MACRO CRYSTALLINE GLAZES
A brief explanation
Similar in construction to frost upon a winter window, and known, probably as a firing fault, in ancient China, this glaze style was accidentally re discovered during the eighteenth century in the great European factories of Sevre and Meissen. This glaze also appealed to the studio potters of the Art Nouveau period and was progressed in nineteenth century America by Taxile Doat and Adelaide Robineau. The industrial kiln masters of the day soon discovered that the whole process was so technically challenging to mass production that they ceased to develop the method, and little or no crystal glazed work was produced between 1919 and the 1960’s. Latterly little industrial production has been attempted, and only a few modern studio potters have chosen to take up the challenge.
The effect created by these glazes is singular, and the loss rate high as the potter breaks all the conventional firing rules to produce this outcome. Sometimes kiln losses can be as great as 70% between the first and last firing stages especially when experimenting with a new glaze base or clay, and the potter has to be phlegmatic and accepting of the outcome when the time comes to open the kiln lid.
A heavily fritted Glaze, containing large amounts of the crystal forming elements - Zinc Titanium or Rutile, plus colouring oxides is applied to the piece by brushing or spraying, and the work glued to a saucer made of matching clay, usually porcelain or very fine white stoneware, which fits the base of the piece as closely as possible, before firing to approximately 1270 degrees celcius in an oxidising atmosphere – a temperature well above the norm for the base ingredients. This saucer is required because it is necessary for the glaze be extremely molten at the top temperature to produce an environment suitable for crystals to grow. This high melt inevitably means that, if not prevented, the glaze runs straight off the pot on to the kiln shelf.
Once top temperature is reached, the firing is stopped abruptly to cause first, a very fast cool down to about 1100 c, after which the very slow, regulated cool down period required to develop the crystals, which form as zinc and silica bond together in the liquid glaze.
After the kiln has reached ambient temperature the work is removed, usually firmly attached to the catcher by the glaze run off. At this point the pot must be cut off the catcher, initially with a glass cutter and jewellers torch, then the excess glaze run ground away with a diamond lap bench grinder to smooth the base. During the course of this process much work can be lost, as the tension between the glaze, the pot and the catcher causes severe stress to the piece.
However, the end results can be spectacular, and always unique. Once attracted to crystalline glazes it is hard for the potter not to remain firmly devoted to continue working in this frustrating, but compelling discipline.
An offshoot of the space industry, and often referred to as 'space age glass', dichroic glass filters light so that the colours within seem to change as the glass moves. Multi layers of coating are placed on the surface using a highly technical vacuum deposition process. It's a complicated process that makes this glass among the most expensive in the world.
Several layers of glass are set together with sterling or fine Silver bails or wires and fused together by firing up to about 810 centigrade. The effect is stunning, and, like an opal, full of light and colour which is definitely not shown at its best in a photograph.