Armchair Potting
Article1 by Mike Kusnik
Medium fire ceramic glazes with above average clay content.
This exercise is a continuation of an article, published some time ago, on eutectic compositions which could be used individually or collectively as medium fire stoneware glazes.
Just, briefly, have a look at three low melting point eutectic mixtures which take part in this investigation.
- Barium carbonate and China clay MP1200°C,
- Calcium carbonate, China clay and Silica MP1170°C and
- Calcium carbonate, Zinc oxide, Alumina and Silica MP 1040°C.
The Barium and China clay mix is not very known one and it is very unusual due to the very low alumina and silica concentration. The percentage of this eutectic is as follows: Barium carbonate 66.6% and China clay 33.3%.
The mix is also, very unusual, for another reason which can be explained as follows:
all eutectic mixtures are determined, by ceramic researchers, using chemically pure raw materials thoroughly mixed (ball milled) together and heated up to a temperature which is above the melting point of the mix. During the cooling period of the, now, perfect mix-glaze the researchers establish the temperature when it solidifies; this temperature point is, of course, melting point (eutectic temperature) when the mix is reheated.
In other words if we mix an eutectic composition using raw materials and heat it up to the expected eutectic temperature not much will happen; the mix is not a perfect one due to impurities and mainly due to the particle size of the constituents and the contact between them.
With Barium carbonate and China clay the story is, remarkably, different.
The mix will melt at the eutectic temperature or very close to it. The reason behind this may be the physical behaviour of barium carbonate on heating. At approximately 1000°C it changes from solid crystalline form into liquid form and now the liquid barium carbonate soaks into the porous china clay (part of the glaze) and thoroughly-completely wets the clay crystals resulting in a perfect blend.
By the way, barium carbonate when heated on its own, just above 1400°C decomposes into barium oxide and carbon dioxide. Barium oxide has melting point of 1920°C. Barium carbonate when heated with other glaze participants decomposes into barium oxide at temperatures well below 1000°C, in raw earthenware glazes.
The Calcium carbonate, China clay and Silica eutectic (1170°C) is very popular one with potters who use it as a clear glaze fired to OC 9-10 [1300C]. No need to tell you the difference between the eutectic temperature and the practical temperature.
The formula of this eutectic is as follows: whiting 33.7%, china clay 30.0% and silica 36.3%.
The Calcium carbonate, Zinc oxide, Alumina an Silica eutectic (1040°C) has to be fired to, at least,1200°C to give you a reasonable glaze. The formula of this eutectic glaze is as follows: whiting 28.1%, zinc oxide 19.1%, china clay 17.8% and silica 35.0%.
As you can see all the three compositions have above average clay content and therefore I call them high clay glazes. In some of the glazes we have to use calcined china clay instead of raw one to prevent crawling and please, correct the weight (raw clay MW 258; calcined.clay MW 222.)
The calcium carbonate, zinc oxide, china clay and silica mix is the only one which can be satisfactory at OC6 (1200°C) temperature.
On the other hand we can mix all three eutectic compositions on a triaxial principal and thus develop, perhaps, two or three satisfactory base glazes.
All the above mentioned glazes are clear transparent wit exception of the zinc one showing slight opacity. Additions of opacifier such as Titanium dioxide, rutile, Tin oxide, Zircon and metallic oxides will give you coloured glazes.
Cheerio for now Mike
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