bob on mon 3 feb 97
I've been using the following cone 10 stoneware claybody, for quite some
time, with a set
of glazes formulated for gas fired reduction atmosphere:
**Goldart- 100, A.P. Green FireClay- 150, Kentucky OM #4- 70, Redart-
60,**********
**Bentonite- 4.5, Custer feldspar- 45, Flint- 36, Sand- 13.5, Grog
(fine)- 13.50***
The claybody's calculated C.O.E is ~55.
The several glazes that have worked well (in both the redux & ox regions
of the
kiln) have COEs from approx. 60 to ~74. A couple of other glazes that
were
randomly successful (in the updraft Bailey that sometimes had 2 cones
difference
in various areas) had COEs around 90.
Due to losing access to the gas kiln, I'm converting to all oxidation
firings
and in an electric kiln (at least until this batch of clay is gone).
The 4 glazes that I've tested
in an electric kiln firing, with success, have COEs ~70. I would like
to add a white liner semi-gloss
and a green/turquoise semi-mat.
QUESTION for the Clayart Community:
------------------------------------
Is there a rule-of-thumb concerning how far a glaze COE can deviate from
the claybody COE and still have a fair chance of fitting (i.e. without
immediate crazing or shivering)? Are there any glaze variables (e.g.
Al:Si ratio of glaze very different from the claybody) which would make
a COE comparison immediately not valid? I've looked for a discussion of
this in Rhoades & Chappell + a couple of other primary sources...no
luck.
My goal/hope is to downselect the glaze candidates via COE and then
focus on color/surface.
Thanks in advance for any help!
David Hewitt on wed 5 feb 97
In message , bob writes
>----------------------------Original message----------------------------
snip
>QUESTION for the Clayart Community:
>------------------------------------
>Is there a rule-of-thumb concerning how far a glaze COE can deviate from
>the claybody COE and still have a fair chance of fitting (i.e. without
>immediate crazing or shivering)? Are there any glaze variables (e.g.
>Al:Si ratio of glaze very different from the claybody) which would make
>a COE comparison immediately not valid? I've looked for a discussion of
>this in Rhoades & Chappell + a couple of other primary sources...no
>luck.
snip
I think you will find that any reference to crazing will say that to
avoid this the glaze should be under slight compression, the optimum
being that the glaze has a 10% lower expansion than the body. Hamer's
'The Potter's Dictionary of Materials and Techniques' and Harry Fraser's
'Glazes for the Craft Potter' are just two examples. For a reference to
the 5% to 15% and optimum of 10%, go to W G Lawrence and his book
'Ceramic Science for the Potter' 2nd Edition page 182.
All this supposes that you know the figure for the clay body and the
glaze and the bases on which they have both been measured or calculated
is the same.
There is an article on this subject, which may be of interest to you, on
the IMC site, which can be accessed on the URL given below.
David Hewitt
David Hewitt Pottery Caerleon, Tel:- 01633 420647
7 Fairfield Road, Caerleon, Newport, South Wales, UK.
URL http://digitalfire.com/magic/hewitt.htm
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