CLAY Clay is a mineral with a crystal structure, originated from the geological weathering of the earth's crust. Erosion, volcanic action and hot gases decomposed hard rock, mostly feldspar. There are two types of clays: primary or residual, and secondary or sedimentary. The first have not moved from where they decomposed from the parent rock, and erosion was performed by ground water and gases from below. They are pure and hardly contain other minerals. Kaolin (main ingredient in porcelain) is a primary clay. The second have been carried by the elements to another location, incorporating impurities in the form of organic matter, minerals and most commonly iron oxide. The action of water grinded the clay in increasingly small particles, leaving all coarse material behind. This is the most common type of clay (terracotta pots are made in this type of clay). My work is made of clay from the secondary type. Each type of clay has its own chemical formula, which varies slightly on each batch. To simplify the work of the ceramic chemist, an ideal formula of one molecule aluminium oxide, two of silica (powdered quartz) and two of water has been accepted; this is the formula of kaolin. Clay, when fired, undergoes a chemical change referred to as ceramic change. The last water not escaped during drying, because trapped in the alumina and silica molecules, evaporates in the form of steam when the pot reaches around 600°C, changing the formula to one molecule alumina and two of silica. GLAZES A glaze is a glass-like substance applied in the form of a layer of powder, fused into place on a pottery body to give it waterproof qualities. It is basically composed of glass-forming oxides or acids (like silica, feldspar or quartz), fluxes or alkaline oxides (like soda, potash, calcium and lead) and stabilizers or neutrals (like alumina). All the oxides in the glaze are solids, but the alkali-acids interaction when exposed to heat results in the formation of a liquid, with the neutrals helping rebalancing it to a perfect fusion. When fused, the glaze interacts with the body; in earthenware only in a superficial way, but in stoneware, glaze and body become one. Various oxides can be used to make glazes in endless colour and texture combinations. For Example Copper oxide produces greens and blues in oxidising atmosphere, and reds in reduction atmosphere, cobalt oxide and carbonate produces blues and Iron oxide browns and oranges. Tin acts as an opacifier to optain white glazes.