There are sands with a very high content of silica which already slag (soften) at about i000° C. and hence readily dissolve in fluxes. Such sands, highly valued for glazing purposes, are, for instance, found at Machlin, in Mecklenberg, and at Fuerstenwalde. These sands contain about 85 per cent. silica, 7.5 alumina, and 4 to 5 fluxing agents (ferric oxide, lime, magnesia, and alkalies), and on account of their ready fusibility and physical constitution (they contain many fine scales of mica, which offer'a large surface to the solution), they dissolve readily and rapidly in fluxing agents. Besides these there are many clays with a comparatively high content of silica which fuse more or less readily. There are clays fusing at from I,oco° to I,Ico° C., which, in case such high temperatures are used in burning, yield by themselves good glazes, provided it does not matter as to color, and that the sole object is to make the articles impervious (earthy glazes for water-pipes, pottery, roofing tile, etc.).
In the great abundance of substances rich in silica and more or less readily fusible, which on the one hand are furnished us by nature, and on the other as waste (slag, broken glass, etc.), by certain industries, there are so many suitable for glazes that with some professional knowledge a suitable choice can be readily made, especially as the readily fusible fluxes (litharge, potash, soda, boric acid, etc.) offer means of bringing the fusi bility of the selected materials within a narrow limit and thus exactly fit the fusing point of the glazing masses to the burning temperature of the kilns. With glazes for the purpose of deco ration the conditions are much more difficult, the choice of sub stances through which the silica can be introduced being very limited. In this case not so much stress can be laid upon the ready solubility in fluxes as upon the purity of the materials. The naturally occurring substances rich in silica which are readily fusible, generally contain more or less iron by which they are colored dark. Hence if pure colors are to be obtained, sands, etc., containing iron have to be avoided and substances free from iron, but more difficult to fuse, selected. For glazes which are to serve for the pupose of decoration and show a determined color, sand free from iron (mica sand of the best quality, finely pulverized quartz or flint, etc.) will have to be used, and the proportion for the substances determined which is suitable for the burning temperature prevailing in the kiln.
When a colorless lead-glass suitable for the special variety of clay and the burning temperature employed has been found, the various shades of color are obtained by mixing the corres ponding metallic oxides in proportions to be determined with the glaze.
For to black, pyrolusite (peroxide of manganese) is used ; for green to black, oxide of copper or chromium ; for blue, cobalt oxide ; for yellow, atimony oxide ; for red, ferric oxide.
The application of the glaze to the tiles is as a rule effected by dipping, or to depressed places in ornamental or shape brick, or in architectural terra-cotta, etc., by means of a brush, the articles being dipped either in a dry state, or still better after having been burnt. If the surface of the tile is not suffi ciently porous for the glaze to penetrate, a cementing agent (paste, gum arabic, etc.) is added to the latter to render its adhesion possible. The condition of the surfaces to be dipped or treated with the brush should be uniform, so that the glaze may everywhere adhere.
The principal defects shown by the glazes after burning are fine cracks. To prevent them, it is advisable to gradually de crease the content of fluxing agent (litharge, etc.) without, however, changing the composition of the mass, and thus make the glaze more difficult to fuse. The temperature must, of course, be increased, but fine cracks will thereby be entirely or almost entirely prevented. If, however, this remedy is not suc cessful, i.e. if fine cracks still occur notwithstanding the utmost possible reduction in the content of fluxing agent in the glaze and a corresponding increase in the temperature, changes, as far as permissible, must be made in the raw materials. It should be endeavored to make the mass meagre with sand or pulverized limestone (chalk) until the formation of fine cracks ceases. The so-called salt-glaze is produced by from time to time throwing salt into the kiln during burning. The salt vaporizes at a high temperature and produces a glaze upon the surface of the articles.
The constitution and preparation of the clay are of special importance, particularly when the tiles are to receive a colored coating of glaze. In order to make the latter durable, it must enter into combination with the clay without the formation of fine cracks, bubbles, etc. The following mixtures which have been established by careful tests in the brickyards of Munich, fulfill these conditions with the glazes belonging to them, and may serve as guiding points.