The above-mentioned lead glazes comprise, according to their composition, a scope of from about 932° F., dark-red heat, to F. (Seger's cone No. ). Above these tem peratures lead glazes cannot well be employed, because they possess the great disadvantage of the oxide of lead readily volatilizing under the influence of the fire-gases, whereby the glazes become more acid and refractory, and no longer turn out bright. The longer the glazes remain in the fire the more apparent this evil becomes, and the stronger, the higher the temperature which is used. The glazes containing tin fuse at about the melting heat of silver, sometimes above and some times below it, according to their composition.
The most important glazes for roofing tiles are those free from lead. They are employed in the form of clay or clay-like masses, which are applied to the tiles and then burned on. It is, however, impossible to give the clays such a composition that the glazes fuse at an early period, a certain degree of heat being required for their fusion. A glaze of 0.2 equivalent potash, o.8 lime, 0.3 alumina, 0.2 ferric oxide and 3.o silica, fuses at about 2100 degrees F. (Seger's cone No. ), this being the lowest fusing point for such glazes which can be at tained ; they frequently require a temperature represented by Seger's cones Nos. 6, 8, and sometimes io. The glazes which, at least for low temperatures, must always contain iron, appear by fusing as a black layer, the surface of which, when for some time exposed to atmospheric influences, acquires a brown color, about that of an ordinary pot glaze. By heating this glaze free from lead under the influence of reducing gases, the ferric oxide is converted into ferrous oxide, and the layer of glaze appears black. By adding to the glaze some oxide of manga nese, copper, cobalt, etc., it also acquires a black color, which is permanent and does not assume the yellowish coloration of iron glazes. They consist of ferrous silicate, corresponding to finery cinders. They are produced by the application of ferric oxide, and sand, generally inpure ferric oxide (yellow ochre) mixed with sand. The ferric oxide (ochre) at disposal for this purpose always contains sand, and sometimes more sand is added.
If the ferrous silicate is burned with the exclusion of reducing gases, it acquires a reddish brown color. But if it is burned in a reducing atmosphere, it becomes pure black, and the surface then assumes a slate-gray color. It does not completely fuse to a liquid mass, but in fact only frits together. The glazes are never quite bright and lustrous, but always dull, which is however, desirable. According to its composition and the proportion of ferrous oxide to silica, the fusing point of this glaze may be as high as that represented by Seger's cones Nos. 10 and I I, or as low as that represented by cone No. 2.
I have recently occupied myself much with glazes, and have found that their higher or lower fusing points are chiefly de pendent on their content of silica. The glazes richest in fer rous oxide are by no means the most readily fusible. The lowest fusing point is possessed by glazes which represent a ferrous bisilicate (72 ferrous oxide, 2x60 silica), the refractori ness increasing with an increase as well as decrease in the con tent of silica. Somewhat more readily fusible than the pure ferrous oxide combinations, are those which, besides ferrous oxide, contain manganous oxide. However, even such a glaze cannot be applied to a tile which has not been burned at at least a temperature represented by Seger's cone No. 3. Such mixtures which, besides ferrous bisilicate, contain considerable quantities of other substances, especially alkalies, lime and alumina, may be more readily fusible, but lose thereby their peculiar character—the slate-gray color—and are converted into glassy, bright, black masses.
It is certain that for glazes free from lead the fusing point should not be lower than Seger's cone No. I (2100 degrees F.), and for those containing lead not higher than that temperature. This establishes a definite boundary between these two kinds of glazes, which cannot be passed. Hence, if a chemist is re quired to compose a glaze free from lead, or furnish a receipt for a slate-gray glaze for a tile which cannot, without danger, be heated above 1832 degrees F., it will be impossible for him to comply with the demand.