Daniel Semler on sun 13 feb 05
Hi all,
I wanted to toss out my thoughts on these three related but different things.
This is a little long, but I hope not entirely pointless.
In general, both recipes and molecular formulae are approximations. The recipe
is an approximation of the raw glaze, and the molecular formula is an
approximation of the fired glaze. Many discussions on this list have borne this
out. Glaze calculation provides a method of conversion between these two
representations. Both representations have their uses.
Strictly, glaze calculation refers to the conversion of a glaze recipe to a
molar representation, such as that developed by Seger, or perhaps to mole %, and
vice versa. Both representations of glaze composition are useful and both are in
some senses incomplete. Converting from one to the other and vice versa can lose
or hide information. It is impossible to tell for example how a glaze will
suspend in the slop just from the Seger formula. One needs to know what
materials are used to source the alumina for example - epk and ball clays will
give suspend differently. In the converse case, it is difficult to tell exactly
what oxides are present in what proportions from the recipe, information which
can tell you a great deal about surface, gloss, melt and so on. It is also more
difficult to perform satisfactory substitutions of complex materials. So it is
important to understand that the knowledge of materials is fundamental to
understanding a recipe, in either form. An understanding of the oxides present
in a fired glaze is important for appropriate manipulation of the seger glaze
formula.
Glaze software is able to perform calculation, and a number of things which
are, to my mind, not strictly glaze calc., but which are applied to the results
thereof. These are things like comparison with limit formulae, and hence
indicate likely melting range, derivation of silica : alumina ratios,
estimation of COE and so on. Others might draw the line elsewhere but that's
how I see it, at least for now.
Finally, to glaze chemistry. The actual chemistry involved in glazes and
claybodies goes far beyond the oxide or recipe representations. How this stuff
actually works, as Ivor often points out, is really not well understood. The
number of variables is very large. Much useful information comes from studying
glass, but the affect of the body substrate on the glaze is particular to
glazes. There is apparently a larger body of knowledge concerning how to
achieve specific results than there is to explain why such results arise.
So what is the value of each thing ?
Glaze calculation will provide you with a way to view two aspects of a glaze.
Glaze software will make doing this a walk in the park. Learning to do it
manually will give you a greater insight into how such calculation works. Glaze
chemistry to the extent that it is understood will explain certain phenomena in
glazes.
It is noticeable in people who use glaze software well, that the decisions
they make in solving problems rely heavily on intimate knowledge of materials.
The software is simply the tool.
Having calculated a glaze tells you little unless you have other knowledge
concerning glaze composition. What does MgO bring to a glaze, why would Na2O be
preferrable ? Under what conditions is there too much alumina or too little ?
These questions are not answered by calculation. They are answered by knowledge
of glaze composition or behaviour. The calculation merely furnishes one with
the raw data of the glaze oxide composition. Likewise the reverse is true.
Given a seger formula why would one choose ball clay over EPK, why source MgO
from talc rather than from dolomite, CaO from whiting rather than from
wollastonite or vice verse ? Knowledge of materials is the basis of decisions
concerning recipes whether in the molecular or recipe forms. Many books
document these things but a few tests really drive a point home.
So if you've got this far, what was the point of it all ?
Given a seger formula, one needs to know which materials provide which oxides
in order to construct a recipe, without which the glaze cannot be made. This
requires a knowledge of materials. To select one of several materials, which
contain the same oxide, in a given situation requires a greater knowledge of
materials to see the trade-offs. Given a recipe and converting it to a seger
representation requires knowledge of the composition of materials. Once it is
in that form an understanding of the behaviour of oxides in the melt is
required, if the information is to be put to use.
Of course, there're all the effects due to firing to consider also, and body
interactions, about which glaze calculation and glaze software say almost
nothing.
Thanx
D
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mailtoandrew@FSMAIL.NET on mon 14 feb 05
Hello Daniel,
Superb post!
Glaze software is a tool, and can be an excellent one, but imparts no more
understanding of the subject than an electronic calculator does about
mathematics.
A concern I ve noted with some, not all, software and books is the quoted
values. An example is feldspar quoted as K2O.Al2O3.SiO2 or kaolin as
Al2O3.2SiO2.2H2O, and the molecular weight being similarly simplified.
Back to your excellent post: whilst I would agree that =94 ... a larger body=
knowledge concerning how to achieve specific results than there is to
explain why such results arise.=94 I think it may be a trifle simple to
say =93How this stuff actually works ... is really not well understood=94
Extensive studies of the technology of ceramics over hundreds of years,
arguable back to Wedgwood in the late C18th, has given a great
understanding. The information may not be centralised or widely
distributed, it is not an extensively taught subject and some may be
confidential, but much understanding does exist within individuals,
universities, research institutes and companies.
Kind regards,
Andrew
Daniel Semler on wed 16 feb 05
Hi,
this is a resend, as I think this disappeared in transit.
Thanx
D
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Hi Andrew,
> A concern I ve noted with some, not all, software and books is the quoted
> values. An example is feldspar quoted as K2O.Al2O3.SiO2 or kaolin as
> Al2O3.2SiO2.2H2O, and the molecular weight being similarly simplified.
I also find often that books show trivial examples of calculation, using such
simplifications, from which the next step to a real example one might actually
use, is just a little large.
>
> Back to your excellent post: whilst I would agree that ” ... a larger body
> knowledge concerning how to achieve specific results than there is to
> explain why such results arise.” I think it may be a trifle simple to
> say “How this stuff actually works ... is really not well understood”
> Extensive studies of the technology of ceramics over hundreds of years,
> arguable back to Wedgwood in the late C18th, has given a great
> understanding. The information may not be centralised or widely
> distributed, it is not an extensively taught subject and some may be
> confidential, but much understanding does exist within individuals,
> universities, research institutes and companies.
This is a good point. I should perhaps have said "completely understood" but
then what is ? It is certainly true that some individuals (and certainly some
manufacturers and research institutes, unis etc.) have much greater depth of
understanding. I guess what I was getting at was what might be called a Grand
Unified Theory of Glaze :) But yes, point taken.
Thanx for your thoughts on this
D
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