Robert Wilt on thu 16 jan 97
....
>(software developers take note: add limit formulas into program)
....
>Thanks, Brad Sondahl, Nezperce Idaho
>bsondahl@camasnet.com
It's actually next on the list for GlazeChem, and should be in the
next release (April or so). It will have a few standard limit formulas
from published sources, and will let you enter your own, generate
limits from sets of glazes, compare glazes to limit formulas, and
those sorts of things. HyperGlaze and (I think) Insight already can
give you help with limit formulas.
As for the "published sources" for limit formulas, I'm still looking
around myself. I'd be happy to hear what other people recommend.
However, generating our own limit formulas from sets of glazes we
know to work is probably the best long-term solution. For example,
Richard Burkett developed custom limit formulas for shinos, celadons,
etc. by analyzing some number of successful glazes of those types.
(These limit formulas are part of HyperGlaze.) More of this could
be done; it would be great to do it collectively and share results.
The GlazeBase project is very important in this regard.
By the way, you can go quite far with GlazeChem (and hopefully other
software) just by comparing your numbers with the numbers for glazes
in the existing ClayArt and GlazeBase databases. You can download the
text versions of these databases, import them into GlazeChem, sort
out the ones in the temperature range of interest, and then sort
them by Al:Si. You'll see the whole range of Al:Si ratios at a glance,
and can make some determination as to where your glaze lies among them.
(Let me know if you need more specific information on how to do all this.)
bob wilt
Richard Burkett on fri 18 apr 97
The current discussion about the source of limit formulas seems to miss
the point, in my opinion. FAR too much credence is often given to limit
formulas, with frequent references to them (on this list at least) as
"rules" not to be broken.
Yes, limit formulas are useful, but they have NEVER been anything more
than guidelines for glazes. Glazes are far too complex to always fit into
a particular limit of the generic kind published. It's entirely possible,
using the typical generic gloss glaze limits for cone 10 say, to make a
glaze that is likely to not be glossy - the limits don't always work that
well if the glaze in question falls too near the extremes of the limit.
They can be useful predictors of likely glaze qualities, BUT using limit
formulas also requires user knowledge of glaze materials.
Limit formulas were originally devised to be generic - to give a broad
picture of the useful range of certain oxides and materials. In this
respect they are quite useful. However they are NOT in my opinion all that
useful in determining whether a glaze is "safe" or not, other than as a
rough suggestion as to whether the glaze is somewhat within the range of a
standard industrial glaze. Certainly testing is advised if there is any
question.
As far as I have seen, much of the work on glaze limits and related
matters was done in the early part of this century when industrial glaze
chemists and ceramic engineers were still putting their efforts into
pottery-oriented research. Ceramic society abstracts from this era contain
many references to work done with this type of glaze or that. Often these
articles will make reference to the working limits of particular oxides
for the particular glaze type in question. Even with these limits, there
is often one or perhaps two formulas that are cited as giving the best
result of all the glazes tested within the limits.
One of the things that I built into HyperGlaze was the ability to pick out
a set of glazes with known characteristics and have the program calculate
limit formulas for that specific type of glaze. I find this a much more
useful approach for certain types of glazes, and it allows for limits that
fall outside the range of the classic ones found in textbooks, limits that
target specic glaze qualities, or custom limits that might be specific to
your particular clay/glaze combination. I still only consider these more
specific limits to be mere suggestions as a starting point or reference
point for glazes.
I would also encourage anyone wanting to work on more specific limit
formulas that might be useful to develop particular types or colors of
glaze to consider sharing these through the web. This could be a very
useful resource to all of us.
Richard
Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
E-mail: richard.burkett@sdsu.edu <-> Voice mail: (619) 594-6201
Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
CeramicsWeb: http://apple.sdsu.edu/ceramicsweb/index.html
Tony Hansen on tue 22 apr 97
A conservative 'formula approach' is appropriate for chemistry beginners
and those lacking access to testing (that's about 95% of us!).
I've never made bread but I'm sure there are some limits, I'm guessing
that
1 teaspoon of yeast is good but that one cup is not. Likewise I've found
glaze limit formulas (or can I call them 'suggestion formulas')
valuable.
If a formula has 3x the max on an oxide (I see this all the time with
manganese, barium, lithium, fluxes, metallics) I think eyebrows should
raise! If the suggested SiO2 min is 3.0 and a formula has 1.5 I question
if
there is enough glass former for chemical stability. My flow tests often
vindicate the SiO2/Al2O3 limits and these are the ones I deal with 90%
of
the time with customers. I think limits are great starting points and
references for liner glazes on food surfaces.
What we have beats the heck out of absolutely nothing. While exisiting
recipes can be averaged to isolate formula profiles for decorative
effects
many of these are controversial for food surfaces, behave erratically,
and
don't travel well.
I find limit formulas a nice starting point for analysing many
troublesome
glazes. Analysing their deviance from the limits often points me in the
right direction to fixing the problem. The fact that you can create a
new
glaze from mid-range limit chart numbers is a testament to their
predictability.
I've often wondered about their sources too, but I now judge them for
content. I find the performance of colorants vindicates the concept of
limit formulas (guidelines). If 2% stain gives a light color and 7.5%
achieves the darkest, then why put in 15%? It is wasted and probably
soluble and contributes to instability or one sort or another. Why
wouldn't
it be the same with many other oxides regarding the property they
contribute?
--
=================================================================
Tony Hansen, IMC thansen@mlc.awinc.com
INSIGHT5/Magic of Fire II demos at www.ceramicsoftware.com
Chris Schafale on tue 10 nov 98
Greetings,
Now that I've finally got my kiln, I'm entering the wild and
wonderful world of glaze chemistry and I'm rapidly gaining respect
for all those incomprehensible discussions I used to use my Delete
key to deal with. Spent all day yesterday mixing tests -- haven't
had so much fun since I was a kid and used to make "concoctions" in
the kitchen. I've been playing with Insight to see what these
recipes really are made of and to try tweaking them in various ways.
I have some idea of what the various materials are used for, and I
get the general concept of limit formulas as a target zone within
which the glaze is most likely to be safe and durable. I understand
the concept of pushing the limits for specific effects, and I'm
enthusiastic about testing, but I'd like to have a better idea of
what's likely to happen when I push a particular limit, and whether
what I'm losing when I go outside the limits is appearance or safety.
My question is, can anyone suggest a reference work that describes
what the various oxides do in relation to the limits? For instance,
Ron Roy frequently says, "this glaze is oversupplied with boron, so
probably will not be durable". What are the equivalent statements
(are there equivalent statements?) for other oxides? For instance,
what is the effect of too much magnesium? Calcium? Lithium? Zinc?
I think I understand that too much alumina will prevent the glaze
from melting, and too little will likely make it run off the pot. Is
that right? You get the idea. Is there a book out there with all
this in it?
Thanks!
Chris
P.S. For those who may remember my wail about "the cracks that won't
quit" last summer, I'm pleased to tell you that, with all your good
suggestions, I've had a 100% success rate in the pieces I've made
since then (with the exception of pieces thrown off the hump, which
I'm still working on). I just got the last batch of casseroles out
of the bisque firing -- 6 out of 6 with no cracks!!!!! By the way, I
haven't changed clay bodies, though I did add fine grog. Now I just
have to go back and figure out which of the eight or so
process changes I made was responsible for the improvement (I was too
frustrated to be systematic at the time.) Thanks again to all who
helped.
Light One Candle Pottery
Fuquay-Varina, NC
candle@nuteknet.com
camoore on wed 11 nov 98
Chris,
Daniel Rhodes's CLAY AND GLAZES FOR THE POTTER (a potter's classic) has
a section that deals colorants. In my 1967 printing of the book, the
section is located on pp. 206-207.
A much newer book, richard Zakin's ELECTRIC KILN CERAMICS gives a
similar section on colorants, p. 94.
Luck.
Charles
John Hesselberth on wed 11 nov 98
Chris,
In all the reading I have done, I have not seen such a summary with
respect ot safety. Taylor or Bull, Ceramics Glaze Technology probably
comes the closest. With respect to appearance and functionality, though,
there is an excellent book by Harry Fraser titled Ceramic Faults. It
should be in every glaze chemist's library. John
Chris Schafale wrote:
>----------------------------Original message----------------------------
>Greetings,
>
>Now that I've finally got my kiln, I'm entering the wild and
>wonderful world of glaze chemistry and I'm rapidly gaining respect
>for all those incomprehensible discussions I used to use my Delete
>key to deal with. Spent all day yesterday mixing tests -- haven't
>had so much fun since I was a kid and used to make "concoctions" in
>the kitchen. I've been playing with Insight to see what these
>recipes really are made of and to try tweaking them in various ways.
> I have some idea of what the various materials are used for, and I
>get the general concept of limit formulas as a target zone within
>which the glaze is most likely to be safe and durable. I understand
>the concept of pushing the limits for specific effects, and I'm
>enthusiastic about testing, but I'd like to have a better idea of
>what's likely to happen when I push a particular limit, and whether
>what I'm losing when I go outside the limits is appearance or safety.
>
>My question is, can anyone suggest a reference work that describes
>what the various oxides do in relation to the limits? For instance,
>Ron Roy frequently says, "this glaze is oversupplied with boron, so
>probably will not be durable". What are the equivalent statements
>(are there equivalent statements?) for other oxides? For instance,
>what is the effect of too much magnesium? Calcium? Lithium? Zinc?
>I think I understand that too much alumina will prevent the glaze
>from melting, and too little will likely make it run off the pot. Is
>that right? You get the idea. Is there a book out there with all
>this in it?
>
>Thanks!
>
>Chris
>
>P.S. For those who may remember my wail about "the cracks that won't
>quit" last summer, I'm pleased to tell you that, with all your good
>suggestions, I've had a 100% success rate in the pieces I've made
>since then (with the exception of pieces thrown off the hump, which
>I'm still working on). I just got the last batch of casseroles out
>of the bisque firing -- 6 out of 6 with no cracks!!!!! By the way, I
>haven't changed clay bodies, though I did add fine grog. Now I just
>have to go back and figure out which of the eight or so
>process changes I made was responsible for the improvement (I was too
>frustrated to be systematic at the time.) Thanks again to all who
>helped.
>
>
>Light One Candle Pottery
>Fuquay-Varina, NC
>candle@nuteknet.com
John Hesselberth
Frog Pond Pottery
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"The whole aim of practical politics is to keep the populace alarmed, and
hence clamorous to be led to safety, by menacing it with an endless
series of hobgoblins, all of them imaginary." H.L. Mencken, 1925
rballou@mnsinc.com on wed 11 nov 98
Chris,
Stoneware Glazes: A Systematic Approach by Ian Currie will help further
your understanding of glaze materials. It's available from the Potters
Shop. The book is a manual for conducting experiments with each of the
major glaze oxides. A series of experiments with each oxide will help
understand the nature of its contribution to glazes and its limit formula.
For example, for CaO, there are 4 experiments, .9 CaO, .8 CaO, .7 CaO and
..6 CaO. These are mixed in a biaxial blend with clay and silica. A biaxial
blend differs from a triaxial and quadraxial in significant way. In a
triaxial, 3 different ingredients are varied making it difficult to tell
which variable is causing the effect you see. Quadraxials present the same
problem, as 4 ingredients are varied. In biaxial, one ingredient is held
constant, and the remaining two ingredients are varied. So in the .9 CaO
experiment, which generates 35 glazes on one divided tile in a 5 x 7 grid,
each glaze contains .9 CaO. The alumina and silica are varied, so you'll
learn about the critical role of these two components of glazes. The book
also tells you how to make up your own experiments. This is the format that
Craig Martell used for the shino test. He gave a very nice description of
the tile and how it's laid out, if you want to check the archives. It's
very clear once you see it, but a little difficult to explain.
If you do all the experiments, you'll have a library of reference tiles.
They will be an invaluable aid in exploring and understanding glazes. The
testing may seem like its a lot of work and I don't mean to minimize the
effort that's required, but in the long run, it will save you a lot time
and increase your understanding of glaze ingredients and limit formula.
Don't know any better way to do it.
Ian also has an informative web page that explains, among other things, how
to apply his method to existing glaze recipes. The address is
http://ian.currie.list.to/index.html
Ruth Ballou
Silver Spring, MD
Ron Roy on thu 12 nov 98
Hi Chris,
I would recommend the Potters Dictionary of Materials and Techniques by
Frank and Janet Hamer to get a good overview of what the different oxides
do in a glaze. Many other texts will have some additional information
depending on the authors experience - you can't be oversuppied with books
when it comes to learning about clay and glazes.
In many cases an oversupply of an oxide will result in that oxide being
"left over" - not able to remain in solution in the glaze as it cools. The
calcium matts are an example of this - an oversupply of CaO will result in
a matte surface - and in this case the glaze will still be somewhat
transparent.
RR
>----------------------------Original message----------------------------
>My question is, can anyone suggest a reference work that describes
>what the various oxides do in relation to the limits? For instance,
>Ron Roy frequently says, "this glaze is oversupplied with boron, so
>probably will not be durable". What are the equivalent statements
>(are there equivalent statements?) for other oxides? For instance,
>what is the effect of too much magnesium? Calcium? Lithium? Zinc?
>I think I understand that too much alumina will prevent the glaze
>from melting, and too little will likely make it run off the pot. Is
>that right? You get the idea. Is there a book out there with all
>this in it?
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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