After the firing, the pottery must be cooled slowly, as too rapid cooling, or the impact of cold air, may cause the pots to split or flake. If this happens the pots join the mass of wasters, or defective pottery, which characterize the rubbish heaps of potteries. However, these waste heaps now form extremely useful information to identify the source of wares when historic potteries have disappeared, often in the development of large cities, notably, in the case of Britain, in London and Liverpool.

7.8 Glaze

The discovery of glaze probably occurred before the discovery of glass, although glaze is in fact a thin covering layer of an amorphous glassy material, predominantly made up of silica. In principle glazes reach their melting point but slips and paints may not. The raw materials of the glaze are applied to the ceramic body and dried, and they form the glaze on the surface of the body during firing. It is likely that glaze was an “accidental” discovery, such as ash with an appropriate composition falling off the kiln roof on to pots and “glazing” them, or a lucky coating of slip with a low melting point. Some clays and some slips become naturally “glassy” on their surfaces when fired and are almost “self glazing”, and some early glazes might well have been due to this effect rather than the deliberate application of glaze. Examples are Egyptian Marl clay with high silicon content, Roman, Samian, polished red ware (using an illitic slip) and the shiny black slip paint used on Greek black and red figure ware. Glaze can have functional and decorative uses. Functionally it can make a porous (earthenware) pot waterproof, making it useful for a greater range of uses, for example holding valuable liquids. It makes the item more pleasing to the eye, more hygienic, more resistant to chemicals and often mechanically stronger. It also permits new forms of decorative effects as well as making the surface glossy, potentially adding a host of colourful effects and variations of glaze texture.

Spanish Majolica Import 16th Century -
source Hampton Court

Dipping Plates into Glaze - courtesy
Worcester Porcelain Museum

Glaze may be formed by adding dry materials directly to the surface of the vessel, or applied as a suspension of fine particles in water. It can sometimes ease preparation and application, and in some cases be essential, for the raw materials that form the glaze to be initially “fritted” or mixed dry and fused in a crucible to form a glass or frit. In ancient times sand, lime, soda or potash and colourants would be heated slowly in open ceramic pans, forming a sintered glassy cake, with the pigments absorbed into the glassy mass. Amongst the advantages are: (1) some useful components of glaze, for example oxides of boron, are soluble in water and therefore difficult to form into a slurry of ground solids, but fritting makes them insoluble, (2) having been pre-melted into a glass the subsequent re-melting temperature is reduced, and (3) fritting poisonous constituents such as lead oxide makes them safer to handle, as many potters suffered from lead poisoning in the early days. The fritted material is then ground to a fine powder, mixed with water and applied to the pot. Suspended glaze can be applied with a brush or the pot can be dipped in a tank of glaze.

When fired, the glaze melts and fuses with the ceramic body. If the raw glaze is applied as a powder, firing can result in a blotchy appearance with a number of small glaze-filled craters where larger lumps of the material have etched into the body of the pot. A smoother, better quality glaze results from its application in suspension. Early glazing was often limited to part of the pot, usually outside on the shoulders.