When it comes to glazing ceramic pieces, potters who use high firing temperatures have a big advantage over us practitioners of terracotta. (You may not think a firing temperature of 1060 deg C is particularly low, but in pottery terms it is.) High temperature glazes can be made from cheap, plentiful materials like feldspar, calcite and wood ash, but most of the materials which can theoretically be used to make low temperature glazes have major practical problems. Four thousand years ago, the potters of ancient Egypt discovered this. They were familiar with a natural mineral called natron which was used in the embalming process for mummies. It melted at a low temperature, but was soluble in water. This fact is significant because the easiest way of applying a glaze to a product involves mixing it with water, but the natron would dissolve in the water and so be separated from the other ingredients. The enterprising Egyptians came up with the answer: melt the natron together with other minerals to form a glass which is insoluble in water but still has a low melting point. Then grind the glass into a fine powder and make your glaze with that. Many centuries later, the potters of mediaeval Italy used the same technique, calling the material “fritta”, meaning “fried”. In English, this became “frit”.
The Romans used the mineral galena for glazing pottery, the active ingredient of which is lead. When pottery was produced industrially in Europe in the 18th and 19th centuries, generations of pottery workers died untimely deaths as a result of lead poisoning. Research by a number of chemists including the New Zealander Joseph Mellor, showed that this scourge could be totally prevented by incorporating the lead into an insoluble frit.
Glaze makers began experimenting with the mineral borax at the end of the 18th century. It was particularly useful in preventing the fault known as “crazing”, and was soon regarded as virtually essential for low firing temperatures. It made practical leadless low temperature glazes possible for the first time. However it is water soluble, so once again it needs to be incorporated in a frit.
Today, glaze technologists can choose from a wide variety of frits produced industrially. However, when I worked in the Philippines during the 1970’s, and was given the job of developing some glazes for terracotta, I discovered that commercial frits were not available there. There was nothing for it but to make my own. The photograph above (left) shows my improvised frit kiln. The raw materials were packed into a fireclay crucible which was then placed in the kiln and heated with gas burners made from pipe fittings. There was a hole at the bottom of the crucible, so the molten material ran through the hole, through a corresponding hole in the bottom of the kiln, and into a bucket of cold water. The resulting glass shattered into small fragments, which were then milled to a fine powder in a ball mill. Commercial frits are produced in much the same way on a larger scale.
The photograph on the right (above) shows the difference between frit after it has been quenched in water, and when it has been milled into a fine powder. These samples came from the Ferro factory in Moorabbin, Australia, which I visited in 1995. Ferro opened this facility (now no longer operating) shortly after Morris and James introduced their first production glazes in the 1980’s.Their technical assistance at the time was much appreciated.
Our Technical Manager, Mike has a rich background in ceramic technologies in NZ and abroad. A true alchemist he is our glaze and kiln specialist. Mike has been with Morris & James since 1985.