Putting aside salt glaze we find a countless number of both lead and leadless glazes. They range from the thin silicious coating of the ancients up to the rich alkaline glazes of the Persians and Chinese; from the raw galena of peasant pottery to the rich Majolicas and fine hard glazes of modern commerce. Salt glaze is obtained by the vaporizing of salt inside the kiln at a great heat. The sodium oxide so formed combines with the silicates in the clay to form a very thin coat of refractory glass, intimately connected with the body. Porcelain glazes, though not differing so much in composition from the ordinary fine earthenware glazes, are extremely hard, being compounded of kaolin, felspar, and quartz, with possibly limestone and ground sherds. It has in common with the salt glaze the close union with the body, so that when fractured the line of demarcation between glaze and body is indeterminable.
With a few minor exceptions the following list comprises the ingredients of all colourless glazes:
Kaolin Quartz Cornish stone Felspar Fluorspar Flint Sand Barytes Bismuth Gypsum Limestone Nitre Borax Bone-ash White lead Red lead Zinc oxide Tin oxide Salt Soda Potash
These materials must be free from all trace of iron. They are pulverized and some are calcined or oxidized. Then they are mixed in varying quantities to form the glaze mass. This mass is easily fusible when lead or borax is present in large proportions, more infusible or harder the more silica it contains, and very refractory if alumina is present in any quantity. The silica forming the glassy part of the glaze is stiffened by the presence of alumina, which stops any tendency to run.
Lead is very largely used as a powerful flux at low temperatures but is unsuited to hard glazes. Borax and boracic acid are important constituents of leadless glazes. They are used to replace some of the silica, than which they are more fusible. Matt or non-reflective glazes are opaque and less vitreous than the glassy glazes. They do not flux or run.
All these minerals are finely ground before mixing. Then those insoluble are mixed and fritted; that is, fused in a crucible or fritting furnace to a greater or less degree, according to the hardness of the glaze. If
fused into a glass, the melted mass is poured into water to facilitate the next process, which is its reduction to a fine powder by re-grinding. Then the completing ingredients are added and the mass coloured by the addition of metallic oxides. Of these the chief are:
Iron Copper Nickel Antimony Cobalt Chrome Manganese Titanium
and the more precious metals, in various forms, as oxides, carbonates, sulphates, and nitrates soluble in the glaze at great heat.
These metals impart the many varied colours found in pottery. Zinc oxide is used to brighten a glaze or to stabilize colour. Tin oxide, which is insoluble at great heat but remains in suspension, gives opacity.