Pottery in Australia Vol 17 No 1 Autumn 1978

LEADLESS, INSOLUBLE,
LOW-TEMPERATURE GLAZES
Leigh Roberts
Since 1968 I have been teaching general art and ceramics in Tasmanian high
schools. From this experience I was acutely aware of the problems facing art
teachers who have not had a thorough background in ceramics, especially in
glaze formulation, as was my case. To rectify this and because of my deep interest
in ceramics I applied for a fourth year at the Tasmanian College of Advanced
Education, Hobart, to study ceramics under Les Blakebrough, Gwyn Hanssen
and Penny Smith.
The Tasmanian Education Department has wisely ruled that lead in any
form may not be used in schools. Because of this ruling and other factors such
as lack of time, expertise, and suitable glaze recipes, teachers have generally used
commercial glazes, usually with most uninspiring results. I found that these were
often faulty and crawled badly, did not melt at the specified temperature, or
cemented in the bottom of glaze containers; some of them deteriorated markedly
when kept for a period of time. I decided, therefore, to eliminate all these undesirable
characteristics, if possible, in any glazes that I planned to develop.
As almost every Tasmanian school that includes ceramics in its curriculum
uses electric kilns, most of which will not fire over IISO°C, I have designed the
glazes to fire in an oxidizing atmosphere over a range from just below 1 100°C up
to l1S0°C, with an ideal maturing temperature of I 120°C. This range is purposely
wide because school conditions are not as closely controlled as in a pottery
studio and because of the variety of kilns in use. The raku glazes are designed to
mature at 900°C but will melt at least 20 degrees below this and will become
rather fluid over about 950°C. All the glazes were tested on red bodies (Relbia
clay, Humes pipeclay, Hayes Gaol Farm clay) and on a white body (Bendigo
clay) and in large and small electric kilns and also in a wood-fired kiln. As well
as being suitable for use on bisqued ware, many of the glazes are suitable for rawglazing
without recipe adjustment-an advantage in the school situation.
During the initial stages of research I investigated literature containing glaze
recipes in search of leadless, insoluble recipes to use as a basis for experiment. I
found no published glazes that were suitable so I used formulae from Dr F.
Singer's book Ceramic Glazes. Daniel Rhodes' book Clay and Glazes for the
Potter and Harry Fraser's book Ceramic Glazes were also informative. The article
by G. Snape which appeared in the Spring issue (Vol. 15, No.2) of this magazine
would also be useful for anyone interested in this area, as it contains theoretical
information which complements this article.
As well as using Singer'S formulae, I noted any recipes I came across that
could have been useful, such as those by Ivan Englund published in this magazine
in an article entitled "Middle Fire Glazes". Any recipes taken in this way were
adjusted to fulfil my criteria. Most of the recipes are based on Cesco Frit 2, a
calcium-borate frit which is readily available and melts on its own at 900°C.
Many of the glazes contain more than four ingredients, but I found this necessary
to maintain glaze qUality. The raku glazes are simpler in composition but greater
difficulty was experienced in developing a workable base for raku glazes than for
an earthenware base glaze. The problem was finding a substance that would flux
at a very low temperature in combination with Cesco Frit 2 and Ball clay "C".
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