=What is Salt glaze?=--We know that many of the stoneware Bellarmines and Rhenish jugs were glazed with salt. It was a process known on the Continent at a very early date, some authorities place it as early as the twelfth century. But it was not until the second half of the sixteenth century that German and Flemish potters used this salt glaze to any great extent. We have seen, in the chapter on stoneware, that the appearance in the mottling and in the orange-skin-like surface is due to the action of salt glaze.
To cover pottery with an outer surface has been practised from earliest times either by the use of some glassy material or by powdered lead. Glazing with common salt was quite a new departure by the English potters. In order for this salt glaze to be used there must be a very high temperature, so high, as a matter of fact, that it would melt or soften in the kiln most English earthenware. This is where stoneware in its body differs from earthenware; it is what is termed "refractory," that is to say, it is not readily fusible. Stoneware is not always glazed. Elers did not glaze his red ware, and Wedgwood did not glaze his basalt or black ware. Stoneware can also be glazed with other processes than the salt glaze, but, as a rule, stoneware is associated with salt glaze.
Without entering too tediously into the exact steps by which salt glazing is performed, it may be roughly described as follows. Other glazes, such as lead, are applied to the surface of the ware prior to its entry into the kiln for firing, but in salt glaze the glaze is incorporated with the ware while it is actually in the kiln. Towards the end of the firing common salt (chloride of sodium) is thrown into the kiln, which is packed with the ware, through apertures in the kiln which has to be specially designed for salt-glaze use. At the high temperature of the kiln (about 2,190° Fahr.) the salt is volatised and its vapour penetrates the saggers (that is, the earthen vessels containing the pieces being made), which have perforated sides to enable this vapour to form on the surface of the pieces being fired. This vapour chemically unites with the silica largely present in the body of the stoneware, and forms a silicate on the surface of the ware. That is to say, the stoneware becomes coated with a thin layer or glaze of sodic silicate or soda-glass.