Paul on thu 14 nov 02
Hello,
does anyone know if there is a formula for comparing/converting the
expansion of a clay body vs the expansion of a glaze? The expansion of my
clay body is 5.5 but the glazes are given in completely different numbers
like 76, etc. So how does one compare these two figures to find out the
compatibility between the two when formulating or adjusting the glazes?
Thanks
Paul B
Dave Finkelnburg on thu 14 nov 02
Hi Paul!
Great question. Hard to answer.
I suspect (and I am only guessing) that the 76 is 76 x 107
inches/inch/degree F. The 5.5 is likely 5.5 x 106 inches/inch/degree F.
The first number can also be expressed as 7.6 x 106, so you see it's not
that different. Sounds like your clay body is a porcelain and your glaze is
a stoneware glaze.
Consider taking a little different approach. Take a glaze recipe that
"fits" your clay. Then calculate the expansion of that glaze. Then you can
use that number as a point of reference to determine if other recipes are
likely to craze, shiver or fit when applied to your clay body.
Just for reference, I can tell you from experience using Insight to
calculate the expansion of my glazes, that if a glaze with an expansion of 7
"fits" well on a clay body I use, then a glaze with an expansion less than
6.5 will shiver on the lip and a glaze of expansion 7.5 will craze pretty
badly.
Good glazing!
Dave Finkelnburg
 Original Message 
From: "Paul"
> does anyone know if there is a formula for comparing/converting the
> expansion of a clay body vs the expansion of a glaze? The expansion of my
> clay body is 5.5 but the glazes are given in completely different numbers
> like 76, etc. So how does one compare these two figures to find out the
> compatibility between the two when formulating or adjusting the glazes?
Jose A. Velez on thu 14 nov 02
Paul:
> does anyone know if there is a formula for comparing/converting the
> expansion of a clay body vs the expansion of a glaze? The expansion of my
> clay body is 5.5 but the glazes are given in completely different numbers
> like 76, etc. So how does one compare these two figures to find out the
> compatibility between the two when formulating or adjusting the glazes?
> Thanks
> Paul B
Paul:
The coefficient of thermal expansion (COE) for a glaze can be calculated
following the Addition Rule, this says, in my words, that the resultant COE
of a glaze is the sum of the individual oxides COE proportional to their
presence (expressed in weight % or mole %). This seems to hold true for
amorphous glass (glaze) better than for a crystalline structure (matt).
Insight and other commercial programs offer ease of computation, but if you
have a math or science background you can do this easily.. The COE is
normally expressed in scientific notation (XEa) to avoid writing too many
zeros in the decimal form. This could be somewhat confusing for those not
familiar with this notation and I have seem quite a bit of errors in this
regard, even in reference books. Also many of the reference books do not
provide the COE in the same units of measure, therefore if you obtain them
from different sources you need to make sure they are the same or be able to
convert them to a common set of UOMs. Digital Fire has a very good
explanation of this in their web site: digitalfire.com. If you do not have
the necessary background, obtaining one of the commercial glaze calculation
programs might be the best way to go.
Good luck, Jose A. Velez
Ababi on fri 15 nov 02
No no no!
It is too complicated!
To translate one system to another!
What if your claybody is 5.5 and so is the glaze after the translations and they are not
fitting?
This is the time to "Mastering Cone Six Glazes" of Ron and Hesslberth for testing the
clay glazes fit and "Cone Six Glazes" of Michael Bailey to understand the calculations
The two excellent books that appeared recently
Ababi Sharon
Glaze addict
I use one recipe from each book yet very happy I have bought them!
Kibbutz Shoval Israel
ababisha@shoval.org.il
http://members4.clubphoto.com/ababi306910/
http://www.milkywayceramics.com/cgallery/asharon.htm
 Original Message 
>Paul:
>> does anyone know if there is a formula for comparing/converting the
>> expansion of a clay body vs the expansion of a glaze? The expansion of my
>> clay body is 5.5 but the glazes are given in completely different numbers
>> like 76, etc. So how does one compare these two figures to find out the
>> compatibility between the two when formulating or adjusting the glazes?
>> Thanks
>> Paul B
>Paul:
>The coefficient of thermal expansion (COE) for a glaze can be calculated
>following the Addition Rule, this says, in my words, that the resultant COE
>of a glaze is the sum of the individual oxides COE proportional to their
>presence (expressed in weight % or mole %). This seems to hold true for
>amorphous glass (glaze) better than for a crystalline structure (matt).
>Insight and other commercial programs offer ease of computation, but if you
>have a math or science background you can do this easily.. The COE is
>normally expressed in scientific notation (XEa) to avoid writing too many
>zeros in the decimal form. This could be somewhat confusing for those not
>familiar with this notation and I have seem quite a bit of errors in this
>regard, even in reference books. Also many of the reference books do not
>provide the COE in the same units of measure, therefore if you obtain them
>from different sources you need to make sure they are the same or be able to
>convert them to a common set of UOMs. Digital Fire has a very good
>explanation of this in their web site: digitalfire.com. If you do not have
>the necessary background, obtaining one of the commercial glaze calculation
>programs might be the best way to go.
>Good luck, Jose A. Velez
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Ron Roy on sun 17 nov 02
Hi Paul,
Craig and others are right  you cannot calculate the expansion of
crystalline glazes  crystals don't obey any expansion rules that I know
of. Clay bodies are mostly crystals  certainly not to be compared with
glossy glazes  of which the expansions can be calculated with some degree
of predictability.
Add to that the role of quartz (and cristobalite in higher fires bodies)
which behaves in a predictable way but you cannot predict how much will be
there in the form of quartz after firing.
You can get a good idea of the expansion of a clay body by using gloss
glazes as indicators. See chapter 5 in Mastering cone 6 glazes for a
complete explanation and a set of cone 6 glazes that can be used to find
the expansion of clays.
Lets say you have 4 glazes  glossy  and you have calculated the expansion
of each.
Glaze #1 crazes badly  and is well crazed before it comes out of the kiln
 the calculated expansion is 8.50 or whatever.
Glaze #2 has a few craze lines when it comes out but crazes more after two
weeks  it has a calculated expansion of 7.5
Glaze #3 occasionally has a craze line after being out of the kiln for a
year and it has an expansion of 7.2
Glaze 4 never crazes and it has a calculated expansion of 6.5.
Guess what the expansion rate of the clay is that these glazes were tested on?
Somewhere between 7.2 and 6.5  if you want to nail it closer do a line
blend of the last two glazes  you will soon come to what I call the magic
number, for that clay, under which most glazes will not craze.
Or send a sample of the fired clay to me (ask me how to prepare the sample)
and I will send you back a chart along with some comparative charts  each
chart will cost $75 American or $100 Canadian. Dilatometers cost about
$20,000.00 American last time I looked.
As you can see in chapter 5 of Master cone 6 glazes  using glossy glazes
to determine body expansion will work  and you can design the glazes using
calculation software  there are some stumbling blocks to watch out for 
don't use Lithium Carb and keep the Boron below 12%  and make sure the
glaze is properly melted  but it is not rocket science.
It is also wise to check the analysis of the materials entered into your
calculation program. Ask your supplier to provide typical analysis for the
materials you buy from them  if they are not current contact the mine that
supplied them  most of the materials we use have a least a little SiO2 in
them. The more accurate your materials analysis are the better  you
certainly don't want to use theoretical data.
RR
>Hello,
>does anyone know if there is a formula for comparing/converting the
>expansion of a clay body vs the expansion of a glaze? The expansion of my
>clay body is 5.5 but the glazes are given in completely different numbers
>like 76, etc. So how does one compare these two figures to find out the
>compatibility between the two when formulating or adjusting the glazes?
>Thanks
>Paul B
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 6134759544
Fax: 6134753513
 
