|Preparation of glaze-frits|
The glaze-frit is the pre-prepared raw material for a glaze, a mixture of two or more materials which have been fused together by heating and then cooled rapidly into solid granules. These are stored for as long as necessary. To make the glaze the granules are ground into a powder and dissolved in water or a weak acid such as vinegar, and applied to the vessel through dipping or painting.
Abul-Qasim describes the process for making glaze-frits for tin glazes. A specially-built glaze kiln is required. Tin and lead are weighed out in the necessary proportions: lead is always in conjunction with tin in equally high quantities until the C12th when the lead content begins to drop; Mason suggests that it is difficult convert metallic tin into tin oxide in the absence of lead. The lead is first put into the kiln and heated, then the tin is thrown in on top of it. They are mixed and fired together at a high temperature until melted. This appears to cool into an earthy white (ie. oxidised) substance which is called sirinj in Persian. This powdery substance is added in certain proportions to a glass frit made of ground quartz and soda. The whole mixture is put back in the frit-kiln for 12 hours to melt into a uniform mass. This molten mixture is quenched in water which causes it to break up into solid granules, which are then stored until required.
The procedure with other types of glazes seems to be very similar to this one, but with different ingredients. Lead in a clay or a glaze acts as a flux, ie. it reduces the melting point of a clay/glaze, allowing it to vitrify at lower temperatures. Lead glazes are generally transparent and clear, though they may be coloured using metallic oxides: copper oxide especially was used, and when added to lead glazes formed a transparent turquoise glaze. Another common colourant was rust (ferric oxide) which gave a yellow or reddish colour.