Fabienne Cassman on sun 1 nov 98
------------------
Today I explored going from a formula to a batch and stumbled over
something interesting :) While using Insight, I entered the following =
formula:
..61 CaO, .10 K2O, .21 Na2O, .08 SrO, .35 B2O3, .35 Al2O3 and 3.5 SiO2
I picked values that seemed to stay within Orton c3-7 limits except for the
CaO which is 0.01 above the upper limit. The Al2O3 and SiO2 are both as
high as they can be and B2O3 is NOT in unity.
I then decided to put B2O3 in unity and to compare it to Green=26Cooper's
limits for a c6 glaze and here is what happened:
CaO 0.45=2A, K2O 0.07=2A, Na2O 0.16=2A, B2O3 0.26=2A, Al2O3 0.26, SiO2=
2.59,
SrO 0.06=2A
There was no indication that CaO was even slightly oversupplied. In fact,
CaO as a long way to go before it's oversupplied, i.e. 0.55=21
Furthermore, I found myself slightly undersupplied with Al2O3 and no matter
how hard I tried, there was no way I could satify both limits for it=3B it
feels like there is a blind spot.
I would like to know how do those of you who make your own glazes decide
which limits to use?
TIA
=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=
=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60
Fabienne
Yes, I have learned from my mistakes...
I can reproduce them exactly.
John Hesselberth on mon 2 nov 98
>----------------------------Original message----------------------------
>------------------
>Today I explored going from a formula to a batch and stumbled over
>something interesting :) While using Insight, I entered the following
>formula:
>
>.61 CaO, .10 K2O, .21 Na2O, .08 SrO, .35 B2O3, .35 Al2O3 and 3.5 SiO2
>
>I picked values that seemed to stay within Orton c3-7 limits except for the
>CaO which is 0.01 above the upper limit. The Al2O3 and SiO2 are both as
>high as they can be and B2O3 is NOT in unity.
>
>I then decided to put B2O3 in unity and to compare it to Green&Cooper's
>limits for a c6 glaze and here is what happened:
>
>CaO 0.45*, K2O 0.07*, Na2O 0.16*, B2O3 0.26*, Al2O3 0.26, SiO2 2.59,
>SrO 0.06*
>
>There was no indication that CaO was even slightly oversupplied. In fact,
>CaO as a long way to go before it's oversupplied, i.e. 0.55!
>
>Furthermore, I found myself slightly undersupplied with Al2O3 and no matter
>how hard I tried, there was no way I could satify both limits for it; it
>feels like there is a blind spot.
>
>I would like to know how do those of you who make your own glazes decide
>which limits to use?
>
Fabianne,
While I am not yet certain of this, your discovery is just one more data
point that tells me there is nothing magic or very scientific about limit
formulas. My working hypothesis as I read abstracts of hundreds of
articles published from 1900 up to recent years is that limit formulas
are integrated observations from hundreds of glaze experiments that made
"good" or "bad" glazes. Good and bad were different in the eyes of every
observer, but were what a particular observer considered to be most
important from a set of things like abrasion resistance, crazing,
crawling, shivering, solubility in (or durability to) acids or bases,
level of gloss, etc. Hence there are several published limit formulas
which are not necessarily consistent with each other--none of them are
necessarily any better than any others and all should probably just be
considered to be only rough guidelines or areas where you might want to
start. If there is any more substance to limit formulas than what I have
outlined above, I have yet to find it in the literature. Test, test,
test yourself to find glazes that work for you and meet criteria that are
important to you.
More later when I get farther into my reading of the literature.
By the way, I'm still looking for someone who has or has access to Volume
2 of the Collected Writings of Herman A. Seger. Can anyone point me
toward a copy to buy or borrow?
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
Paul Lewing on wed 4 nov 98
Fabienne,
Limit formulas are only the most general of parameters. I think if
you were designing hotel china, you'd want to keep everything within
the limits, but almost all the really cool glazes have at least one
ingredient outside the limits. Tony Hansen's 5x20 glaze, which is a
great glaze that doesn't craze on most clay bodies, and is probably
food-safe, has almost.7 moles of CaO. Go figure. In fact, now, if I
see a glaze where everything is within the limit formulas, I don't
even test it. I figure it will be too much like a lot of other glazes
I already have.
As far as the boron, and whether to include it in unity or not, that's
hard to say. Boron is a very strange beast, especially in the
mid-range of temperature. It always fits by chemical formula into the
R2O3 column, but it acts sometimes like a flux, and sometimes like a
glass-former. In some cases, it lowers expansion, and in others, it
raises it. Different calculation programs let you include it in some
way in all three columns. It's one of the numbers that always has to
be taken into consideration when you look at a glaze formula. And
it's one of the numbers that makes interpreting this stuff an art as
well as a science.
Paul Lewing, Seattle
Fabienne Cassman on thu 5 nov 98
------------------
Thank you to those of you who answered so far. I really hope that more of
you would give me some insight on how you decided which limit to use.
I can deal with the fact that limits don't hold all the answers, that they
are a good guideline/headstart, and that testing needs to be done
ultimately. However, right now, I get a feeling that potters are not taking
them seriously.
Does everyone agree that glaze limits are only a scientific disguise for
the art of pottery? Does it really matter which limit I start with then?
=2ALooking at the unglazed pieces piling up in the basement. Help=21=2A
=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=
=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60
Fabienne
Yes, I have learned from my mistakes...
I can reproduce them exactly.
Barney Adams on fri 6 nov 98
I would like to hear about this also. I'd feel a lot more comfortable
using the limit tables especially at the level of knowledge I have
at the moment. Does any one who follows the limits ever utilize more
than one for diffrent situations? I under stand the argument between the
two camps on this, but it clouds up learning either way for us trying
to pick it up.
Barney (where all this is about as clear as a rich tenmoku)
>
> ----------------------------Original message----------------------------
> ------------------
> Thank you to those of you who answered so far. I really hope that more of
> you would give me some insight on how you decided which limit to use.
>
> I can deal with the fact that limits don't hold all the answers, that they
> are a good guideline/headstart, and that testing needs to be done
> ultimately. However, right now, I get a feeling that potters are not taking
> them seriously.
>
> Does everyone agree that glaze limits are only a scientific disguise for
> the art of pottery? Does it really matter which limit I start with then?
>
> *Looking at the unglazed pieces piling up in the basement. Help!*
> $:0`0:$x,88,x$:0`0:$x,88,x$:0`0:$x,88,x$:0`0:$x,88,x$:0`
> Fabienne
> Yes, I have learned from my mistakes...
> I can reproduce them exactly.
>
David Hendley on fri 6 nov 98
At 09:19 AM 11/5/98 EST, Fabienne wrote:
>I can deal with the fact that limits don't hold all the answers, that they
>are a good guideline/headstart, and that testing needs to be done
>ultimately. However, right now, I get a feeling that potters are not taking
>them seriously.
I take the limit formulas very seriously when deciding what glazes
to use on food-bearing surfaces.
For the ousides of pots and for decorative pieces....who cares?
A technically flawed glaze is often more interesting.
>
>Does everyone agree that glaze limits are only a scientific disguise for
>the art of pottery? Does it really matter which limit I start with then?
>
I've found the limit formulas from different sources to be remarkable
similar. There is a general consensious as to what makes a technically
good glaze. That's where to start.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
John Hesselberth on sat 7 nov 98
I am happy to see all the interest in limit formulas and their relation
to glaze safety. As many of you know I am deeply involved in a
literature search to try and put the subject in proper historical
perspective. My work starts in the mid-1880s with Herman Seger's work
(the original guru of the unity formula, limit formulas and the father of
modern glaze chemistry in my opinion) and proceeds to the current time.
It is just too early to say any more than I already have about the nature
of the work behind limit formulas--I'm still trying to keep an open mind
and would prefer not to have to eat too much crow. I promise to post an
extended article (several pages) with a complete bibliography within 2
weeks. Even then, it will still be "a work in progress", but I should be
far enough along to be down to dotting "i"s and crossing "t"s.
In the meantime, follow Dave Hendley's advice and stay within the limit
formulas if you are exposing the glazed surface to food. I would add to
that to have your fired glaze tested by the Alfred Analytical Lab for
extractable metals. Roland Hale, the lab director, gives good service
and the cost is modest ($10 extraction fee + $10 for each metal
analyzed). If anyone is interested in specific instructions on how to do
this, I'll be happy to post them with up-to-date addresses and phone
numbers.
John
>----------------------------Original message----------------------------
>I would like to hear about this also. I'd feel a lot more comfortable
>using the limit tables especially at the level of knowledge I have
>at the moment. Does any one who follows the limits ever utilize more
>than one for diffrent situations? I under stand the argument between the
>two camps on this, but it clouds up learning either way for us trying
>to pick it up.
>
>Barney (where all this is about as clear as a rich tenmoku)
>
>>
>> ----------------------------Original message----------------------------
>> ------------------
>> Thank you to those of you who answered so far. I really hope that more of
>> you would give me some insight on how you decided which limit to use.
>>
>> I can deal with the fact that limits don't hold all the answers, that they
>> are a good guideline/headstart, and that testing needs to be done
>> ultimately. However, right now, I get a feeling that potters are not taking
>> them seriously.
>>
>> Does everyone agree that glaze limits are only a scientific disguise for
>> the art of pottery? Does it really matter which limit I start with then?
>>
>> *Looking at the unglazed pieces piling up in the basement. Help!*
>> $:0`0:$x,88,x$:0`0:$x,88,x$:0`0:$x,88,x$:0`0:$x,88,x$:0`
>> Fabienne
>> Yes, I have learned from my mistakes...
>> I can reproduce them exactly.
>>
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
Ron Roy on sun 8 nov 98
------------------
Hi Fabienne,
Been out of town and now swimming in clayart messages.
I wondered how long it would take for someone to notice this.
Because of my experience - looking at cone 6 glazes with significant
amounts of boron - I have come to the conclusion that the limits with boron
in unity are more appropriate for them.
If you want to see why I think so - increase the alumina in your glaze to
..28, .32, and .34 - you will find an increase in gloss.
The other reason I think they are more appropriate, at least for this mid
range - cone 5 to 7 - boron is a strange oxide when compared to all the
others - it is a glass maker which melts so low that it also acts like a
flux - that is not the whole story however - If the boron content in a
glaze is high enough, phase separation will result, which in turn will have
a detrimental effect on durability. Having some extra silica and alumina
seems like a good idea to me.
I should also add - if the glaze satisfies either set of these limits I
would tend to think of them as durable - unless there is an oversupply of
certain colouring oxides.
I should also add - every time I check a glaze that has demonstrated an
inability to resist acid attack - against these limits - I have always
found then to be lacking in silica.
I keep hearing that there are glazes, which fall outside the limits, which
are durable. What I have never seen is an example of such a glaze. If
anyone has such information I would like to hear about it.
RR
=3EToday I explored going from a formula to a batch and stumbled over
=3Esomething interesting :) While using Insight, I entered the following =
formula:
=3E
=3E.61 CaO, .10 K2O, .21 Na2O, .08 SrO, .35 B2O3, .35 Al2O3 and 3.5 SiO2
=3E
=3EI picked values that seemed to stay within Orton c3-7 limits except for =
the
=3ECaO which is 0.01 above the upper limit. The Al2O3 and SiO2 are both as
=3Ehigh as they can be and B2O3 is NOT in unity.
=3E
=3EI then decided to put B2O3 in unity and to compare it to Green=26Cooper's
=3Elimits for a c6 glaze and here is what happened:
=3E
=3ECaO 0.45=2A, K2O 0.07=2A, Na2O 0.16=2A, B2O3 0.26=2A, Al2O3 0.26, =
SiO2 2.59,
=3ESrO 0.06=2A
=3E
=3EThere was no indication that CaO was even slightly oversupplied. In =
fact,
=3ECaO as a long way to go before it's oversupplied, i.e. 0.55=21
=3E
=3EFurthermore, I found myself slightly undersupplied with Al2O3 and no =
matter
=3Ehow hard I tried, there was no way I could satify both limits for it=3B =
it
=3Efeels like there is a blind spot.
=3E
=3EI would like to know how do those of you who make your own glazes decide
=3Ewhich limits to use?
=3E
=3ETIA
=3E
=3E
=3E
=3E
=3E=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=
=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=
=60
=3EFabienne
=3E Yes, I have learned from my mistakes...
=3E I can reproduce them exactly.
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
Craig Martell on sun 8 nov 98
Hi:
Limit formulas may not make a hell of a lot of sense until you gain an
understanding of what you're limiting and why. How do you gain a sense of
glaze materials and what they do, and how much to use, and will this stuff
cause me to go blind or what?
Do some reading, or ask a colleague or a clayarter, and get an idea of what
the different materials are used for. Which are fluxes, what contributes
alumina or silica, what makes color, what opacifies, what is toxic?
Start with some simple tests of melting or firing individual materials to
the temp you glaze fire at. See what happens, some stuff melts and some
stuff doesn't. Do some more reading and questioning. Then do some simple
line blends of feldspar and other fluxes and see how they interact. Use the
fusion button method to determine the rate of fusion. If you just pile
stuff on a tile, you won't know the difference in fusion rates between one
mixture or another. They all might be glassy, but you can't really
determine the stiffness of the melt unless all tests start with a "known"
shape. There is a photo of some spar fusions in Robin Hopper's book the
Ceramic Spectrum on pg. 49. This is a good text to peruse in general, for
simple testing methods to develop some familiarity with glaze making. From
line blends, you can move to biaxial, triaxial, and quadraxial blends and
you'll eventually develop good glazes on your own.
As far as limits are concerned, use them as guidelines for making glazes.
They give general parameters for all the different materials at whatever
temp you are working at. You can use limits to make glazes, or you can go
beyond the limits. That's what testing is all about. You are trying to see
what the possibilities are. Push it!
I stongly recommend Ian Curries methods for developing an understanding of
what fluxes, in controlled proportions do to influence glazes when they are
systematically blended with silica and alumina. Ian's method is a very
powerful tool for understanding glaze development and investigation.
Once you get some glazes that you want to use run them through some
calculation software and compare the composition to recommended limits.
Have them tested if there is any question. Check it all out and become an
authority in your own right.
now get out there an' do it to it, Craig Martell in Oregon
Ron Roy on sun 8 nov 98
Hi Fabienne,
I just got some feedback on a cone 6 maj glaze I had designed for someone.
She had it tested for release at Alfred and it passed - even when mixed
with the glaze for painting over the same glaze.
When I compare this glaze with the limits that come with Insight the silica
is within but the alumina is a bit over.
When I use the boron in unity with fluxes limits the silica and alumina are
within.
To answer your question - which to use - just make sure you use limits
designed for durability - the limits in the Zakin book for instance are
designed to help predict surface - matte, shiny etc.
I am sorry I can't give the recipe but the glaze was commissioned - I can
say I do believe the limits you are using are a good bet to give durable
glazes based on feed back over the last few years.
I will keep the list informed of additional information as it comes in -
anyone who is interested in the subject and would consider pooling our
resources to do some serious investigation - let me know.
RR
>----------------------------Original message----------------------------
>Thank you to those of you who answered so far. I really hope that more of
>you would give me some insight on how you decided which limit to use.
>
>I can deal with the fact that limits don't hold all the answers, that they
>are a good guideline/headstart, and that testing needs to be done
>ultimately. However, right now, I get a feeling that potters are not taking
>them seriously.
>
>Does everyone agree that glaze limits are only a scientific disguise for
>the art of pottery? Does it really matter which limit I start with then?
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
Chris Schafale on tue 10 nov 98
Ron--
When you get a chance, could you expand on the following?
"Because of my experience - looking at cone 6 glazes with significant
amounts of boron - I have come to the conclusion that the limits with
boron in unity are more appropriate for them.
If you want to see why I think so - increase the alumina in your glaze
to .28, .32, and .34 - you will find an increase in gloss."
I'm not sure I get it -- I thought more alumina would tend to make
the glaze more matte?? Or is that the point? And if so, why?
Also, is it possible to get Insight to include the boron in unity?
If so, how?
Chris
Light One Candle Pottery
Fuquay-Varina, NC
candle@nuteknet.com
Chess Denman on tue 10 nov 98
thanks for your post but who is Ian Curries and how does one discover his
method?
CHess (from UK)
-----Original Message-----
From: Craig Martell
To: CLAYART@LSV.UKY.EDU
Date: 09 November 1998 03:51
Subject: Re: Choosing Limits to Make Glazes
>----------------------------Original message----------------------------
>Hi:
>
>Limit formulas may not make a hell of a lot of sense until you gain an
>understanding of what you're limiting and why. How do you gain a sense of
>glaze materials and what they do, and how much to use, and will this stuff
>cause me to go blind or what?
>
>Do some reading, or ask a colleague or a clayarter, and get an idea of what
>the different materials are used for. Which are fluxes, what contributes
>alumina or silica, what makes color, what opacifies, what is toxic?
>
>Start with some simple tests of melting or firing individual materials to
>the temp you glaze fire at. See what happens, some stuff melts and some
>stuff doesn't. Do some more reading and questioning. Then do some simple
>line blends of feldspar and other fluxes and see how they interact. Use
the
>fusion button method to determine the rate of fusion. If you just pile
>stuff on a tile, you won't know the difference in fusion rates between one
>mixture or another. They all might be glassy, but you can't really
>determine the stiffness of the melt unless all tests start with a "known"
>shape. There is a photo of some spar fusions in Robin Hopper's book the
>Ceramic Spectrum on pg. 49. This is a good text to peruse in general, for
>simple testing methods to develop some familiarity with glaze making. From
>line blends, you can move to biaxial, triaxial, and quadraxial blends and
>you'll eventually develop good glazes on your own.
>
>As far as limits are concerned, use them as guidelines for making glazes.
>They give general parameters for all the different materials at whatever
>temp you are working at. You can use limits to make glazes, or you can go
>beyond the limits. That's what testing is all about. You are trying to
see
>what the possibilities are. Push it!
>
>I stongly recommend Ian Curries methods for developing an understanding of
>what fluxes, in controlled proportions do to influence glazes when they are
>systematically blended with silica and alumina. Ian's method is a very
>powerful tool for understanding glaze development and investigation.
>
>Once you get some glazes that you want to use run them through some
>calculation software and compare the composition to recommended limits.
>Have them tested if there is any question. Check it all out and become an
>authority in your own right.
>
>now get out there an' do it to it, Craig Martell in Oregon
>
Michael Banks on tue 10 nov 98
Hi Ron,
Here's a possible durable glaze way outside the limits. A few years ago I
fired a stoichiometric mix of zinc oxide and boric acid to produce pure zinc
borate glaze. It produced a hard, extremely low expansion glass which
appeared to be durable but I never tested it. Even if this composition
isn't actually durable, I predict that it could be made so by additions of
alumina. This would be an interesting silica-free glaze. Alumina seems to
have greater solubility in boron oxide melts than silica, hence the glossing
effect you mention.
The phase separation with more than a relatively small amount of boric oxide
in silica glasses is caused by liquid immiscibility, but it would be
interesting to see how much silica will go into stable borate glasses such
as zinc borate, before this occurs.
----------------------------Original message----------------------------
(Abridged)
I keep hearing that there are glazes, which fall outside the limits, which
are durable. What I have never seen is an example of such a glaze. If
anyone has such information I would like to hear about it.
RR
Ron Roy
Ron Roy on thu 12 nov 98
Good questions Chris,
I have noticed in cone 6 glazes with boron - if get enough alumina into the
glaze to satisfy the limits that come with Insight - when I add more
alumina very often that glaze will become more glossy. This means to me
that there is something going on between the boron and the alumina. I have
found no references to this in my library. I have even tried to make some
sense of it looking at phase diagrams but nothing there that I can see.
This makes me feel the limits with boron in unity are more appropriate for
those types of glazes because they demand more alumina.
There is a point when, as you add more and more alumina, the effect is a
stiffer and stiffer glaze with less gloss - the normal expectation.
The get boron into unity with the fluxes - I am still using version 4 for
the Mac - I simply double click on B2O3 in the formula column and a dialog
box appears. There are two check boxes marked 1 and 2 and labled
"Participates in Unity. The first one (1) is the one you want to check and
boron will then be included in flux unity.
It is very easy to switch the unity back and forth so you can easily check
glazes both ways.
I am noe running a windows emulator on my Mac and have Insight 5 for
windows installed so if you need instructions for that I can do it for you.
It is very easy to switch the unity back and forth so you can easily check
glazes both ways.
Later - RR
>----------------------------Original message----------------------------
>Ron--
>When you get a chance, could you expand on the following?
>"Because of my experience - looking at cone 6 glazes with significant
>amounts of boron - I have come to the conclusion that the limits with
>boron in unity are more appropriate for them.
>
>If you want to see why I think so - increase the alumina in your glaze
>to .28, .32, and .34 - you will find an increase in gloss."
>
>
> I'm not sure I get it -- I thought more alumina would tend to make
>the glaze more matte?? Or is that the point? And if so, why?
>
>Also, is it possible to get Insight to include the boron in unity?
>If so, how?
>
>Chris
>
>Light One Candle Pottery
>Fuquay-Varina, NC
>candle@nuteknet.com
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
Michael Banks on fri 13 nov 98
Ron,
I think it has something to do with polymerisation. In a boron-free glaze,
adding alumina results in a rapid increase in long chain Si-Al polymers with
consequent rise in viscosity, surface tension and matting of the surface.
In borosilicate glazes the degree of polymerisation is reduced to a degree
related to the activity (concentration) of boron, but still has an effect on
the final glaze surface. Adding still more alumina may decrease viscosity
and devitrification tendancy (surface matting on cooling) because this
excess forms a relatively fluid boro-aluminate liquid, increasing the fluid
melt/polymerised melt ratio. Polymerisation, devtrification etc effects do
not appear on phase diagrams because they are due to properties other than
chemical equilibria, for example crystal field energies.
----------------------------Original message----------------------------
(Abridged)
I have noticed in cone 6 glazes with boron - if get enough alumina into the
glaze to satisfy the limits that come with Insight - when I add more
alumina very often that glaze will become more glossy. This means to me
that there is something going on between the boron and the alumina. I have
found no references to this in my library. I have even tried to make some
sense of it looking at phase diagrams but nothing there that I can see.
Later - RR
Ron Roy on sat 14 nov 98
Hi Michael,
Thanks for this - I have a few questions if you have the time and inclination.
What is a stoichiometric mix? Do I assume this example of a glaze would
simply run off any pot it was on?
Can it be that phase separation is limited to boron/silica glasses - this
can't be so - is not crystals surrounded by amorphous glaze a form of phase
separation?
Thanks for any light you can spare on this - RR
>----------------------------Original message----------------------------
>Here's a possible durable glaze way outside the limits. A few years ago I
>fired a stoichiometric mix of zinc oxide and boric acid to produce pure zinc
>borate glaze. It produced a hard, extremely low expansion glass which
>appeared to be durable but I never tested it. Even if this composition
>isn't actually durable, I predict that it could be made so by additions of
>alumina. This would be an interesting silica-free glaze. Alumina seems to
>have greater solubility in boron oxide melts than silica, hence the glossing
>effect you mention.
>
>The phase separation with more than a relatively small amount of boric oxide
>in silica glasses is caused by liquid immiscibility, but it would be
>interesting to see how much silica will go into stable borate glasses such
>as zinc borate, before this occurs.
>
>----------------------------Original message----------------------------
>(Abridged)
>I keep hearing that there are glazes, which fall outside the limits, which
>are durable. What I have never seen is an example of such a glaze. If
>anyone has such information I would like to hear about it.
>
>RR
>Ron Roy
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
Michael Banks on sun 15 nov 98
Hi Ron,
Stoichimetric mixtures are those where the each ingredient is in the right
molar proprtion to the other(s) to produce a pure end product compound. For
example to produce water (H2O), a stoichiometric mix of hydrogen and oxygen
is 2 mols of hydrogen to one of oxygen. When ignited.....booom! it should
produce one mol of water with no other byproducts except for shattered
windows, assorted shrapnel etc. A practical application of this is for that
fantastic piece of engineering, the space shuttles main engines, which burn
liquid hydrogen with liquid oxygen. When they fill the tanks, I would guess
there would be no point in putting more of either liquid in than was
neccessary to produce water, as anything else is just dead weight.
Zinc borate is a chemical compound with a fixed formula, so exact
proportions of zinc oxide and boric oxide (in a 1:1 stoichiometric ratio)
were weighed out after allowing for their differing molecular weights. An
excess in either ingredient may have resulted in exsolution, paticularly if
boric oxide was in excess. Boric oxide, being soluble and hydroscopic will
react with atmospheric water vapour and produce surface crystals of boric
acid.
The answers to the other questions are yes and yes. Adding alumina might
stiffen the melt sufficiently to stay on a pot. Quite right Ron, phase
separation of two immiscible liquids is not limited to borosilicate mixes,
opalescence and the optical blue colour of classic jun glazes has been
credited to phosphatic glass/silica glass immisciblity. Many other
attractive glaze effects are due to phase separations, eg: crystalline
glazes (precipitation/exsolution) and lustres (immiscibility) are two large
groups.
I've since come across two other silica-free glassy insoluble compounds,
which might make strange glazes; boron phosphate and aluminium phosphate
(the latter in combo with copper is the mineral turquoise!).
Cheers,
Michael
Nelson,
New Zealand
mjbanks@clear.net.nz
----------------------------Original message----------------------------
Hi Michael,
Thanks for this - I have a few questions if you have the time and
inclination.
What is a stoichiometric mix? Do I assume this example of a glaze would
simply run off any pot it was on?
Can it be that phase separation is limited to boron/silica glasses - this
can't be so - is not crystals surrounded by amorphous glaze a form of phase
separation? Thanks for any light you can spare on this - RR
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