Ron Roy on sun 22 dec 96
Bob wrote
>In my naivete I've probably reduced the problem to some ridiculous
>impossibility and I anticipate I'll be pummeled to dust by some of you ceramic
>engineers, but think about my challenge. Is this problem sooooo complex that
>it can't be reduced to some fairly simple rules that a non-chemist/non-engineer
>dummy like me can follow and get it right, say 80% of the time?
Hi Bob,
Unfortunately you have asked the BIG question and it requires a BIG answer
- my predilection for brevity is being taxed.
First of all shrinkage (wet to dry and fired) has nothing to do with what
we refer to as "expansion." Expansion means expansion & contraction caused
by changes in temperature - glazes contract when cooled and expand when
heated. Clay bodies do the same thing but always at a different rate than
the body - this is further complicated by free quartz (silica) and
cristobalite (silica) in a body. They go through "inversions" at certain
temperatures. Lets look at the quartz inversion as an example. The clay
body is cooling and contracting - the free quartz suddenly gets a bit
smaller at 573C (the inversion point for quartz). Lucky for us because we
use it to help stop crazing.
Most glazes don't have any free silica crystals so it keep contracting at a
more or less even rate so we can use the free quartz in a body to help keep
the glaze under "compression" and avoid crazing - a cure for crazing is to
add more silica to a body and thus increase the amount of free silica and
thus increase the contraction at 573C. Too much contraction in a body (in
relation to the glaze) and you get the opposite of crazing - glaze too big
- and shivering and/or dunting can be the result.
Melted silica (amorphous silica) does not go through the (reversible)
inversion at 573C but has one of the lowest coefficient of all the oxides
we use - hence adding silica to a glaze can be a cure for crazing.
Facts: It is never the fault of the clay or the glaze - it's always the
fault of their combination.
Fact: Differences between clay and glaze fit only become a problem AFTER
the glaze has frozen. My cone 10 glazes start to soften at about 800C -
lower fired glazes will soften at even lower temperatures.
Fact: It is possible to get an idea of the rate of expansion and
contraction of a glaze by calculation - but not in all cases. Mat glazes
don't go by the rules and certain oxides behave differently when in certain
combinations and percents.
We can use calculation for glazes - I do it all the time - to estimate the
"right" expansion for our glazes. You find out the "right" general
expansion for your clays by testing them with glazes of known expansion.
Fact: All clay bodies have a different rate of expansion and contraction.
Test each clay body to see which of your glazes fit.
Would some one post the date of the article from CM with the glazes for
testing cone 6 bodies - I left mine at my studio.
The problem is we can't calculate the amount of free silica or cristobalite
in a body and crystallized material (bodies, mat glazes) are difficult
(impossible?) to predict.
So Bob, I think the answer is yes - you can be right if you do the testing
with glazes of know expansion for the cone you work at for many glazes.
There is the set I referred to for come 6 - I have a set for cone 10 - what
we need now is a set for cone 04.
The real answer is to have precise, actual, measurements of our glazes and
clays. I now have a dilatometer and am working on a set of charts for each
clay Tuckers sells. That, combined with calculated expansion for most
glazes, will help in the selection of appropriate clays for your glazes. An
even more precise method would be to measure your glazes as well.
Overlaying the charts for any specific clay/glaze/slip combination will
give a visual check on what will happen.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Craig Martell on mon 23 dec 96
Hi: I believe this is the cone 5-6 article in CM that Ron Roy referred to in
his post about glaze fit. This article covers the 5-6 glaze series (all
glaze coefficients of expansion are given) that you can use to evaluate any
cone 5-6 claybody. I copied this from a post I submitted a week or so ago.
Craig Martell-Oregon
<that applies Jim's concepts to cone 5 work. Jim told me that this is an
INCREDIBLE article and he gives it the highest reccomendation possible.>>
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