Malone & Dean McRaine on sat 18 apr 98
Aloha all:
Much has been made of late of bringing glazes into agreement with limit
formulas to insure durability and food safety. I am beginning to work with
the GlazeChem software. Out of curiousity I compared the limit formulas
given on the Magic of Fire web site with those in GlazeChem and they are
different. Not hugely different but enough to make me wonder about the
inconsistency, for instance, for silica (^10) Glazechem has .2-.5 (matt &
satin glazes) or .2-.6 (gloss glazes) Magic of Fire has .3-.5(unspecified
as to glaze type). So here are a few questions:
Why are these programs different?
How are these standards set and by whom?
Is there a universal standard?
Is Magic conservative and GlazeChem Liberal?
Mahalo,
Dean
Paul Lewing on sun 19 apr 98
Aloha, Dean,
No, there is no standard for limit formulas. I think they were
originally set by different people just based on years of observation.
One thing to look for when using limit formulas is whether the person
who put them out has included Boron in the fluxes or not. It is
sometimes useful to do it that way, and some books will actually give
you two different sets of limits, one with B in the fluxes, and one
with it not.
Limit formulas are the most general kind of parameters. We all know
of gorgeous glazes, maybe even some safe and durable ones, that have
some element or other outside the limit formulas. HyperGlaze even
allows you to "customize" your limit formulas, so you know it's pretty
subjective if that works.
At the temperature I work at (c5) the limit for Ca is .6. But I know
from experience that if I see a glaze formula with .6 Ca, unless all
the other fluxes are very active fluxes, or there is lots of Boron in
there, it isn't going to melt.
There's a lot of information in glaze calculation, but that's
different from knowledge or wisdom.
Paul Lewing, Seattle
Gavin Stairs on tue 21 apr 98
Hi All,
This question of varying limit formulae goes to the heart of the problem in
glaze calculation. No matter which program (or hand calculation) we use,
our calculations of glaze analysis are only a starting point. Limit
formulae are an aid in making the next step.
Here's what they are based on: given a mixture of Al2O3 and SiO2 plus flux
(I'm going to ignore complications like the other glass formers and
amphoterics), the resulting mixture will glaze at a temperature determined
by: the initial sintering/melting temperatures of the individual fluxes in
contact with silica, and the solidus (lower melting temperature) of the
whole glaze mixture.
The initial sintering temperature defines when the glaze begins to melt,
and is largely a matter of the binary phase behavior of the various fluxes.
For example, what is the melting temperature of a mixture of (Na,K)CO3.
In a real glaze, this temperature is determined normally by the KNa content
of the glaze.
The solidus of the glaze is determined by both the type and number of the
fluxes in the glaze, so long as all of the fluxes at least partly dissolve
in the melt, and the number of fluxes. The more fluxes, the lower the
solidus. So, the alkali earths can contribute, even though they are more
refractory, if they are not in concentrations larger than those which fully
dissolve at the maturation temperature of the glaze.
And this is where it comes apart slightly. The amount of any constituent
that will dissolve varies according to the temperature and the
concentrations of other constituents. So, for each list of fluxes, there
will be an optimum mix at every maturation temperature, and also varying
with the silica and alumina content. Clearly, a limit formula based on a
single list of fluxes and covering a range of maturation temperature must
be approximate.
Now begin the rest of the complication: add the other glass formers and
amphoterics, and ask for the full range of surface textures and opacifiers,
and you will see that very quickly the problem becomes quite complex. The
Seger formulations were defined during an era before digital computers, so
they were necessarily simplified for hand calculation. Now, we have
powerful personal computers which could, in principle, account for all
these variations that I have sketched. However, this would entail an
enormous data compilation problem, which no-one has yet performed. So we
have the "limit formulae", which are guesstimates based on experience and
not very precisely defined criteria.
As Paul Lewing pointed out, a limit formula that permits 0.6 of CaO does
not imply that every glaze with 0.6 CaO will mature over the full range of
the limit. Nor does the lower limit on flux imply that such a glaze will
mature at the lower bound of maturation of the limit. In fact, at the
lower end of the range, there will need to be a higher proportion of
alkalis, and lower alkali earths, while at the higher end, the alkalis will
need to be diminished somewhat, while the alkali earths increase.
Similarly the proportions of Al2O3 to SiO2 will need to increase slightly
as the maturation increases. And, to make a matte, the proportion of
alkali earth and Al2O3 will need to increase slightly at any given
maturation. All this within any given limit range.
Similarly, if not all the fluxes on the list are present, the maturation
temperature will rise, even for the same total flux proportion. You can
see this by looking at the difference between an 06-01 limit and an 8-10
limit. The lower cone limit will have more ingredients. Those extra
ingredients are needed to force the maturation temperature down. The same
is true of B2O3.
Now add the additional complication of thermal expansion, dynamic phase
behavior during heating/cooling, colorants, encapsulants, opacifiers,
texture modifiers, carbon traps, lustres, etc., and the problem begins to
get interesting. The wonderful glaze calculation programs we now have are
only scratching the surface. They are wonderful tools for taking the labor
out of calculation, but they are no substitute for judgement and experience.
Incidentally, this very problem of determination of better limits came up
about a year ago in relation to a glaze of dubious quality. Ron Roy and I
discussed for a while the idea of doing some work to better define the
limits. We didn't get very far with it, because it is an enormous task. I
still hope one day to make a start on it, which is one reason I began to
promote the idea of a Technical Institute for Studio Potters. Properly
managed and shared out amongst many participants (e.g., at universities),
this project might become tractable. One of the benefits might be better
glaze calculation programs, with less guesswork at the other end.
Gavin
Technical Institute for Studio Potters (proposed)
c/o Gavin Stairs
Toronto, Ontario, Canada
Ron Roy on tue 21 apr 98
Hi Dean,
I wondered when this was going to come up.
Here is what I think - I wish I knew more about this or could at least
point to some papers on the subject. Perhaps others on the list can point
us in the right direction.
I think your first question should be - why are two different kinds of
limits being used - all programs are different in many other ways.
It seems to me the two different limits are being used for different
purposes. The glaze Chem limits are a guide to help you get a particular
glaze surface - as do the limits in Mastering the Craft by Zakin - they do
not apply to durability. The limits in Insight are to help make durable
glazes.
How are these standards set? I can only surmise that Industry -
particularly the dinnerware industry - had to do a lot of testing in the
past - and even more now. I assume the limits having to do with durability
are the result of the data gathered during that testing. In other words -
what do durable glazes have in common?
A universal standard? I am not sure what the question is - does it have to
do with what is allowed? In that case different countries and even states
have different standards - but if the question has to do with degree of
leaching - I have to assume the standard has to do with threshold limits of
living organisms - but what is it in relations to each oxide - most of the
time the answer must be - we don't know and/or it varies with each
individual.
I have answered the liberal/conservative question I think - they are to be
used for different purposes. If it is durability you want - use the limits
included with Insight.
>Much has been made of late of bringing glazes into agreement with limit
>formulas to insure durability and food safety. I am beginning to work with
>the GlazeChem software. Out of curiousity I compared the limit formulas
>given on the Magic of Fire web site with those in GlazeChem and they are
>different. Not hugely different but enough to make me wonder about the
>inconsistency, for instance, for silica (^10) Glazechem has .2-.5 (matt &
>satin glazes) or .2-.6 (gloss glazes) Magic of Fire has .3-.5(unspecified
>as to glaze type). So here are a few questions:
>Why are these programs different?
>How are these standards set and by whom?
>Is there a universal standard?
>Is Magic conservative and GlazeChem Liberal?
>
>Mahalo,
>Dean
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
Tony Hansen on tue 21 apr 98
> Out of curiousity I compared the limit formulas
>....
> Why are these programs different?
Check the web page at
http://digitalfire.com/magic/limits.htm
for more information.
--
T o n y H a n s e n thansen@digitalfire.com
Don't fight the glaze dragon alone
INSIGHT, Magic of Fire at http://digitalfire.com
Richard Burkett on thu 23 apr 98
Thanks, Gavin, for giving an good overall view of limit formulas. As you
mention, the standard limit formulas promoted by some are exactly that, a
VERY limited generalization. They were devised to produce industrial glazes
of very limited color range and surface qualities. There are very durable,
workable glazes that fall outside these limits - glazes that may have much
more interesting surface and color reactions. Limit formulas offer good
advice to the knowledgeable ceramist - no more. They are certainly not the
law or to something to which one must mindlessly adhere.
One of the features that I wrote into my HyperGlaze software was the
ability to compute and install custom user-defined limit formulas so that
glazes with specialized needs to produce a particular color or surface
could be made available. I had in mind exactly what Gavin has proposed - a
systematic analysis by a large number of ceramists investigating a wide
range of glazes, colors, and surfaces and the subsequent compilation of
limit formulas to produce these colors. These could then be shared on the
web.
Quite a lot of this type of research has actually been done and is buried
in the Journal of the American Ceramic Society publications from early in
the century when the ceramic industry at large was still concerned with
pottery as a research topic. Sadly, most of it concerns only the oxidation
glazes favored by industry, but there are some interesting tidbits there.
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/
HyperGlaze@aol.com & http://members.aol.com/hyperglaze/
_ Richard Burkett, Associate Professor of Art
_ The School of Art Design & Art History, SDSU, San Diego, CA 92182-4805
_ http://www.sdsu.edu/art
_ E-mail: richard.burkett@sdsu.edu - voice mail: (619) 594-6201
_ The CeramicsWeb: http://apple.sdsu.edu/ceramicsweb/
David Hewitt on thu 23 apr 98
I agree with all that you say, Gavin.
One of the most useful things I have found to look at are phase
equilibrium diagrams. With just two materials these can be relatively
simple, in that there is just one eutectic point, but they also show the
phases through which a given mixture would go when heated or cooled.
Some mixtures, of course have more than one eutectic point so that you
may be able to get similar glaze results with different mixtures. A
triaxial phase equilibrium diagram shows what happens with three
components, which can be silica, alumina and a flux (or acidic,
amphoteric and basic oxides). Much interesting data has been published
by the American Ceramic Society, but again, this does not give a total
answer to all your glaze problems. Far from it. Just one more bit of
knowledge to add to all the other matters you have raised. But we do get
closer by using all these tools.
David
In message , Gavin Stairs writes
>----------------------------Original message----------------------------
>Hi All,
>
>This question of varying limit formulae goes to the heart of the problem in
>glaze calculation. No matter which program (or hand calculation) we use,
>our calculations of glaze analysis are only a starting point. Limit
>formulae are an aid in making the next step.
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
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