Tom Buck on sun 17 nov 02
A question arose on Clayart last week:
"My C10R base glaze worked fine on my porcelain body, but crazed
on my stoneware body. I did what is commonly advised: I added more
silica/flint, and it still crazed. Is there an explanation?"
Yes, there is one.
In 1921, a ceramic researcher, H.H. Sortwell explored this problem
and reported on it - "High Fire Porcelain Glazes" Journal of The American
Ceramic Society 4:718 (1921). Later, 1970, F.H.Norton included part of
Sortwell's report in the McGraw-Hill book, "Fine Ceramics", pages 188/9.
Norton wrote: The basic porcelain glaze firing at cone 8 and
9 (1225 to 1250 oC) is expressed by:
0.3K2O 0.4Al2O3 4.0SiO2
0.7CaO
(Note: This Seger Formula could yield a batch recipe made of potash, kaolin,
silica sand, and limestone/whiting. Glzcalc could suggest other mixes).
Sortwell learned that as the silica content increased, the zone of
crazing enlarged, even if the alumina increased in step.
While not specifically stated, the crazing was likely due to
a change in the glaze/glass by the formation in the molten liquid
("liquidus") of crystals of cristobalite, a rigid structure of
SiO2/silica that separates from the glassy phase and makes the glaze
non-uniform on cooling. Hence, adding more silica simply makes more
cristobalite.
One of Sortwell's graphs shows how small the zone of good glass
really is at C10, all quantities in "moles":
0.48-0.52Al2O3 and 3.0-5.0SiO2 will yield a bright gloss glaze;
0.52-0.8Al2O3 and 5.0-7.0SiO2 will yield a semimatt;
0.8-1.2Al2O3 and 7.0-9.0SiO2 will yield a matt glaze.
Larger values will make the glaze immature at C10.
If one fires to C12, one can enlarge the zone of bright glaze,
allowing for more silica, 4.0-7.0SiO2, at alumina levels of 0.48-0.70.
However, in the case of the potter who has a problem with
a porcelain glaze on a stoneware body, firing to C12 may not be an option.
Possible avenues that then might provide a solution would be:
Follow Ron Roy's advice and shorten the firing cycle to limit
formation of cristobalite, and
Include a small amount of B2O3 in the glaze, say 0.05-0.10 moles,
which may inhibit formation of crystals.
and for a revised batch recipe, the potter can request same from many
on Clayart who do glzcalc.
an aside: members of the Potters Council of ACerS can order a
reprint of Sortwell's paper at a reduced fee.
til later. Peace. Tom B.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
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