Gregory D Lamont on thu 15 jul 99
Dear glaze and clay body chemistry gurus,
I'm trying to get a handle on coefficient of expansion as it relates to
glaze fit.
By wording the materials used in my claybody (a Cone 6 stoneware), I have
been able to get an analysis of it on the glaze chemistry program I use
(GlazeChem). According to the program, my claybody has an expansion of
55.6 x 10e7 per degree C. Here's the recipe:
Goldart clay 14
Hawthorne Bond clay 26
OM4 Ball Clay 20
Redart clay 10
Flint 10
Custer feldspar 15
Pyrophyllite 5
Is there a way to predict a "window" of potential glaze fit based on this?
For example, with the knowledge of the coefficient of expansion of my
claybody, can I predict that a glaze that has a coefficient of expansion
falling within a certain range will likely fit (prior to actual testing, of
course)? Can this be expressed as a ratio of the coefficient of expansion
of a claybody to one of a glaze?
Your help is greatly appreciated!
Greg Lamont
gdlamont@iastate.edu
http://www.ourwebpage.net/greglamont/
3011 Northwood Drive
Ames, IA 500104750
(515) 2333442
Richard Burkett on fri 16 jul 99
Greg Lamont asks:
>I'm trying to get a handle on coefficient of expansion as it relates to
>glaze fit.
>By wording the materials used in my claybody (a Cone 6 stoneware), I have
>been able to get an analysis of it on the glaze chemistry program I use
>(GlazeChem). According to the program, my claybody has an expansion of
>55.6 x 10e7 per degree C. Here's the recipe:
>
recipe deleted...
>
>Is there a way to predict a "window" of potential glaze fit based on this?
>For example, with the knowledge of the coefficient of expansion of my
>claybody, can I predict that a glaze that has a coefficient of expansion
>falling within a certain range will likely fit (prior to actual testing, of
>course)? Can this be expressed as a ratio of the coefficient of expansion
>of a claybody to one of a glaze?
Well, Greg, first of all glaze calculation programs which also calculate
estimated thermal expansion are for exactly that: GLAZES only. While it may
be useful to calculate clay bodies for the purpose of substituting
materials, thermal expansion calculations are far from accurate (to the
point of being nearly useless) for crystalline materials like claybodies.
All thermal expansion calculations expect completely melted,
noncrystalline glass as the final result. Even with glazes, if the glaze
is matte then thermal expansion calculations are not likely to be accurate,
as crystalline materials expand differently than do glasses.
Your best bet is to use a set of gloss test glazes of a number of different
thermal expansions (like those James Robinson came up with in his article
in Studio Potter a few years back) which are completely glassy and
predictable, and use those to estimate the thermal expansion of your clay
body in terms of what glazes are most likely to fit it.
Yes, there is a little variation possible from the best match (slight
compression of the glaze) to the clay body as most glazes have some
elasticity after cooling, but that too varies from glaze to glaze,
depending on the oxides present.
Richard Burkett
Richard Burkett  School of Art, SDSU, San Diego, CA 921824805
Email: richard.burkett@sdsu.edu <> Voice mail: (619) 5946201
Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
CeramicsWeb: http://art.sdsu.edu/ceramicsweb/
HyperGlaze@aol.com & http://members.aol.com/hyperglaze/
Ron Roy on fri 16 jul 99
Hi Greg,
Not possible as far as I can see. What calculation cannot do is determine
the free quartz and/or cristobalite  and both affect expansion/contraction
dramatically.
Best thing to do is find or make a glaze that crazes on your clay and one
that does not  do a line blend between the two  you will get some idea of
where the crazing stops. That calculated expansion number will help you
choose or make glazes that will not craze. The number will probably not
apply to all the clays you use.
Any glazes in the line blend that don't craze when just out of the kiln may
still graze eventually  to make any of these craze  freeze for 24 hours
and dip in boiling water while still frozen.
It will be best to test any glazes that don't craze for too low an
expansion  glaze a bottomless cylinder  just on the inside  keep the
clay on the thin side and the glaze on the thicker side  if these do not
crack after freezing and pouring in boiling water (while frozen) then you
can be fairly certain they are OK.
Remember  the worst combination is  low expansion glaze on the inside and
a crazing glaze on the outside.
Let me know if you have any questions  RR
>I'm trying to get a handle on coefficient of expansion as it relates to
>glaze fit.
>By wording the materials used in my claybody (a Cone 6 stoneware), I have
>been able to get an analysis of it on the glaze chemistry program I use
>(GlazeChem). According to the program, my claybody has an expansion of
>55.6 x 10e7 per degree C. Here's the recipe:
>
>Goldart clay 14
>Hawthorne Bond clay 26
>OM4 Ball Clay 20
>Redart clay 10
>Flint 10
>Custer feldspar 15
>Pyrophyllite 5
>
>Is there a way to predict a "window" of potential glaze fit based on this?
>For example, with the knowledge of the coefficient of expansion of my
>claybody, can I predict that a glaze that has a coefficient of expansion
>falling within a certain range will likely fit (prior to actual testing, of
>course)? Can this be expressed as a ratio of the coefficient of expansion
>of a claybody to one of a glaze?
>
>Your help is greatly appreciated!
>
>Greg Lamont
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 4164392621
Fax: 4164387849
Web page: http://digitalfire.com/education/people/ronroy.htm
gambaru on sat 17 jul 99
Greg: Cannot answer your question as posed. If I recall correctly I think
Ron Roy has made past comments that the formulas of clay bodies do not
really give indications of possible expansion numbers for glazes but
certainly can be the guide to solving problems.
BUT, on Tony Hansen's web site digitalfire.com there is an article about
finding the best glaze expansion range for your clay body. It is a line
blend that should take you from shivering to crazing . With alot of test
tiles throughout your kiln and a few firings you should be able to nail down
the range of expansion that will be correct for your clay body. You can
subject the most promising tiles or small pot versions to the
heating/icewater or freezing/boiling water tests.
Once you have determined the optimum range this will enable you, using your
software to note any changes in expansion when additions are made to a base
glaze i.e. opacifiers and colouring oxides and adjust the recipe
accordingly.
Not quite the answer you were asking for but hope this is of some
assistance. MB
Original Message
From: Gregory D Lamont
To: CLAYART@LSV.UKY.EDU
Date: Thursday, July 15, 1999 9:13 AM
Subject: Coefficient of expansion info sought
Original message
Dear glaze and clay body chemistry gurus,
I'm trying to get a handle on coefficient of expansion as it relates to
glaze fit.
By wording the materials used in my claybody (a Cone 6 stoneware), I have
been able to get an analysis of it on the glaze chemistry program I use
(GlazeChem). According to the program, my claybody has an expansion of
55.6 x 10e7 per degree C. Here's the recipe:
Goldart clay 14
Hawthorne Bond clay 26
OM4 Ball Clay 20
Redart clay 10
Flint 10
Custer feldspar 15
Pyrophyllite 5
Is there a way to predict a "window" of potential glaze fit based on this?
For example, with the knowledge of the coefficient of expansion of my
claybody, can I predict that a glaze that has a coefficient of expansion
falling within a certain range will likely fit (prior to actual testing, of
course)? Can this be expressed as a ratio of the coefficient of expansion
of a claybody to one of a glaze?
Your help is greatly appreciated!
Greg Lamont
gdlamont@iastate.edu
http://www.ourwebpage.net/greglamont/
3011 Northwood Drive
Ames, IA 500104750
(515) 2333442
David Hewitt on sun 18 jul 99
Greg,
Ron Roy has given you an excellent way of finding out if your glaze will
fit your clay.
May I use this thread to ask about data on clay bodies. All glaze
calculation programs can calculate a coefficient for a glaze. We may
argue over how meaningful the figure is and which ceramists coefficients
to use, but it is better than nothing. What we then need to compare it
with is a coefficient for the clay body. I have often wondered why such
coefficients have not been made readily available from suppliers. In
the UK, for example, I have only come across one supplier, Potclays,
that gives such data in their catalogue. What is the practice in other
parts of the world? Should we not expect suppliers to give such data as
a matter of routine?
If anyone is interested in the data from Potclays and the general
question of glaze fit they may like to look up the articles written by
Mike bailey and myself on the IMC web site:
http://digitalfire.com/education/people/hewitt.htm
or through my web site:
http://www.dhpot.demon.co.uk Pottery techniques/calculating crazing
In message , Gregory D Lamont writes
>Original message
>Dear glaze and clay body chemistry gurus,
>
>I'm trying to get a handle on coefficient of expansion as it relates to
>glaze fit.
>By wording the materials used in my claybody (a Cone 6 stoneware), I have
>been able to get an analysis of it on the glaze chemistry program I use
>(GlazeChem). According to the program, my claybody has an expansion of
>55.6 x 10e7 per degree C. Here's the recipe:
>
>Goldart clay 14
>Hawthorne Bond clay 26
>OM4 Ball Clay 20
>Redart clay 10
>Flint 10
>Custer feldspar 15
>Pyrophyllite 5
>
>Is there a way to predict a "window" of potential glaze fit based on this?
>For example, with the knowledge of the coefficient of expansion of my
>claybody, can I predict that a glaze that has a coefficient of expansion
>falling within a certain range will likely fit (prior to actual testing, of
>course)? Can this be expressed as a ratio of the coefficient of expansion
>of a claybody to one of a glaze?
>
>Your help is greatly appreciated!
>
>Greg Lamont
>gdlamont@iastate.edu
>http://www.ourwebpage.net/greglamont/
>
>3011 Northwood Drive
>Ames, IA 500104750
>(515) 2333442
>

David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel: +44 (0) 1633 420647
FAX: +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Tom Wirt on sun 18 jul 99
From: Gregory D Lamont
Subject: Coefficient of expansion info sought
> Original message
> I'm trying to get a handle on coefficient of expansion as it relates to
> glaze fit.
> By wording the materials used in my claybody (a Cone 6 stoneware), I have
> been able to get an analysis of it on the glaze chemistry program I use
> (GlazeChem). According to the program, my claybody has an expansion of
> 55.6 x 10e7 per degree C. Here's the recipe:
Greg...
My understanding of trying to mathematically calculate clay body expansion
is that, while in theory it should work, with clay, the melt of the blend,
and thus the expansion, is heavily influenced by the particle sizes of the
various constituents. Smaller particles give greater melt and thus greater
expansion. Further complicating this is the fact that the particle size
distribution of many of the clays we use will vary considerably batch to
batch.
Thus as Ron has posted, about the only ways are either through dilatometric
measures or by setting up a glaze set with known expansion, and then trial
and erring your way to an approximation. For reference, besides Ron's
suggestion, Jim Robinson published a methodology in Studio Potter, Vol. 16,
No 2 June 88. This article, "Body Building for Potters: A ClayBlending
Formulary", was published before the days of much glaze calculation
software, so it's easier now using Ron's method....but the article is an
excellent read on developing clay bodies.
Tom Wirt
 
