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## glaze coefficients of expansion

### Bob Cyr on sun 8 feb 98

Hi Good People of CLAYART,

I am developing a computer program (based on HyperCard for Mac) for
converting a glaze recipe to a unity formula. In addition to computing the
unity formula, I am attempting to include a calculation of the glaze
expansion.

I have found that there is not agreement among the references on the
numerical values for the temperature coefficients of common glaze
materials. In fact, there is a considerable range of values that are
listed. Take silica as an example:

Rhodes ("Clay and Glazes for the Potter," page 242) lists silica as having
an expansion coefficient of 0.05 x 10^-7 per degree C, linear.

Fournier ("Illustrated Dictionary of Practical Pottery," page 60) lists
two references as follows -

English and Turner: Silica = 0.5
Winkelmann and Scott: Silica = 2.7

INSIGHT (Beta 5.0i January 15, 1998) indicates as follows -
Silica = 3.50
Flint = 3.70
Quartz = 3.63

I would appreciate knowing if there is a standard reference for obtaining
the expansion coefficients for common glaze materials. Is there a
"standard" that is used by the various commercial software programs that
compute the glaze expansion? And, what are the units of the expansion
calculated by INSIGHT? (For example, is it per degree C, x 10^-7?)

Thanks in advance,
Bob

Reaching out from Townsend Hill, Springville, NY, USA

### David Hewitt on tue 10 feb 98

In message , Bob Cyr writes
>----------------------------Original message----------------------------

>I would appreciate knowing if there is a standard reference for obtaining
>the expansion coefficients for common glaze materials. Is there a
>"standard" that is used by the various commercial software programs that
>compute the glaze expansion? And, what are the units of the expansion
>calculated by INSIGHT? (For example, is it per degree C, x 10^-7?)
>
>
I am not aware of any of the different Ceramist's sets of coefficients
being accepted as 'Standard', and as you say, there are quite wide
differences on some oxides. Also there is a problem that no one
Ceramists has given figures for all oxides. Presumably only those that
he or they tested.
In my GLAZE WORKBOOK, see URL below, I include a page which enables you
to calculate the expansion for a recipe using 7 different sets of
coefficients.
My preferred set, however, is based on English & Turner and I give
figures which are per degree C x 10^-6 linear.
David
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://digitalfire.com/education/people/hewitt.htm