Technique and Composition of Glazes

Glaze was obtained by adding to the glass paste between 6 to 15 percent tin oxide, which remained in suspension and gave it a whitish color. Various metal oxides were used as dyes. First a mixture of tin and lead was heated, the resulting oxidation of which left behind a yellow powder, called "Glass Earth".

Substances forming the glaze and glass earth were then added to molten glass, which continued to be heated. This "low temperature" method was one of the most difficult stages of potters work, because it required two firing, each in a different temperature.

Vessel was first covered with glaze and fired for the first time. Then, back from the kiln, it was painted over and decorated before being fired the second time, below 600 degrees Celsius, so it gives solidity to colors. Presence of metal oxides in glaze, which acts as flasks, cause the firing temperature to decrease.

In another type of production, perfected by Iranian artists, copper sulfate, silver nitrate, gold chloride and bismuth nitrate, and also more rarely copper and silver carbonate were used. One part of these ingredient was mixed with three parts of ocher and some Arabic gum. Mixture was thoroughly pulverized and applied in a thin layer to vessels.