise as we wish it would be!
Now that is not to say that you cannot reproduce glazes from published
recipes or recipes from friends, it happens every day! (I have written an
entire book doing just that.) Is it the same precise glaze they made up,
certainly not. But it doesn=92t need to be. There is a wide latitude in
glaze making. Am I saying that UMF is irrelevant, NO. It is a good tool
but it is only model.
Just my opinion,
John Britt
www.johnbrittpottery.com
Lili Krakowski on sun 12 sep 04
Ivor: As you wrote me directly, as well as to the List, I am replying in
kind.
I have followed the exposition of all the variables in silica from the
start. All interesting, most known to me. "We" are all heading in two
directions at once. The scientist, and I say this with the utmost respect
and admiration, equips herself with the best in scales, the finest "meters"
the most controlled kilns and so on. Every ingredient can be analyzed,
every variable calculated in.
The potter on the other hand accepts as givens rough analyses, adequate
scales or volume measures, and the like. As a result there is a wide
(within bounds of course) variation in the content of our glazes. We open a
new bag of clay or neph sy or silica. We do not bother to wash the scoop
between bags. We left our reading glasses at home so we do not see the
scale's finer gradations so well. The last time we mixed the glaze it was
in a dry dry spell of winter; we mix it now on a muggy day, after a long
spell of rain.
I have puttered with glaze since 1951 and some of my glazes remain perfectly
fine, and others have fallen into disuse because they don't work with the
ingredients at hand. Furthermore the clay I use has changed. A lot, or a
little. Obviously anything that was swell on a Jordan/Dalton/Fetzer body
may not be happy on my current Tucker or Standard clay. BUT what was happy
two years ago on a Tucker or Standard clay, may, just may, be unhappy now,
if the clays in the boughten blends have changed.
I think it fabulous when potters like Mel develop glazes akin to ancient
ones, esp. that their magic seems rather broadly adaptable and the secret
is heavily in the firing. But I think we should impress on students,
newbies that AS POTTERS we are not exact scientists, and that they should
focus ON GLAZE FAMILIES.
To repeat what I often have said on Clayart. Someone develops a wonderful
glaze depending on whiting, dolomite, neph sy, gerstley borate, and clay.
This glaze he passes on to students. One student moves to some desert, one
to Brazil, one to Alaska etc. Each finds the original recipe is not
working. The recipe then is modified. It is passed on again. You get the
idea.
I once spend a Summer finding all the Calcium borate zinc glaze I could.
There were maybe 30 and I made test tiles. I made two tiles for each
glaze, one one a white body, one on a dark one. I then showed these to my
students. They were predictably impressed that I had gotten like 4 or 5
different "looks" and that the difference between the tests on white clay
and those on dark were greater than those between the zinc glazes.
Lili
----- Original Message -----
From: "Ivor"
To: "Ceramic Arts Discussion List"
Cc: ; "Ron Roy" ; "Vince
Pitelka" ; "Steve Mills"
; "John Britt" ; "Lily
Krakowski"
Sent: Sunday, September 12, 2004 4:50 AM
Subject: Re: Glaze formulae was - Quartz, flint and silica
> The discussion on water content of silica sources exposes a
> fundamental issue for glaze analysis.
>
> It is a given that the same glaze can be made up from different
> mixtures of raw materials.
>
> Since our materials each contain a range of elements, it follows
> that the same Seger formula can be formulated from different
> source compounds. Even the same chemical glaze formula can be
> derived from different mixtures of ingredients.
>
> The recent discussion of the use of Wollastonite (Calcium Silicate)
> not only as a source of Calcium but also to replace some of the
> Quartz or Flint in a recipe is a good example. Zircon, Zircopax
> (Zirconium silicate) for Zirconium and Silica is another.
>
> We now learn that our sources of silica can contain different
> amounts of water. Up to 12% according to Steve Mills. My
> impression is that Wollastonite and Zircon and often china or ball
> clay is dry whilst flint is wet.
>
> This means that the amount of silica getting into our glazes is
> determined by the particular source of silica used.
>
> Different individuals using different raw materials could conceivably
> arrive at different proportions of silica in the actual glaze made up
> despite using the same recipe!
>
> However, unless they allowed for the water in their analysis they
> would all believe they had the same formula!
>
> Indeed one individual could arrive at different glazes by using
> different chemicals to arrive at a common formula. This would
> occur despite balancing his recipe using a glaze analysis
> programme unless he allowed for differing proportions of water in
> his ingredients.
>
> This is a separate source of variability in our calculations although I
> doubt that it is new.
>
> Thank you David Hewitt for starting off this discussion.
>
> Ivor
> Ivor J Townshend
> Macclesfield UK
>
>
David Hewitt on mon 13 sep 04
I don't disagree with what John says below or with what Lili Krakowski
says in another post on this subject. I would, for example, never bother
to dry out a glaze material before weighing and adding to a mix and I do
not use the most accurate of scales. However, I do get requests from
people who have gone on to my web site and ask for recipes. These I
readily provide, but I appreciate, as you do, the problems of
'transporting' recipes.
The question is, how to make the information given in response to such
requests as meaningful and useful as possible. Part of this is surely to
give an analysis of the recipe (together, of course, with as much
information as to the body you use, the method of applying the glaze and
the firing as you can put together)
I have always advocated that a recipe should always be accompanied by an
analysis based on the raw materials that are being used use. I am
fortunate in that my supplier, Bath Potters' Supplies makes public the
analysis of the materials that it supplies. If the recipient has a glaze
program and knows his raw material data, then he can make adjustments
that will give him a better start on testing this new recipe. It was in
this context that I enquired about quartz and the moisture content. I
gather from Roy that in North America it is supplied dry. Is the UK
alone in having a moisture content of up to 12%?
It is true that over time raw materials change as, for example, when new
sources are opened up. Fortunately being in the UK I have never had to
bother with something as variable as gertsley borate. This, however, to
my mind only emphasise the need for potters to press their suppliers for
data on the materials that they are supplying. I believe that it should
become the norm, not the exception, that this information be made
available to the user if they so request.
Glaze programs are by no means the answer to all ones glaze problems,
but I would imagine that you agree that they are a useful tool. To this
end they should surely provide the best possible information. The raw
material data on any program that I have come across allows one to alter
or add raw materials and hence the question regarding quartz and
allowing a percent LOI to cater for the moisture content. I would be
interested in the views of other authors of glaze programs if they would
like to comment.
David
In message , John Britt writes
>Ivor,
>
>What you say is correct that “Different individuals using different raw
>materials could conceivably arrive at different proportions of silica in
>the actual glaze made up despite using the same recipe!” But it is not
>just the silica that is a potential problem, it is almost every materials
>we use! And the silica is probably the least of the problems.
>
>The question is how to correct the problem. How to standardize
>ingredients! You cannot. There are too many producers, too many countries,
>too many materials, too much variation in each material. Even if you did
>manage to standardize all the materials, potters in arid climates would
>have drier materials than those in the tropics, just because of the
>humidly levels. Also I guarantee you that potters will not dry out all
>their materials every time that they make up a glaze, and frankly, it is
>not necessary.
>
>If potters use the same silica supplier (in the US or in a region of the
>US) they are getting a very consistent product. So if they share recipes
>their will be a relatively high level of success. If they share recipes
>from other countries or regions with different suppliers then they will
>have to test. This has always been the case and will continue to be the
>case. The water in silica is the least of the problems. Just take a look
>at the analysis of gerstley borate and how it changes over time. But
>people are still using the same recipes they use 20 years ago and putting
>in gerstley borate at the same percentage. Also people buy materials and
>keep them stored, sometimes for 20 years without using them up. If they
>give you a recipe based on their materials it will certainly differ from
>ones produced today. (Just based on advances in grinding technology alone).
>
>
>The UMF only gives the illusion of accuracy. It gives us a ball park range
>which is helpful, but it is only that, a ballpark. Using glaze programs
>and analysis is not really accurate we just want to think it is.
>
>To keep this short, I will list some of the variables: water in the
>materials(including silica, clays (some are air-floated) zinc oxide,
>gerstley borate, borax, etc) , accuracy of analysis from the mine and if
>you don’t get the most recent analysis you many be using a 2-year old
>analysis, (bear in mind that this is an average, not the exact composition
>of each bag or each scoop and there are often errors in that analysis, we
>are dealing with humans here), UMF measures materials above the line which
>does not include opacifiers, suspenders (bentonite) or colorants, it
>doesn’t account for volatile materials or soluble materials or
>crystallization (so slow cooling may create concentrations of oxides on
>the surface not reflected it UMF as with crystalline glazes), and it
>doesn’t account for variations in atmosphere, like reduction or wood, salt
>or soda firing. (Even though people want to say that they count for
>reduction in the programs you merely click a button reduction or
>oxidation. But there is no possible way to account for the incredible
>variability of atmosphere conditions in a kiln-- at what rate of
>reduction, for how long, at what time did the reduction begin, what type
>of fuel, etc. None of these questions are asked nor could they be pinned
>down. Merely the wind blowing across a chimney will cause a variation in
>the reduction/oxidation of a kiln. And I won’t even get into the variation
>in scales and ability to use the scales (this is probably the largest area
>for variation), or the need for reading glasses and their lack of use, or
>the sieving (which removes weight out materials), mixing, and application
>part of glazing.
>
>I could go on but this is probably more than was necessary already in
>order to say that the water content of silica is of little consequence
>when considering the overall variability of ceramic glaze materials and
>analysis. And UMF analysis is not as precise as we wish it would be!
>
>Now that is not to say that you cannot reproduce glazes from published
>recipes or recipes from friends, it happens every day! (I have written an
>entire book doing just that.) Is it the same precise glaze they made up,
>certainly not. But it doesn’t need to be. There is a wide latitude in
>glaze making. Am I saying that UMF is irrelevant, NO. It is a good tool
>but it is only model.
>
>Just my opinion,
>
>John Britt
>www.johnbrittpottery.com
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
John Hesselberth on mon 13 sep 04
On Monday, September 13, 2004, at 01:34 PM, David Hewitt wrote:
> The raw
> material data on any program that I have come across allows one to
> alter
> or add raw materials and hence the question regarding quartz and
> allowing a percent LOI to cater for the moisture content. I would be
> interested in the views of other authors of glaze programs if they
> would
> like to comment.
Hi David,
It would certainly be easy to account for in GlazeMaster and, I
suspect, in all the other glaze calculation software programs. You
could easily have as many variants of silica (or any other material) as
you wish with the LOIs being set to accommodate the moisture. You would
of course have to maintain separate versions of each recipe depending
on which moisture level you had at hand--but that is no big problem
either.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Britt on mon 13 sep 04
John,
Tell me if I am wrong here, but giving people the illusion that it is
actually possible to account for the moisture level of their materials
with any glaze program is overstating things. You may be able to punch in
a number but you would have to retest it and repunch it in every time you
used the material.
Knowing the moisture content when a bag leaves the plant is no guarantee
that the material will have that moisture content in a week or two months,
let alone 3 years. It will be constantly gaining and losing moisture
depending on the humidity of the environment. It would be like knowing and
recording the gas tank levels in all the cars that leave GM throughout
their lifetime.
And trying to do it for silica is only the tip of the ice berg. The same
is true of feldspars, clays, etc. They have moisture content when leaving
the plant but that is also subject to daily changes in the humidity of our
studios. And what about zinc oxide, gerstley borate, and any other
materials that are deliquescent.
In my mind, it is simply impossible and unnecessary to try to control this
variable.
John
Christy Pines on mon 13 sep 04
Since the moisture content must affect the weight of the material, if I
were trying to weigh to 1/100th of a gram (as in a recipe that calls for
5.01 grams of something) could I assume any kind of accuracy? If the
moisture content could affect the weight so that I might be getting more
or less than my assumed accurate measurement, how important is it that I
accurately measure that 5.01 grams? If 100th's of a gram is too precise,
is tenth's of grams also too precise? Or even 1 gram. If I'm making a
large batch, how important is it that I measure out 2209 grams versus
2210 grams? What kind of water weight will there be that would account
for the 1 gram difference? Here in Connecticut, where my dehumidifer is
full after 6 hours of running in the summer, should I be measuring out
my raw materials differently than when I mix the same glaze in the dead
of winter when the house is as dry as a bone?
christy in connecticut, about to start making glazes and thinking that
tenth's of grams are too precise to worry about in mixing a glaze since
I also believe that 1/8 of a teaspoon is usually more precise than one
needs to be in cooking, and thinking that the two have got to be
related, since they're both based on chemistry and the interaction of
elements.
cpines at ix.netcom.com
John Britt wrote:
>
>Tell me if I am wrong here, but giving people the illusion that it is
>actually possible to account for the moisture level of their materials
>with any glaze program is overstating things. You may be able to punch in
>a number but you would have to retest it and repunch it in every time you
>used the material.
>
David Hewitt on tue 14 sep 04
In message , Christy Pines writes
>Snip<
> should I be measuring out
>my raw materials differently than when I mix the same glaze in the dead
>of winter when the house is as dry as a bone?
Christy,
Clearly the answer is, no.
The question raised related to materials such as Quartz and Cornish
Stone which in the UK at least are deliberately supplied with a
significant moisture content. I am still interested to know if the UK
is alone in this practice.
The question is not related to whether or not materials may take up
moisture or not according to way they are stored and the atmosphere in
which they are stored.
It is like asking if we are content to use generic analyses in our glaze
calculations or the actual analyses as given by our suppliers. Is it
satisfactory to just say that China clay is 1 AL2O3, 2 SiO2 and ignore
that it may actually contain some K2O, Na2O, Fe2O3 and TiO2. Or say that
Potash Feldspar is 1 K2O, 1 Al2O3, 6 SiO2 when it actually contains some
Na2O, CaO, and Fe2O3
In my CeramDat program I provide some materials, such as the above, with
generic analyses and also with the actual analysis that is supplied by
Bath Potters'. The generic materials are clearly indicated in their
title so the user has a clear choice. I have not, though, done this for
quartz or Cornish stone in respect of their possible moisture content
and it was this that prompted me to enter this thread and seek views.
The consensus so far would seem to be don't bother.
David
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
John Britt on tue 14 sep 04
christy,
I think a 1/10 of a gram is sufficient.
John Britt
Dorothy Feibleman on tue 14 sep 04
Dear Clayart,
What I was trying to say is that industry uses machines to measure the
water all the time and do this so that they can come up with consistant
products.
By the time I look at the clayart stuff, I am quite tired and am maybe not
clear when I write.
Many suppliers in the US and Canada supply many dry materials but in other
countries, as I said before, they supply moist materials and some around
20% water. If a company is making a run of stuff they could have problems
if they didn`t take the water into account.
I made a new, very reliable clay for a product in Japan and it makes a big
difference. You either make something you really can repeat--that is if
they mined it in the same place+++++etc, or you don`t. And you do make
tests of each batch of materials too.
Best,
Dorothy
If you want to see more pictures of my personal work, look on this site,
but unless you have a Japanese language facility on your computer, you
won`t be able to read the written stuff, but it is only an interview and
the pictures are more interesting, maybe.
http://www.yakimono.net/monthly/interview/interv010.html
the two pics above are a large piece I did a few years ago, not with my new
clay but with 4 white porcelains which are different whites naturally, a
bit translucent and textured. It is about3mm thick.
It is laminated white clays or "nerikomi".
--------------------
the below is in response to the message below*
The same mining companies that own mines all over the world process wet and
dry materials. I guess wherever there is a larger demand for dry for
certain industries, they process it to suit their largest buyers.
The UK is not alone, in supplying these materials with a moisture content,
i.e. some suppliers in the Far East, but some in the Far East also supply
dry. There has been moisture of around 20% from some of the materials I
have been using in Japan (of course many are imported from China, Korea,
Australia, India, New Zealand, Malaysia as well as North America and Europe
etc.)
Remember that we are at the bottom of the pecking order.
The largest suppliers of our china clays are mainly tin producers and the
china clay is not their main business, tin is. It makes tin mining
economic to also sell the china clay---I am being sarcastic and extreme
here, but it is basically the situation.
Best,
Dorothy
-------------------------------------------------------
>In message , Christy Pines writes
>>Snip<
>> should I be measuring out
>>my raw materials differently than when I mix the same glaze in the dead
>>of winter when the house is as dry as a bone?
>Christy,
>
>Clearly the answer is, no.
>
>The question raised related to materials such as Quartz and Cornish
>Stone which in the UK at least are deliberately supplied with a
>significant moisture content. I am still interested to know if the UK
>is alone in this practice.
>
>The question is not related to whether or not materials may take up
>moisture or not according to way they are stored and the atmosphere in
>which they are stored.
>
>It is like asking if we are content to use generic analyses in our glaze
>calculations or the actual analyses as given by our suppliers. Is it
>satisfactory to just say that China clay is 1 AL2O3, 2 SiO2 and ignore
>that it may actually contain some K2O, Na2O, Fe2O3 and TiO2. Or say that
>Potash Feldspar is 1 K2O, 1 Al2O3, 6 SiO2 when it actually contains some
>Na2O, CaO, and Fe2O3
>
>In my CeramDat program I provide some materials, such as the above, with
>generic analyses and also with the actual analysis that is supplied by
>Bath Potters'. The generic materials are clearly indicated in their
>title so the user has a clear choice. I have not, though, done this for
>quartz or Cornish stone in respect of their possible moisture content
>and it was this that prompted me to enter this thread and seek views.
>The consensus so far would seem to be don't bother.
>
>David
>
>--
>David Hewitt
>
>Web:- http://www.dhpot.demon.co.uk
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
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>
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>melpots@pclink.com.
John Hesselberth on tue 14 sep 04
On Monday, September 13, 2004, at 07:32 PM, John Britt wrote:
> Knowing the moisture content when a bag leaves the plant is no
> guarantee
> that the material will have that moisture content in a week or two
> months,
> let alone 3 years.
Hi John
I keep most of my materials in sealed containers. I doubt the moisture
level varies very much over time. If you measure the moisture level of
the material when you get it and keep it sealed it should be fairly
constant. That said, I'm not aware that North American materials are
sold wet like they apparently are in the UK. After reading this thread,
though, I'm going to do a little checking.
Certainly if I had a material I knew was wet I would keep it sealed and
adjust its composition in my glaze program (or enter a new variant of
the material) to accommodate that moisture. I don't see where that
would be a big deal--at least not with GlazeMaster and I suspect not
with other programs.
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Hesselberth on tue 14 sep 04
On Monday, September 13, 2004, at 11:12 PM, Christy Pines wrote:
> If I'm making a
> large batch, how important is it that I measure out 2209 grams versus
> 2210 grams?
Hi Christy,
Its not important at all. With 2 kg of material you probably wouldn't
see a difference if you were off by 10 or 20 grams.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
mailtoandrew@FSMAIL.NET on tue 14 sep 04
Hello all,
Just throwing my comment into the debate:
All powdered or lump raw materials will contain an amount of water. This
may be residual from the production process, such as found in China stone
and many kaolins, or may be as a result of exposure to the atmosphere such
as with dry milled feldspars.
The presence of such moisture can be advantageous, for example the
suppression of harmful dusts, silica, or to allow the easy dispersion of
clays.
Whilst materials are likely to be produced to known moisture this will be
subject to a tolerance that can be as wide as say 8 to 14%. And of course
the level can alter during storage, though surely good practice dictates
that materials are kept in sealed containers.
The measurement and subsequent allowance for the moisture content is easy
to perform, and very commonly undertaken for both bodies and glazes:
1. Take sample: near representative as possible
2. Weigh
3. Dry: ideally to 110oC for 24 hours but a domestic oven, or even
microwave, works perfectly well
4. Reweigh & then simple percentage calculation
5. Make allowance in recipe. For example if 150 g dry is needed and
moisture content is 5% then 150 divided by 0.95 gives the required damp
mass of 157.9g
What could the consequence be of not allowing for moisture content?
Consider the typical moisture contents of some raw materials: kaolin 10%,
ball clay 16%, silica 5%, feldspar 0%. So if not allowed for in a generic
25, 25, 25 & 25% body the actual recipe would be 24.4, 22.8, 25.7 & 27.1%.
Such differences can be very significant to the performance of bodies and
glazes.
Glaze calculations, be it computer based or the traditional hand written
variety, work on the assumption of dry materials. The only time moisture
needs to be allowed for is when batching out.
Modifying an LOI value is over complicated, and flawed: a chemical
analysis does not include free water as samples are dried before testing.
Thats all .....
Regards,
Andrew
Linda Ferzoco on tue 14 sep 04
I think Christy has hit it on the head.
1. Those triple beam balances do not have the precision to accurately
weigh 5.01 grams, or, I don't think, even 50.1 grams and certainly
not under potter's studio conditions.
2. You're talking about working to three significant figure precision
while weighing and then simply add enough water to make a creamy
consistency? (How many of you really batch to a certain specific
gavity?) That kind of accuracy calls for volumetric blending in lab
conditions. Makes no sense to be so precise in one area and then
blow that precision and accuracy later. Save yourself the trouble.
3. It reminds me of making pie crust: the water the flour has
absorbed varies with ambient humidity. That's why good bakers make
their dough by the feel and consistency and not by measure or even
weight.
Bottom line: you can't control the water of hydration without a great
deal of trouble, which would affect your own bottom line as a
production potter. I think the most I would do would be to make
small batches when new raw materials arrive, just to confirm that my
old recipe is working. If not, then do some line blends to achieve
what I needed.
Once you get a transported glaze recipe to work by doing the
appropriate experiments, don't mess with success. Just thank the
potting gods.
Hmmmmm. Who are the potting gods? I mean it: who did ancient
potters thank for their good fortune?
Cheers, Linda
--- Christy Pines wrote:
> Since the moisture content must affect the weight of the material,
> if I
> were trying to weigh to 1/100th of a gram (as in a recipe that
> calls for
> 5.01 grams of something) could I assume any kind of accuracy?
Dorothy Feibleman on tue 14 sep 04
depending on the source, if anyone is in Australia or New Zealand, the
percentages for feldspars are quite high sometimes and Kaolin quite low.
not as listed below.
Dorothy Feibleman
>Hello all,
>
>Just throwing my comment into the debate:
>
>All powdered or lump raw materials will contain an amount of water. This
>may be residual from the production process, such as found in China stone
>and many kaolins, or may be as a result of exposure to the atmosphere such
>as with dry milled feldspars.
>
>The presence of such moisture can be advantageous, for example the
>suppression of harmful dusts, silica, or to allow the easy dispersion of
>clays.
>
>Whilst materials are likely to be produced to known moisture this will be
>subject to a tolerance that can be as wide as say 8 to 14%. And of course
>the level can alter during storage, though surely good practice dictates
>that materials are kept in sealed containers.
>
>The measurement and subsequent allowance for the moisture content is easy
>to perform, and very commonly undertaken for both bodies and glazes:
>
>1. Take sample: near representative as possible
>2. Weigh
>3. Dry: ideally to 110oC for 24 hours but a domestic oven, or even
>microwave, works perfectly well
>4. Reweigh & then simple percentage calculation
>5. Make allowance in recipe. For example if 150 g dry is needed and
>moisture content is 5% then 150 divided by 0.95 gives the required damp
>mass of 157.9g
>
>
>What could the consequence be of not allowing for moisture content?
>
>Consider the typical moisture contents of some raw materials: kaolin 10%,
>ball clay 16%, silica 5%, feldspar 0%. So if not allowed for in a generic
>25, 25, 25 & 25% body the actual recipe would be 24.4, 22.8, 25.7 & 27.1%.
>
>Such differences can be very significant to the performance of bodies and
>glazes.
>
>Glaze calculations, be it computer based or the traditional hand written
>variety, work on the assumption of dry materials. The only time moisture
>needs to be allowed for is when batching out.
>
>Modifying an LOI value is over complicated, and flawed: a chemical
>analysis does not include free water as samples are dried before testing.
>
>Thats all .....
>
>Regards,
>
>
>Andrew
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
Ivor on tue 14 sep 04
John Britt is absolutely right in citing many of the sources of
variability in our materials. It happens and its a problem, both for
the suppliers and for potters who make up their own materials.
Any thing dug out of the ground will vary. It is difficult to keep
things consistent.
But ...
Most suppliers do a brilliant job blending different batches to
ensure consistency. The ceramic manufacturers will not tolerate
variability in supply and we benefit since we share many of our
materials with the trade makers.
If we are honest most of us try to reduce the variability in the work
we do. Surely most of us weigh more accurately than plus or
minus 5%! That's half a pound in ten.
What bothers me is that flint (in the UK) can contain 12% of water
possibly more. My work is more accurate than that. My glazes
respond to a 10% change in a constituent and I can see the result.
I keep most of my materials in plastic which minimizes any loss or
gain of water to/from the atmosphere.
In developing glazes to get the best result, it is important to be
aware of changes in the materials. Thus, a particular result can be
attributed to a particular change.
Its important not to blindfold ourselves.
If I am to pass on my good fortune to other potters then I need to
know about the water so that others can minimize the chances of
things going wrong when they try my glaze.
The prize for everyone is - only by successful sharing will we make
progress.
Ivor
Ivor J Townshend
Macclesfield UK
John Britt on tue 14 sep 04
Andrew,
Ok, let me see if I understand what you are saying because I think it has
significant consequences to many on this list. All those who use glaze
calculation software to develop recipes must allow for moisture in the
materials when mixing a batch up otherwise they will be off 8 =96 14% or
so. This is because the percent chemical analysis sheets are based on dry
materials while our raw materials have moisture in them.
I have never read this in any software users guide nor have I ever heard
of any potters or anyone on this list compensating for it. It makes sense
but have we been living under the illusion of precision all this time?
I can also say that no potters I know use hermetically sealed containers,
in fact, many schools have the Bailey storage bins (which are open to the
air) and many potters just use the bags the materials come in and scoop
out of those. This means that for accuracy they will have to test water
content of the stored materials at least here in North Carolina in the
summer ( when it is damp) and winter (when the furnace dries things out)
and to be totally accurate they will have to do it every time the mix
unless the bins are sealed?
Thanks,
John Britt
www.johnbrittpottery
Dorothy Feibleman on wed 15 sep 04
>Hello Dorothy
>
>Your comment to my post was .... depending on the source, if anyone is in
>Australia or New Zealand, the percentages for feldspars are quite high
>sometimes and Kaolin quite low. not as listed below.
>
>The values I noted were not intended as a definitive list of the moisture
>contents of raw materials, not that such a thing could exist, but simply
>as the data for a calculation to illustrate the consequence of ignoring
>the presence of water.
thanks.
>
>The values you quote do illustrate that the amount of moisture in
>materials can be very different: Australian feldspars I have used have all
>been supplied at low moisture, less than 1%, whilst clays I know that are
>imported into the country are, depending on type, produced at levels
>ranging from 0 to 35%.
I wonder why the ones I have had access to in Japan have had high water
contents.
Maybe different mines and end users or maybe producers like exporting water.
North American materials that are imported to Japan are dry. Or very low
water content.
The kaolins I have had access to have been quite dry on imported ones even
from China
The feldspars from Australia have been wet. That is general.
>
>Moisture contents are not fixed, with influencing factors that include:
>
>Different materials
>Different ranges between manufacturers
>Differences in tradition, such as flint in the UK is typically around 5%
>whilst quartz in the US is near zero (and what of the differences in the
>meaning of flint in the UK and in the US!)
>Duration of storage
>Atmospheric humidity
>
>The effect of ambient conditions is readily appreciated if a piece of clay
>is dried to zero moisture then immediately placed on a balance of high
>precision and accuracy ... the displayed numbers increase almost like a
>stop watch! Of course given constant temperature and humidity an
>equilibrium will be reached, but what of seasonal effects, especially in
>extreme climates?
>
>Testing is the only way to know the moisture content.
>
>Referring back to the body recipe I mentioned in my last post of 25%
>kaolin, 25% ball clay, 25% silica and 25% feldspar. If the moistures I
>noted were ignored the actual recipe would be 24.4, 22.8, 25.7 & 27.1%.
>One consequence may be, depending on the firing schedule, the actual mix
>would bloat whilst the target version would not.
>
>Should all potters test the moisture of their raw materials? Some glazes
>and bodies are sensitive to small variations in raw materials whilst
>others are quite tolerant: the consequences of not allowing for moisture
>may be less for a stoneware body prepared from crude clays than a
>porcelain made from purified components. Also whilst some potters welcome
>the unpredictability resulting from variable materials others seek
>repeatability, and separate batches of body or glaze prepared from the
>same materials though with different moisture contents will not
>necessarily exhibit identical behaviour.
I guess I like testing all the variable materials in the same recipes and
seeing how they differ.
I like using that. And then figuring out how to use it.
I think it also depends on if you have another job or if you have to sell
everything to pay the bills.
I don`t have another job or a private income, so, I have no choice I have
to experiment within certain limits. It helps to have reliable sources of
raw materials.
I do like to have the use of a nice material I have made and use it in a
new way. I like understanding how materials work and if I came up with
something stunning and hadn`t taken into account all the factors such as
water, where all the stuff was mined and noted the source and particle size
etc, I would be broken hearted if I wanted to use the same effect again for
something which I got all excited about and it failed due to something like
the feldsapar from the same supplier which just happened to be substituted
with something from another mine in another country and the melting
temperature was lowered. If I hadn`t tested the new batch. It could cause
someone like me a big cut in income. I would have lost too much time.
Sorry, my English is not so great tonight, I have been working in Japan too
long and my language is rather stunted at the moment. It has been a long
day today.
But, considering that many recipes are handed out with good will, I think
it is interesting that this type of question has only surfaced now.
I for some reason, had always assumed that all recipes I read were dry
weight and that dry meant dry.
But, as I am someone who looks at cook books and then does something else
that feels right, at the time, that is also what I do with clay. I use them
only as a vague idea of where I might want to start for an investigation of
something I want to make. I do make most of my materials and approach it
like jazz but note what I am doing so I can continue with an idea.
So, maybe this is why so many people seem to have a problem translating
recipes. Maybe recipes can only be guides and everyone has to start
understandiing their materials better. It is more exciting that way anyway
and much more satisfying in the end when you get a stunner. I think that
many people think that if they read a recipe in a book, it is absolute.
Maybe most people don`t use it as a starting point and forget to question
the factors that can change what they think they are making. I do feel that
there is a problem in the schools and that is general and world wide, that
students don`t really have good materials courses any more and are a bit
dependent on processed, ready made. Ready made is fine, but the problem (in
Japan) is no one questions the clay manufacturers, they just accept what is
available and most don`t have a clue about what they are working with at
least under the age of 40-. I am a bit shocked. That is just my view.
>
>As has been discussed the art and craft of ceramics does not follow a
>single path; knowledge and understanding allows decisions and choices to
>be made.
>
>Thats all,
>
>Andrew
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
mailtoandrew@FSMAIL.NET on wed 15 sep 04
Hello John
To your statement ... let me see if I understand what you are saying ...
I would answer yes
A percentage recipe however achieved, guess work, experientially or by
sophisticated calculation, does by convention assume dry ingredients.
This does not mean that some formulations that require a certain amount of
damp material, measured by mass, volumetrically or even hands fulls, do
not exit or can not be successful ... just they will be, be definition,
variable, difficult to modify, difficult to replicate or use substitute
raw materials. Certainly though those looking to exercse control or
procedures such as Seger formulae do need to consider dry materials and
then make allowance.
I do not know why compensation is unknown in your experience as to me, and
many others I know, it is routine. Perhaps the raw materials in your area
are produced at zero moisture and the effects of absorbence from the
atmosphere has been found to be negligible ...again I dont know. What is
definite is that non liquid raw materials are produced at ranges from zero
to high moisture, the highest Ive known neared 40%, and these will not be
completely stable.
Certainly chemical analyses, which these days are almost exclusively by
XRF, provided by suppliers and test houses are from samples at 0%. These
results are quoted in books, websites and software programs.
As far as storage ... again practices vary but sealed, though perhaps not
hermetically, is common. Not only does it reduce water absorbence but
helps prevent contamination and minimises health hazards.
Whether your materials and conditions, summer/winter/kiln on or off, lead
to significant changes is unkown without testing.
Hope that helps,
Regards,
Andrew
mailtoandrew@FSMAIL.NET on wed 15 sep 04
Hello Dorothy
Your comment to my post was .... depending on the source, if anyone is in
Australia or New Zealand, the percentages for feldspars are quite high
sometimes and Kaolin quite low. not as listed below.
The values I noted were not intended as a definitive list of the moisture
contents of raw materials, not that such a thing could exist, but simply
as the data for a calculation to illustrate the consequence of ignoring
the presence of water.
The values you quote do illustrate that the amount of moisture in
materials can be very different: Australian feldspars I have used have all
been supplied at low moisture, less than 1%, whilst clays I know that are
imported into the country are, depending on type, produced at levels
ranging from 0 to 35%.
Moisture contents are not fixed, with influencing factors that include:
Different materials
Different ranges between manufacturers
Differences in tradition, such as flint in the UK is typically around 5%
whilst quartz in the US is near zero (and what of the differences in the
meaning of flint in the UK and in the US!)
Duration of storage
Atmospheric humidity
The effect of ambient conditions is readily appreciated if a piece of clay
is dried to zero moisture then immediately placed on a balance of high
precision and accuracy ... the displayed numbers increase almost like a
stop watch! Of course given constant temperature and humidity an
equilibrium will be reached, but what of seasonal effects, especially in
extreme climates?
Testing is the only way to know the moisture content.
Referring back to the body recipe I mentioned in my last post of 25%
kaolin, 25% ball clay, 25% silica and 25% feldspar. If the moistures I
noted were ignored the actual recipe would be 24.4, 22.8, 25.7 & 27.1%.
One consequence may be, depending on the firing schedule, the actual mix
would bloat whilst the target version would not.
Should all potters test the moisture of their raw materials? Some glazes
and bodies are sensitive to small variations in raw materials whilst
others are quite tolerant: the consequences of not allowing for moisture
may be less for a stoneware body prepared from crude clays than a
porcelain made from purified components. Also whilst some potters welcome
the unpredictability resulting from variable materials others seek
repeatability, and separate batches of body or glaze prepared from the
same materials though with different moisture contents will not
necessarily exhibit identical behaviour.
As has been discussed the art and craft of ceramics does not follow a
single path; knowledge and understanding allows decisions and choices to
be made.
Thats all,
Andrew
Ron Roy on wed 15 sep 04
Hi John,
Some say the glass is half full, some say it's half empty and others say
the glass is too small - we are currently talking in relative terms and I'm
thinking that everyone will interpret them according to what they want to
believe.
Those who do not wish to learn how to use calculation software will take
some of what you say as a good reason to ignore that tool.
So you will understand why I think it important to clarify some of the
statements below - I hope!
There are some very good reasons to try and standardize materials - and
there are many which are more reliable than others - it is why we chose the
materials we used in our glazes in Mastering Cone 6 Glazes.
Anyone having the analysis of their local materials can duplicate our
glazes because we have provided the analysis of the materials we used. We
have provided the Unity formula and the firing schedule needed to duplicate
them.
To say that is impossible to address these issues is to take a step back in
time.
To say we should not be concerned with water added to materials is like
saying we should not be concerned with LOI - it would make a difference -
not only with silica but with any wet material - including wollastonite and
Cornish Stone.
If the atmosphere varies between wet and dry - all materials will be
affected in the a similar way - it will not make much of a difference - but
ignoring added water will.
The UMF gives much more than the allusion of accuracy - and is much better
than any other tool we have - If you had my experience with the technique -
as I have over the last 15 years - you would understand that.
I have used it to solve problems that would have taken me months of
experimentation - and made the necessary changes in as little as one time.
Some of the bodies I originally developed were made with Calvert Clay -
same mine that produced Jordan. The mine closed and we could not get any
more Calvert - a main constituent of a whole family of bodies from cone 04
up to cone 10.
I found a clay that had about as much iron but was different in that it was
more refractory and not as fine.
I reformulated using the new clay - making adjustments based on the
molecular formula (and my experience) and in many cases - only had to do it
once - most potters using the new clays never noticed the difference. Can
you imagine the time and effort it would have taken if that tool was not
used?
My point is - Calculation gives us infinitely more accuracy than any other
system we have at hand.
To plug in wet materials and pretend they are dry is just beyond my
comprehension - why not? Easy to find out the percent of water - weigh it
wet - dry it out - weigh again - you calculate the percent of water - and
you have the LOI - why not?
Many of the examples you give that contribute to the inaccuracy are not
true - at least in my case - I do include and exclude zirconium silicates
depending on whether I am looking at ratio or expansion. I do include
bentonite as a material that is included in the glaze melt. It can predict
oxide devitrification on the surface. It also can predict general stability
and expansion. If reduction is a factor then iron can be shown as a flux.
There is a way to compensate for material lost during sieving.
I'm not saying it's perfect - but the advantages are awesome compared to
what we used even 20 years ago. And I not saying that experience does not
count - but experience does not come out of the air - and the more you use
calculation the stronger your "experience" is.
I will be giving a talk at the next NCECA - on how I used calculation to
come up with my own version of a shino type glaze. I have a full hour and
there will be time to ask questions - if you (or anyone else) have a
question there will be time to answer some of them - preferable questions
that fit the subject. If anyone one submits a question before hand I may
also to be able to provide an illustrated answer.
Sorry to go on like that - it just seems to obvious to me - it would be so
simple to say - when you pass on a recipe - this silica had 12% water - if
you are using dry silica use this much.
RR
>The question is how to correct the problem. How to standardize
>ingredients! You cannot. There are too many producers, too many countries,
>too many materials, too much variation in each material. Even if you did
>manage to standardize all the materials, potters in arid climates would
>have drier materials than those in the tropics, just because of the
>humidly levels. Also I guarantee you that potters will not dry out all
>their materials every time that they make up a glaze, and frankly, it is
>not necessary.
>
>If potters use the same silica supplier (in the US or in a region of the
>US) they are getting a very consistent product. So if they share recipes
>their will be a relatively high level of success. If they share recipes
>from other countries or regions with different suppliers then they will
>have to test. This has always been the case and will continue to be the
>case. The water in silica is the least of the problems. Just take a look
>at the analysis of gerstley borate and how it changes over time. But
>people are still using the same recipes they use 20 years ago and putting
>in gerstley borate at the same percentage. Also people buy materials and
>keep them stored, sometimes for 20 years without using them up. If they
>give you a recipe based on their materials it will certainly differ from
>ones produced today. (Just based on advances in grinding technology alone).
>
>
>The UMF only gives the illusion of accuracy. It gives us a ball park range
>which is helpful, but it is only that, a ballpark. Using glaze programs
>and analysis is not really accurate we just want to think it is.
>
>To keep this short, I will list some of the variables: water in the
>materials(including silica, clays (some are air-floated) zinc oxide,
>gerstley borate, borax, etc) , accuracy of analysis from the mine and if
>you don=92t get the most recent analysis you many be using a 2-year old
>analysis, (bear in mind that this is an average, not the exact composition
>of each bag or each scoop and there are often errors in that analysis, we
>are dealing with humans here), UMF measures materials above the line which
>does not include opacifiers, suspenders (bentonite) or colorants, it
>doesn=92t account for volatile materials or soluble materials or
>crystallization (so slow cooling may create concentrations of oxides on
>the surface not reflected it UMF as with crystalline glazes), and it
>doesn=92t account for variations in atmosphere, like reduction or wood, sal=
t
>or soda firing. (Even though people want to say that they count for
>reduction in the programs you merely click a button reduction or
>oxidation. But there is no possible way to account for the incredible
>variability of atmosphere conditions in a kiln-- at what rate of
>reduction, for how long, at what time did the reduction begin, what type
>of fuel, etc. None of these questions are asked nor could they be pinned
>down. Merely the wind blowing across a chimney will cause a variation in
>the reduction/oxidation of a kiln. And I won=92t even get into the variatio=
n
>in scales and ability to use the scales (this is probably the largest area
>for variation), or the need for reading glasses and their lack of use, or
>the sieving (which removes weight out materials), mixing, and application
>part of glazing.
>
>I could go on but this is probably more than was necessary already in
>order to say that the water content of silica is of little consequence
>when considering the overall variability of ceramic glaze materials and
>analysis. And UMF analysis is not as precise as we wish it would be!
>
>Now that is not to say that you cannot reproduce glazes from published
>recipes or recipes from friends, it happens every day! (I have written an
>entire book doing just that.) Is it the same precise glaze they made up,
>certainly not. But it doesn=92t need to be. There is a wide latitude in
>glaze making. Am I saying that UMF is irrelevant, NO. It is a good tool
>but it is only model.
>
>Just my opinion,
>
>John Britt
>www.johnbrittpottery.com
>
>___________________________________________________________________________=
___
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.=
com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
John Britt on wed 15 sep 04
Hey Ron,
First of all, I am not degrading UMF, or glaze calculation software! Nor
do I doubt your expertise or successes. I actually applaud your efforts
every time I can and insist that you are one of the best things about
Clayart!
But to have a discussion about the details of the UMF system and its
benefits or flaws is not to say it is bad, wrong, stupid or useless. It is
a way to understand it better so we, potters at large, can use it as a
tool for greater benefit.
I am very happy that you clarify the discussion. I know that everyone
will gather ammunition for their point of view, but discussing it in a
rational way will convince some, clarify for others and turn other off.
To simply brush the inconsistencies of the UMF as insignificant is not
going to sway anyone to use the system. This is the place that real
understanding can occur, not reading out of books with no reply button.
For greater clarity let me put in snips of your post and reply to them so
we keep on track. I am not doing this to be petty or condescending in any
way. I just want to keep on track and have you respond to the points. I
think sometimes we, on Clayart, paint with a broad brush and then the
discussion goes nowhere.
You say: =93I do include bentonite as a material that is included in the
glaze melt.=94 Now I am saying that this shows the problem with the UMF.
Bentonite supplies oxides which are then unaccounted for, like alumina,
silica, magnesium oxide, iron oxide, etc. If you use Macloid then add
lithium to the list of items left out. It is not a major thing but it is
left out.
You say: =93If reduction is a factor then iron can be shown as a flux=94. I =
am
saying if iron is below the line it is not included in the UMF for the
glaze. Am I wrong there? But even if it is included how does the glaze
program know what the level of reduction is? Is half of the red iron
reduced? All of it? 2/3 of it? How much is reoxidized during cooling? What
if it is reduction cooled? How much is reduced thermally? I am saying that
reduction is an unknown factor which is not taken into account in the UMF
and it can never be fully accounted for because of it unpredictability.
This is not a major thing but it is not included in the UMF.
Speaking of coloring oxides which are below the line, the UMF doesn=92t
account for the fluxing action of cobalt or the refractory nature of
chrome. These can really affect the glaze surface and structure causing
running or matting, etc.. And coloring oxides can sometimes account for
10% of the glaze recipe. Now, in this case I may have to say that this is
significant factor that is left out of the UMF.
You say =93I do include and exclude zirconium silicates depending on whether=
I am looking at ratio or expansion.=94 I am a bit unclear on what you are
saying here, but what I am saying is that =96 at least in my understanding
which is obviously deficient- opacifiers go below the line, therefore they
are not calculated in the UMF. This means that the silica in the zircopax
is not recorded in the formula. This can be 8%, which is significant. It
could even bring a glaze that was thought to be outside the limits, into
the target range, or it could also push a glaze that was at the upper edge
outside the limits, into the immature area..
You say =93It can predict oxide devitrification on the surface=94. I have no=
t
seen anything or read anything about this. I find this hard to believe
because how does the program know what the firing cycle is. How does the
program know if I am firing in a 100 cu. ft. hard brick kiln with 15 inch
walls =96 which would cause calcium matts?
You say =93There is a way to compensate for material lost during sieving.=94=
I
have never heard of this and find it hard, if not impossible, to believe.
How can you know what sieve I am using. 60, 80 100 or 200. How do you know
if the sieve was 10 years old and very worn allowing more materials
through than normal. How about a small slit in the screen? I know these
are minor variables but they are unaccounted for in the UMF.
You say =93It also can predict general stability and expansion=94. I guess t=
he
key words here are =93predict general=94. These are very loose terms in a
system that purports to be an atom for atom model. I know that the
expansion and contractions numbers are ballpark at best as the formation
of crystals through slow cooling is impossible to predict for many of the
same variables listed above.
You say=94 To say we should not be concerned with water added to materials
is like saying we should not be concerned with LOI - it would make a
difference - not only with silica but with any wet material - including
wollastonite and Cornish Stone.=94 I am not saying we should not be
concerned with water in materials!! Maybe I was not clear but let me
clarify now. I am saying that the water in a material varies with ambient
temperature and humidity and the length of time stored. Then, through this
discussion - Andrew informed me/us that all the chemical analysis of the
materials we use are based on dry materials. These are the numbers that
are put into the glaze programs. So when a glaze is constructed from glaze
software, in order for it to accurately reflect that UMF, the batch should
be weighted out with DRY materials or the water compensated for.
This is a significant statement! This was enlightening! But the problem is
I have been using glaze software for years and have never seen this
mentioned. I have never heard this mentioned on Clayart and we have done
lots of manipulating of glazes for countless potters. Correct me if I am
wrong here=97--But doesn=92t this mean that unless everyone accounted for th=
e
water in their materials that the glazes they mixed up are not the glaze
that was analyzed? Now it may be a very small bit off or as Andrew said
it may be 5 -15 % off. But unless they accounted for this the batch recipe
differed from the formula. Am I mistaken on this?
You say, =93Sorry to go on like that - it just seems to obvious to me - it
would be so simple to say - when you pass on a recipe - this silica had
12% water =96 if you are using dry silica use this much.=94 I agree with thi=
s
statement completely but I have never heard this mentioned in 10 years on
Clayart. Am I the only one who has missed this? I have never read it in
any books or users manuals either.
You say, =93And I not saying that experience does not count - but experience=
does not come out of the air - and the more you use calculation the
stronger your "experience" is. I agree with this completely. I know that
the UMF is a tool and a model of glaze behavior not the glaze behavior
itself. So, for example it does not and cannot account for the physical
properties, as opposed to the chemical properties, of the materials, like
for example solubility. So looking at a carbon trap shino formula will
never tell you that the soluble soda will rise to the surface and trap
carbon if it is reduced properly. It just says: Na - 0.513. For that you
must look to the recipe and see the soda ash and have experience which
tells you this will happen.
You say, =93I'm not saying it's perfect - but the advantages are awesome
compared to what we used even 20 years ago.=94 I agree with this statement
completely. It is not perfect. The problem is that there is the perception
out there, (and maybe it is just me??), that it is an atom for atom
formulation of glazes. This is because you take the number of moles (6.98
x 10 23) times the molecular weight =85yadda, yadda, yadda,.... to end up
with 4.55 moles of silica. That is pretty precise. And I think potters
have the impression that it is that accurate, when in fact it is not. I
really think that by knowing how and why it is not perfect we can explain
what the advantages are in a realistic manner.
So all the little things discussed above may be minor (some are major) but
cumulatively they become significant. For example, if I had a glaze like
Leach 4,3,2,1, that was clear, white (zircopax 8%), or black (chrome,
cobalt, red iron, and nickel totaling 8%), wouldn=92t they all have the same=
UMF, the same Exp. /Cont rate, the same surface tension, the same Si/Al
rate, etc.?
In closing=85.. I would like to thank the academia=85=85..I hope that was no=
t
too long? I am, truly, only trying to learn about the intricacies of the
system since I spend a good bit of time teaching it and I don=92t want to
misinform my students. So setting me straight will protect the lives of 20
students starting September 23 at Penland .
Help me!
John Britt
www.johnbrittpottery.com
Ron Roy on fri 17 sep 04
Hi David,
What a wonderful topic this is and a special thanks to you for bringing up
the subject of wet and dry materials.
Will we ever look at a recipe including Cornwall Stone and not wonder if it
was wet or dry again. One way around this is to assume the worst (best)
case do it with both wet and analysis - the problem now is - we need to
know - with wet materials - what percent of water to use - do the mines
supply that info? Is the water content consistant? Perhaps someone should
compile the numbers for everyone - and they can become yet another table in
our glaze books.
If your software thinks the materials you use are dry - and it does if
there is no allowance for it (like using LOI to take it into account) -
then is not the % analysis and molecular formula in fact wrong?
Best regards - RR
>The question is, how to make the information given in response to such
>requests as meaningful and useful as possible. Part of this is surely to
>give an analysis of the recipe (together, of course, with as much
>information as to the body you use, the method of applying the glaze and
>the firing as you can put together)
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on fri 17 sep 04
Hi John,
If - as most of us seem to think - variable materials are at least part of
the problem for potters - why not develope a strategy that will at least
give us an advantage. Eliminating errors when we can - if we choose to -
when it is an advantage - seems to me approriate.
If we are trying to duplicate a glaze - and we know there is a possibility
that some raw materials may be supplied wet - why not test it both ways -
or better still - do a line blend using the numbers for wet material - and
dry.
I am pleased this subject has come up - just another factor we had not
thought of before - and should have.
We are not talking about moisture absorbed from the air here - we are
talking about lots of water being added during processing - to only some of
the materials used. It will be useful to at least understand the
implications and compensate for material which are not there to take part
in the final glaze.
I just calculated out a recipe using 20% silica - then again with 10 % less
silica (the result of having 10% water in the silica) - the ratio went down
and the expansion went up of course - would anyone notice - well if the
glaze was just below the expansion need to prevent crazing - it could make
a difference.
It is an interesting process and will give those - who care to do it - some
insight into how different amounts of any material in a glaze will affect
results - expecially if they actually mix the tests and can see what those
differences are.
RR
>Tell me if I am wrong here, but giving people the illusion that it is
>actually possible to account for the moisture level of their materials
>with any glaze program is overstating things. You may be able to punch in
>a number but you would have to retest it and repunch it in every time you
>used the material.
>
>
>Knowing the moisture content when a bag leaves the plant is no guarantee
>that the material will have that moisture content in a week or two months,
>let alone 3 years. It will be constantly gaining and losing moisture
>depending on the humidity of the environment. It would be like knowing and
>recording the gas tank levels in all the cars that leave GM throughout
>their lifetime.
>
>And trying to do it for silica is only the tip of the ice berg. The same
>is true of feldspars, clays, etc. They have moisture content when leaving
>the plant but that is also subject to daily changes in the humidity of our
>studios. And what about zinc oxide, gerstley borate, and any other
>materials that are deliquescent.
>
>In my mind, it is simply impossible and unnecessary to try to control this
>variable.
>
>
>John
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on fri 17 sep 04
Hi Linda,
Your comment below made me think of something.
I wonder if anyone has ever done a survey of the chard heaps associated
with potteries through the ages - and when did they start to get
significantly smaller.
And I wonder - if reduced losses were ever the result of a pottery god and
if so - which one - we would all like to know that.
RR
>Once you get a transported glaze recipe to work by doing the
>appropriate experiments, don't mess with success. Just thank the
>potting gods.
>
>Hmmmmm. Who are the potting gods? I mean it: who did ancient
>potters thank for their good fortune?
>
>Cheers, Linda
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Linda Ferzoco on fri 17 sep 04
I wonder too. Also, I've been re-thinking what I said earlier about
the dryness of raw materials. I stand by my contention that it's
silly to worry about 3 significant-figure accuracy, but if a material
has water of hydration, then you certainly have the potential for a
significant error in the glaze recipe and must account for it. I'm
guessing that any materials potters buy are not anhydrous; it would
be too expensive. Some materials could be mixtures of different
materials, some with 2 water molecules or others with 3 H2O.
As someone said, you can certainly check each batch as it arrives
and, using glaze calculation software certainly, factor the LOI into
the formula. This also assumes storage in air/watertight containers.
In just about any part of the country, humidity varies and a
material left in a bag will test differently depending upon relative
humidity.
So, when I set up my studio (I should live so long), I will practice
a little quality control on the incoming raw materials, do the LOI,
put that number into the glaze calculation software for my glaze
recipes and store ALL materials in airtight containers. Think I even
have an old Coors porcelain crucible around here somewhere and can
certainly buy more.
Now Ron, this begs the question of what standards you and others put
into your programs for LOI. Was it based on a suppliers analysis?
Cheers, Linda
Ah, about that god. I imagine that in Graeco/Roman times, potters
prayed to Colcanus/Hephaistos, the god of fire. Check this quote
from this site: http://www.theoi.com/Olympioi/Hephaistos.html
Volcanus [Hephaistos], at Joves [Zeus] command, made a womans
form from clay, Minerva [Athene] gave it life, and the rest of the
gods each gave some other gift. Because of this they named her
Pandora. Hyginus Fabulae 142
AND
[One] who has learned the works of Athena and Hephaistos, the god of
many crafts, gathers in his livelihood with his hands. Solon Frag
13
--- Ron Roy wrote:
> Hi Linda,
>
> Your comment below made me think of something.
>
> I wonder if anyone has ever done a survey of the chard heaps
> associated
> with potteries through the ages - and when did they start to get
> significantly smaller.
>
> And I wonder - if reduced losses were ever the result of a pottery
> god and
> if so - which one - we would all like to know that.
>
> RR
>
>
> >Once you get a transported glaze recipe to work by doing the
> >appropriate experiments, don't mess with success. Just thank the
> >potting gods.
> >
> >Hmmmmm. Who are the potting gods? I mean it: who did ancient
> >potters thank for their good fortune?
> >
> >Cheers, Linda
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
>
______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
> subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
John Britt on fri 17 sep 04
Ron,
I am glad we had this discussion too! It has been very enlightening.
>I am pleased this subject has come up - just another factor we had not
>thought of before - and should have.
I just find it amazing that all this time and no one knew that the glazes
we formulated were not being batched out correctly for the formula. And I
guess that the more amazing thing is that they seemed to work !
>
>We are not talking about moisture absorbed from the air here - we are
>talking about lots of water being added during processing - to only some
of
>the materials used. It will be useful to at least understand the
>implications and compensate for material which are not there to take part
>in the final glaze.
I know that we are talking about moisture added in processing which is
often done to keep the danger from dust down, but I think we cannot ignore
the water absorbed by some materials (ZnO) or the water lost in transport
and storage. I know that some people keep their materials in plastic
containers but certainly most people do not, at least, not in my
experience. (I am also thinking about countries other than the US) And
certainly anyone in a group studio with Bailey type storage bin is subject
to variation in moisture level which will affect weight. So to compensate
for this with glaze software you would have to keep a close watch on it
and update your program frequently rather than have a set number from the
factory. That is what I was referring to in the post to John.
The following statement has me puzzled though:
>
>I just calculated out a recipe using 20% silica - then again with 10 %
less
>silica (the result of having 10% water in the silica) - the ratio went
down
>and the expansion went up of course - would anyone notice - well if the
>glaze was just below the expansion need to prevent crazing - it could make
>a difference.
Leaving out 10% silica - "would anyone notice", it =93could make a
difference=94? I think my world is spinning out of control!! If 10%
doesn't matter, why use the glaze calculation programs!? We could simply
estimate. The confusing thing to me is that with the very tight targets
you set for "stability" this would surely affect the results. Wouldn=92t it?=
I am still waiting on your reply from my last monster post, but in the
meantime, I guess I am coming to the conclusion that I am wrong, UMF is
not meant to be an atom for atom method of glaze calculation in spite of
the fact it uses moles and atomic weights, rather it is just a "general
predictor". I come to this conclusion because no one else seems to share
my view of UMF, or at least has not spoken up.
It seems to me that if we go to the trouble of all this calculation and
then -- it can be plus or minus 10% on the silica; +/- 2 percent on the
bentonite; +/- 5 to 8% on the opacifer and +/- 1 to 10 % on colorants we
are just looking for general predictors not really precision at all. (That
could amount to a total of +/- 30%)
Do you think that is a reasonable conclusion?
Thanks,
John
www.johnbrittpottery.com
Earl Krueger on sat 18 sep 04
One of the wonderful things about Clayart is the
exposure you get to how differently people think.
It never occurred to me that people would _not_
take into account the amount of water in their
materials. Having a research chemistry background
it has been difficult for me to adjust to working with
impure materials. In the lab, most compounds you
use are something like 99.9+% pure and are stored
in air-tight bottles. If you are doing the most precise
work you store chemicals in a climate controlled
environment.
Glaze calculation, whether performed by software or
by paper and pencil, is only a tool that attempts to
model the outcome of the complex reactions that occur
inside our kilns. As we learn more about these reactions
and the materials we use it is indubitable that these
calculated models will improve. However we may find
that to get better predictions the models require us to
switch to purer (read more costly) materials or the model
will require more data than the current analysis provide.
Perhaps we will need to not only specify the amount of
CaO, SiO2, Al2O3 equivalents but also the chemical and
physical structure of the materials providing these
elements.
At what point does a potter become an industrial chemist?
Earl K...
Bothell, WA, USA
"You may be disappointed if you fail,
but are doomed if you don't try"
Beverly Sills (1929- )
Dorothy Feibleman on sat 18 sep 04
Dear any one,
You have been discussing water in processed raw materials for glaze.
I have personally been interested in water in materials in clay recipes,
where it does seem to have a big impact on if something is good enough for
an industrial or studio repeat of the type of work and products I have been
working on in Japan.
It might be interesting if someone was willing to do a small batch sample
tests on your glazes and see what the difference is now that pandora`s box
has been opened? It might take less time than imagining by email (although
it is free entertainment for us audience here) what might happen if someone
just tried a few samples of their favorites or what failed to be something
gorgeous that you thought you were making. Maybe all those terrible glazes
could turn out bingo after all. Or maybe more cull to recycle for the road
system.
Just a thought as it is quite a relief walking away from the computer and
going into the shop from time to time--or the other way around.
I will continue to go by dry weight for myself, but it might be interesting
to see how the glazes are different or not.
Best,
Dorothy
>Ron,
>
>I am glad we had this discussion too! It has been very enlightening.
>
>
>>I am pleased this subject has come up - just another factor we had not
>>thought of before - and should have.
>
>
>
>I just find it amazing that all this time and no one knew that the glazes
>we formulated were not being batched out correctly for the formula. And I
>guess that the more amazing thing is that they seemed to work !
>
>>
>>We are not talking about moisture absorbed from the air here - we are
>>talking about lots of water being added during processing - to only some
>of
>>the materials used. It will be useful to at least understand the
>>implications and compensate for material which are not there to take part
>>in the final glaze.
>
>
>
>I know that we are talking about moisture added in processing which is
>often done to keep the danger from dust down, but I think we cannot ignore
>the water absorbed by some materials (ZnO) or the water lost in transport
>and storage. I know that some people keep their materials in plastic
>containers but certainly most people do not, at least, not in my
>experience. (I am also thinking about countries other than the US) And
>certainly anyone in a group studio with Bailey type storage bin is subject
>to variation in moisture level which will affect weight. So to compensate
>for this with glaze software you would have to keep a close watch on it
>and update your program frequently rather than have a set number from the
>factory. That is what I was referring to in the post to John.
>
>
>The following statement has me puzzled though:
>>
>>I just calculated out a recipe using 20% silica - then again with 10 %
>less
>>silica (the result of having 10% water in the silica) - the ratio went
>down
>>and the expansion went up of course - would anyone notice - well if the
>>glaze was just below the expansion need to prevent crazing - it could make
>>a difference.
>
>Leaving out 10% silica - "would anyone notice", it =93could make a
>difference=94? I think my world is spinning out of control!! If 10%
>doesn't matter, why use the glaze calculation programs!? We could simply
>estimate. The confusing thing to me is that with the very tight targets
>you set for "stability" this would surely affect the results. Wouldn=92t it=
?
>
>I am still waiting on your reply from my last monster post, but in the
>meantime, I guess I am coming to the conclusion that I am wrong, UMF is
>not meant to be an atom for atom method of glaze calculation in spite of
>the fact it uses moles and atomic weights, rather it is just a "general
>predictor". I come to this conclusion because no one else seems to share
>my view of UMF, or at least has not spoken up.
>
>It seems to me that if we go to the trouble of all this calculation and
>then -- it can be plus or minus 10% on the silica; +/- 2 percent on the
>bentonite; +/- 5 to 8% on the opacifer and +/- 1 to 10 % on colorants we
>are just looking for general predictors not really precision at all. (That
>could amount to a total of +/- 30%)
>
>Do you think that is a reasonable conclusion?
>
>Thanks,
>
>John
>www.johnbrittpottery.com
>
>___________________________________________________________________________=
___
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
Ron Roy on sat 18 sep 04
I think it is better to have lids on bins - there is a better chance that
materials will not get contaminated. Open bins are an invitation to any one
with some left over material in a scoop - to make a mistake.
This is especially so in a common studio situation. I do glaze courses as
you all know - and it has been the case - several times - when materials
were contaminated and the testing was for nothing - even if there was an
interesting result - it was impossible to duplicate it.
When working in that situation I always stipulate - only one bin open at a
time - which means the excess material - after weighing - goes back into
the right bin.
I use the same system in my own studio -anyone who has gone through the
experience of having contaminated material will understand exactly what I
am talking about.
RR
>I can also say that no potters I know use hermetically sealed containers,
>in fact, many schools have the Bailey storage bins (which are open to the
>air) and many potters just use the bags the materials come in and scoop
>out of those. This means that for accuracy they will have to test water
>content of the stored materials at least here in North Carolina in the
>summer ( when it is damp) and winter (when the furnace dries things out)
>and to be totally accurate they will have to do it every time the mix
>unless the bins are sealed?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sat 18 sep 04
Hi John,
I welcome the opportunity to discuss the merits of clay and glaze
calculation - and I thank you for your views, questions and support.
And I thank you for clarifying your views - it is most welcome.
I thought I made it clear - I do include bentonite in my calculations - I
include everything I can - and probably more than others do - the more you
include the better the picture you get. How significant is including
bentonite - when only 2% is added to a glaze or clay body - not much I must
admit - but why not include it?
Calculation software is still evolving - we will see improvements in the
way the programs work. Our discussions will help that happen.
We must remember - that calculations are just that - a representation of
the fired material. How we use the information - and how accurate it is -
is up to us. At the same time there are limits - as you point out - to the
accuracy. It is also true - according to my experience that complete
accuracy is not necessary. On the other hand - when we can - we should
strive for the best we can get. That is why I say - if we can - we should
take into account the weigh of added water to those materials that come
wet.
Perceived inconsistencies: The question of significance can be seen as a
reason to disregard the value of calculation. In some cases those perceived
inconsistencies are exaggerated and that is what I was trying to explain.
There are ways to get around shortcomings - it depends on your point of
view and your creativity.
An example: I am working on a glaze that has a frit for which the analysis
is not available. The glaze is crazing - I can work on the glaze without
the frit - lower the expansion and then include the frit back in at the
same level.
There are many work arounds - it depends on how your mind works and how
determined you are to make it work. If you focus on the shortcomings of a
program you limit you chances of discovering different ways to make it tell
you what you need to know. Experience is essential because it is the users
thinking that is the important factor. The more you use it - the better you
get - the program and the mind.
Please explain what you mean by below the line - It is important that you
have control about what is a flux and what is not. If I include iron oxide
as a flux - for reduction firing - does that not address what you are
talking about. I take CaO and MgO out of unity for a cone 04 glaze for
instance. One of the limitations I have now is that I don't have a choice
about which CaO I take out. CaO in whiting will not help the melt - but CaO
in a frit will for instance - it makes working on low fired glazes more
difficult.
I don't know what program you use but if you want the iron oxide in unity
for reduction firing - contact the author and ask how to do that. How much
is reduced is where your experience comes in - in a body - if it is
properly reduced - all the iron can be counted as reduced for instance. If
you are firing oil spots in reduction and oxidation - then you will need to
do more research and have a sample analysed to get a better picture. What I
am trying to say here is - if you perceive a shortcoming in a calculation
program - that perception can limit you.
Yes - not having the colouring oxides included is a problem - and it will
be fixed - in the meantime I have iron and rutile and titanium oxide
included in my materials table - I suggest you do the same. As for cobalt -
because so little is used it is usually not a factor - copper can sometimes
be significant. We can still understand this - Hamer gives good info on
which a colouring oxides are fluxes or refractories at what ever
temperature you are working at.
What surprises me about your question is - why not work on the glaze
without the oxides included - as I do most of the time - just treat it as a
separate issue. If your glaze is over fluxed by Cobalt - you need to make
the glaze more refractory - there is nothing complicated about that. As you
learn you will be able to anticipate what is going to happen and find the
answers quicker.
Zircopax is about 35% SiO2 - most of which is unavailable to the glaze melt
- if you are trying to make a stable glaze - and you know you need a
certain amount of SiO2 - you should not include the SiO2 that is in the
Zircopax - so look at the unity formula with it out of the calculation.
Zircopax does have a low expansion however - so when you are looking at the
expansion number include it in the calculation.
None of this is addressed in the manuals that come with calculation
programs - it is up to use to find the ways that give us the answers we
need - how we use our tools is part of our craft - the better we are with
our tools the better the results.
Well this is getting too long - sounds like a whole workshop to me - I
don't have any problem with answering questions about any of this - but it
would be better if there is just one question at a time. I have the feeling
that only a few will get this far and that I am wasting effort - and there
is so much else to do.
Good questions John - thanks - RR
>First of all, I am not degrading UMF, or glaze calculation software! Nor
>do I doubt your expertise or successes. I actually applaud your efforts
>every time I can and insist that you are one of the best things about
>Clayart!
>
>But to have a discussion about the details of the UMF system and its
>benefits or flaws is not to say it is bad, wrong, stupid or useless. It is
>a way to understand it better so we, potters at large, can use it as a
>tool for greater benefit.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
daniel on sat 18 sep 04
Hi Ron, John,
I've stayed out of this so far but have found it very interesting.
Ron, I was thinking about the zircopax issue the other night and pulled out
Hamer. They mention that it does happen that zirconium silicates can result
in more silica in a glaze than you might like. They show a pair of recipes,
with one being adjusted (reducing the flint) from the other to allow for
about 1/3 of ZrO2.SiO2 weight being silica. This was interesting to me
because we'd discussed this before and I was under the impression that the
silica in zircopax was all pretty much insoluble. They unfortunately, do not
quote any source for this and they do not suggest why it might be. The glaze
is cone 10 semi opaque. I have not done any testing on this, but offer it
up for discussion.
> Calculation software is still evolving - we will see improvements in the
> way the programs work. Our discussions will help that happen.
I can already think of some that would be nice.
> Perceived inconsistencies: The question of significance can be seen as a
> reason to disregard the value of calculation. In some cases those perceived
> inconsistencies are exaggerated and that is what I was trying to explain.
> There are ways to get around shortcomings - it depends on your point of
> view and your creativity.
One thing that is often discussed on this list is the development of ones
own tools. Software for glaze calc. is little different. John Hesselberth
for example, added the ability to classify glaze formulae by the sillica
alumina content in an effort to screen for probably durable glazes. Such
screening could be applied other glaze attributes, surface tension
indicators for crawling glazes for example, matting caused by high flux
content and so on. This would allow one to focus on glazes that will likely
posses the properties that one is after.
Thanx
D
Belmont, California, USA
(ex terra australis)
John Britt on sat 18 sep 04
Dorothy, Edouard, Ivor, John and Ron,
I think I have gone on long enough about this, but it has been very
enlightening. Thanks for your interest, time and energy in answering all
my questions! I appreciate it.
We may meet again,
John Britt
www.johnbrittpottery.com
John Britt on sat 18 sep 04
Daniel,
Just one more thought before I stop - It has been my experience that
zircopax does contribute silica to the melt and that is why you use it to
get glosser whites than with tin - that is it adds silica (glass).
John
Earl Brunner on sun 19 sep 04
This might be a problem in other parts of the country, or in
transportability of a recipe from one local to another, but it isn't much of
a problem here in Las Vegas.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Earl Krueger
Sent: Saturday, September 18, 2004 9:40 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Glaze formulae was - Quartz, flint and silica
One of the wonderful things about Clayart is the
exposure you get to how differently people think.
It never occurred to me that people would _not_
take into account the amount of water in their
materials. Having a research chemistry background
it has been difficult for me to adjust to working with
impure materials. In the lab, most compounds you
use are something like 99.9+% pure and are stored
in air-tight bottles. If you are doing the most precise
work you store chemicals in a climate controlled
environment.
Glaze calculation, whether performed by software or
by paper and pencil, is only a tool that attempts to
model the outcome of the complex reactions that occur
inside our kilns. As we learn more about these reactions
and the materials we use it is indubitable that these
calculated models will improve. However we may find
that to get better predictions the models require us to
switch to purer (read more costly) materials or the model
will require more data than the current analysis provide.
Perhaps we will need to not only specify the amount of
CaO, SiO2, Al2O3 equivalents but also the chemical and
physical structure of the materials providing these
elements.
At what point does a potter become an industrial chemist?
Earl K...
Bothell, WA, USA
"You may be disappointed if you fail,
but are doomed if you don't try"
Beverly Sills (1929- )
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Ron Roy on mon 20 sep 04
Hi Linda,
Thanks for the note on gods - got any ideas - or maybe experience about how
to choose the right one. This seems fraught with danger - pick one and
watch out for the others? Perhaps what is needed is one of each - or would
they all resent that?
There are some materials that are anhydrous - like borax - best to probably
calcine it and keep it sealed. I certainly keep my zinc properly covered -
had it for about 40 years now - took some to a workshop this summer and it
was fine.
Both are supposed to be supplied calcined - but who knows how l they have
been stored at the suppliers - or how long?
Also remember - if all our materials are affected by humidity they will
more or less be affected in the same way - except for a few exceptions - so
the results are evened out - that is if you are storing them in the same
way - like sealed or not - frankly - I don't think it's much a concern but
why not be consistent?
Yes - LOI is usually gotten off the analysis sheet from the miner -
sometimes you need to specify it to get it. Some times you can calculate it
out from the formula - Like CaCO3 - it's east to do if you use the
molecular weights - it's one of the things I do in my workshops - it helps
potters understand.
There are times when it is not provided - the analysis comes with the
oxides adding up to 100 - or close - even though you know there is some
combined water - like with clays. Then you need to calcine a known amount
of clay and re weigh to get the loss.
As I said before - materials should be stored covered - it helps prevent
the accidental contamination of materials - just one of the potters
nightmares.
RR
>I wonder too. Also, I've been re-thinking what I said earlier about
>the dryness of raw materials. I stand by my contention that it's
>silly to worry about 3 significant-figure accuracy, but if a material
>has water of hydration, then you certainly have the potential for a
>significant error in the glaze recipe and must account for it. I'm
>guessing that any materials potters buy are not anhydrous; it would
>be too expensive. Some materials could be mixtures of different
>materials, some with 2 water molecules or others with 3 H2O.
>
>As someone said, you can certainly check each batch as it arrives
>and, using glaze calculation software certainly, factor the LOI into
>the formula. This also assumes storage in air/watertight containers.
> In just about any part of the country, humidity varies and a
>material left in a bag will test differently depending upon relative
>humidity.
>
>So, when I set up my studio (I should live so long), I will practice
>a little quality control on the incoming raw materials, do the LOI,
>put that number into the glaze calculation software for my glaze
>recipes and store ALL materials in airtight containers. Think I even
>have an old Coors porcelain crucible around here somewhere and can
>certainly buy more.
>
>Now Ron, this begs the question of what standards you and others put
>into your programs for LOI. Was it based on a suppliers analysis?
>
>Cheers, Linda
>
>Ah, about that god. I imagine that in Graeco/Roman times, potters
>prayed to Colcanus/Hephaistos, the god of fire. Check this quote
>from this site: http://www.theoi.com/Olympioi/Hephaistos.html
>
>=ECVolcanus [Hephaistos], at Jove=EDs [Zeus=ED] command, made a woman=EDs
>form from clay, Minerva [Athene] gave it life, and the rest of the
>gods each gave some other gift. Because of this they named her
>Pandora.=EE =F1Hyginus Fabulae 142
>
>AND
>
>=EC[One] who has learned the works of Athena and Hephaistos, the god of
>many crafts, gathers in his livelihood with his hands.=EE =F1Solon Frag
>13
>
>
>--- Ron Roy wrote:
>
>> Hi Linda,
>>
>> Your comment below made me think of something.
>>
>> I wonder if anyone has ever done a survey of the chard heaps
>> associated
>> with potteries through the ages - and when did they start to get
>> significantly smaller.
>>
>> And I wonder - if reduced losses were ever the result of a pottery
>> god and
>> if so - which one - we would all like to know that.
>>
>> RR
>>
>>
>> >Once you get a transported glaze recipe to work by doing the
>> >appropriate experiments, don't mess with success. Just thank the
>> >potting gods.
>> >
>> >Hmmmmm. Who are the potting gods? I mean it: who did ancient
>> >potters thank for their good fortune?
>> >
>> >Cheers, Linda
>>
>> Ron Roy
>> RR#4
>> 15084 Little Lake Road
>> Brighton, Ontario
>> Canada
>> K0K 1H0
>> Phone: 613-475-9544
>> Fax: 613-475-3513
>>
>>
>___________________________________________________________________________=
___
>> Send postings to clayart@lsv.ceramics.org
>>
>> You may look at the archives for the list or change your
>> subscription
>> settings from http://www.ceramics.org/clayart/
>>
>> Moderator of the list is Mel Jacobson who may be reached at
>> melpots@pclink.com.
>>
>
>___________________________________________________________________________=
___
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.=
com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
Ron Roy on mon 20 sep 04
Hi John,
Let me restate my little demonstaration of more and less silica - I can see
I was less than clear.
I took a glaze with 20% silica (as in a bag of quartz) in the recipe. I
calculated the glaze like that so I would have something to compare the
next example. The next example was -the same glaze but assuming the quartz
was wet -with 10% water added - I knew I would be getting less quartz -
because some of it would be water so I estimated 10% less and entered that
amount of quartz as 18 (instead of 20. Then I compared the two glazes.
The first glaze was a little over the number of mols John and I found to be
necessary to have a stable glaze - the second glaze was closer to the lower
limit than the first - so it would be less likely to be a stable glaze.
Also the silica/alumina ration was lower - so it ment the glaze would
probably be a little less shiny.
To carry this further - say 3 of the materials ina glaze came wet - lets
say Wollastonite, Cornwall Stone and silica - all of these materials have
silica in them - so you would wind up with even less silica.
All I am saying is - it's just another way that recipies can go wrong if
the information is not provided.
It is easy to determine how much extra water is in a material - weigh it
when it arrives - weigh1000 grams out - dry it and weigh it again - then
you have the number you need to adjust your glaze either way by using more
- or less of any wet materials - all you need to do is keep them in sealed
containers - or weigh and dry some again each time you mix up any glazes
that may be affected.
I don't think the molecular formula is all that percise - depending on the
care we take though - My experience is certainly that it's good enough for
what we usually do - my experience with subbing in new raw materials in a
clay body proved that. I also have reformed hundreds of glazes putting in
frit for GB and the great majority work. Try doing that without
calculation.
I work at trying to keep the variables to a minimum - get yearly analysis
of the materials we use - and keep an eye on those that do not have a good
track record for instance.
We test all the raw clays that Tuckers used for instance - so I know which
have analysis that are close each year. When I have to reformulate a body -
because it is not within tolerances - I try to avoid those erratic
materials - or at least cut down on their use. I have not had to alter a
clay body for well over a year now so I think the practice has resulted in
more stable clays.
RR
>It seems to me that if we go to the trouble of all this calculation and
>then -- it can be plus or minus 10% on the silica; +/- 2 percent on the
>bentonite; +/- 5 to 8% on the opacifer and +/- 1 to 10 % on colorants we
>are just looking for general predictors not really precision at all. (That
>could amount to a total of +/- 30%)
>
>Do you think that is a reasonable conclusion?
>
>Thanks,
>
>John
>www.johnbrittpottery.com
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
mailtoandrew@FSMAIL.NET on mon 20 sep 04
Hello Earl K,
I certainly agree with your last post.
Some problems Ive seen with the use of calculations for bodies and glazes
is the use of idealised chemical compositions for raw materials, such as
Al2O3.2SiO2.2H2O for clay or K2O.Al2O3.6SiO2 for feldspar, rather than the
actual composition.
Perhaps semantics, or having a different perspective than yourself, but Im
a little uncomfortable with describing ceramic raw materials as impure ...
Theoretically kaolinite can be pure but kaolin never can be.
Invariably ceramic raw materials are minerals, and even more likely rocks,
and not chemicals ... sure many will be treated and processed to remove,
or perhaps more accurately, reduce the amounts of unwanted material but an
assay standard is, and often can not be, produced. Calcined alumina may be
approaching 99.9+% purity it is highly unlikely that practical amounts of
structurally and chemically perfect kaolinite, feldspar and many other raw
materials will be available. However industrially mined and treated
minerals are controlled to tight specifications.
And to your question: At what point does a potter become an industrial
chemist ... is a, I think, a valid observation. Perhaps its just a
continuum as whilst many are happy to make pots without becoming immersed
in science chemists are employed by ceramic manufacturers and material
suppliers.
Regards,
Andrew
Ron Roy on mon 20 sep 04
Hi John,
Adding zircopax increases the viscosity as well, and is an aid to
recrystalization - and a matting agent if enough is added.
Small amounts can be added to a glaze to help gloss - and because it
provides increased durability.
RR
>Just one more thought before I stop - It has been my experience that
>zircopax does contribute silica to the melt and that is why you use it to
>get glosser whites than with tin - that is it adds silica (glass).
>
>John
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on mon 20 sep 04
Hi Daniel,
Thanks to Tom Buck for helping clear that point up.
I have been reading about zirconium opacification and thought couple of
extra points might be of interest.
Zircionia - and zircon and the opacifying zirconium silicates we use - like
Zircopax, super pax, opax etc. - all have a positive effect on the
stability (read resistance to acid and alkaline leaching) of glazes -
provided those glazes are properly melted.
The finer the grind of those opacifiers the better they opacify.
RR
> Ron, I was thinking about the zircopax issue the other night and pulled out
>Hamer. They mention that it does happen that zirconium silicates can result
>in more silica in a glaze than you might like. They show a pair of recipes,
>with one being adjusted (reducing the flint) from the other to allow for
>about 1/3 of ZrO2.SiO2 weight being silica. This was interesting to me
>because we'd discussed this before and I was under the impression that the
>silica in zircopax was all pretty much insoluble. They unfortunately, do not
>quote any source for this and they do not suggest why it might be. The glaze
>is cone 10 semi opaque. I have not done any testing on this, but offer it
>up for discussion.
>
>> Calculation software is still evolving - we will see improvements in the
>> way the programs work. Our discussions will help that happen.
>
>I can already think of some that would be nice.
>
>> Perceived inconsistencies: The question of significance can be seen as a
>> reason to disregard the value of calculation. In some cases those perceived
>> inconsistencies are exaggerated and that is what I was trying to explain.
>> There are ways to get around shortcomings - it depends on your point of
>> view and your creativity.
>
>One thing that is often discussed on this list is the development of ones
>own tools. Software for glaze calc. is little different. John Hesselberth
>for example, added the ability to classify glaze formulae by the sillica
>alumina content in an effort to screen for probably durable glazes. Such
>screening could be applied other glaze attributes, surface tension
>indicators for crawling glazes for example, matting caused by high flux
>content and so on. This would allow one to focus on glazes that will likely
>posses the properties that one is after.
>
>Thanx
>D
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on mon 20 sep 04
Hi Earl,
I think we rely on the contamination to help melt the materials we use at
the temperatures we work at.
In fact we might consider - which contaminants help the most - and
considering the side effects - we may find ourselves adding them in larger
amounts.
The fact that any new material add to the batch will lower the temperature
somewhat - at what point (or amount) is that extra melting optimized.
I wonder how this all fits into the frit making process - do they consider
this? I do know they use some of the same materials we use. Do they seek
out pure materials or find it an advantage to use less pure materials?
RR
>One of the wonderful things about Clayart is the
>exposure you get to how differently people think.
>It never occurred to me that people would _not_
>take into account the amount of water in their
>materials. Having a research chemistry background
>it has been difficult for me to adjust to working with
>impure materials. In the lab, most compounds you
>use are something like 99.9+% pure and are stored
>in air-tight bottles. If you are doing the most precise
>work you store chemicals in a climate controlled
>environment.
>
>Glaze calculation, whether performed by software or
>by paper and pencil, is only a tool that attempts to
>model the outcome of the complex reactions that occur
>inside our kilns. As we learn more about these reactions
>and the materials we use it is indubitable that these
>calculated models will improve. However we may find
>that to get better predictions the models require us to
>switch to purer (read more costly) materials or the model
>will require more data than the current analysis provide.
>Perhaps we will need to not only specify the amount of
>CaO, SiO2, Al2O3 equivalents but also the chemical and
>physical structure of the materials providing these
>elements.
>
>At what point does a potter become an industrial chemist?
>
>Earl K...
>Bothell, WA, USA
>"You may be disappointed if you fail,
>but are doomed if you don't try"
> Beverly Sills (1929- )
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
David Hewitt on wed 22 sep 04
In message , Ron Roy writes
>Hi David,
>
>What a wonderful topic this is and a special thanks to you for bringing up
>the subject of wet and dry materials.
>
>Will we ever look at a recipe including Cornwall Stone and not wonder if it
>was wet or dry again. One way around this is to assume the worst (best)
>case do it with both wet and analysis - the problem now is - we need to
>know - with wet materials - what percent of water to use - do the mines
>supply that info? Is the water content consistant? Perhaps someone should
>compile the numbers for everyone - and they can become yet another table in
>our glaze books.
>
>If your software thinks the materials you use are dry - and it does if
>there is no allowance for it (like using LOI to take it into account) -
>then is not the % analysis and molecular formula in fact wrong?
>Best regards - RR
It is certainly not as good an analysis as it could be.
I am altering my glaze program, CeramDat, to include two raw materials
for flint/quartz and Cornish stone, labelling them ' x% wet' and 'dry'.
The recipes that I include with the program will also show which has
been included. The user of the program can choose which one to use
according to their particular supply conditions.
From tests with my stock of quartz and Cornish stone it would seem
sensible to say that quartz has an 8% LOI (8% wet) and Cornish stone 10%
LOI (10% wet) because of its moisture content. This does of course
depend on how it is stored. Users of glaze programs can of course enter
their own raw materials and label them according to their supply
conditions.
David
>
>
>
>
>>The question is, how to make the information given in response to such
>>requests as meaningful and useful as possible. Part of this is surely to
>>give an analysis of the recipe (together, of course, with as much
>>information as to the body you use, the method of applying the glaze and
>>the firing as you can put together)
>
>Ron Roy
>RR#4
>15084 Little Lake Road
>Brighton, Ontario
>Canada
>K0K 1H0
>Phone: 613-475-9544
>Fax: 613-475-3513
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
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