GLAZING OF ROOFING TILES, ETC.* Most varieties of clay, particularly those which burn red, permit, even when thoroughly burnt, the percolation of more or less water. Hence, it is frequently observed, that new roof ing tile, especially when not previously soaked, permit the rain to pass through to such an extent that the water not only appears in drops on the lower surface, but in sufficient quantity to wet the floor below. To be sure, the interspaces in the clay are gradually closed by mud deposited by the water, whereby the tiles after some time become impervious. But the same mud also produces upon the entire surface of the tiles, especi ally if the latter are not perfectly smooth, a coating which takes up dust and dirt. Hence, it frequently happens that nearly all the tiles, particularly upon low roofs, are covered with thick moss, which retains moisture, and the tiles on such places must necessarily rot. The partial object of glazing roofing tiles is to prevent this evil.
Besides, the great roof surfaces of deep buildings, such as churches, are monotonous, they offering to the eye but little interruption. This is still more the case in brick-work, where the colors of the building are repeated in the roof. In the middle ages this evil was also recognized and overcome by covering the roof with glazed tiles of different colors. In build ing the Ludwig church, at Munich, V. Gaertner pursued the same course, and in covering the roof with glazed tile produced an effect which admits of comparison with a pearl-embroidered carpet.
Glazing consists in giving the exterior surface of the tiles a coating, which at the temperature required for burning is con verted into a glass-like mass. Hence the glaze is a substance entirely different from the mass of the tile, and its durability and solidity depend much on the manner of its combination with the tile. The object of the glaze is, therefore, on the one hand, to decorate the tile, and on the other, to render its sur face impervious. The glaze mostly consists of a lead-glass col ored as desired by the addition of metallic oxides. For ordi nary pottery the proportion of silica to lead oxide is about I :2 ; i. e. mix about i part by weight of sand (frequently mixed with some clay) with double the weight of litharge or minium. The
more sand in proportion to lead oxide is taken the more diffi cult to fuse the glaze becomes. Litharge or minium melts at a comparatively low temperature and in a melted state dissolves the admixed sand, forming with it, if the substances are pure, a white transparent glass. According to the constitution of the sand a varying quantity by weight of it is at the same time dis solved by the melting lead oxide. If the sand is coarse grained, so that the surface exposed to the attack of the lead oxide is a small one, as compared with the mass of the sand, far less of it will, of course, be in the same time dissolved than when the exposed surface is greater. Hence the finer the sand, the greater the quantity which will in the same time be dissolved by the lead oxide. For this reason sand as finely di vided as possible is a chief condition in preparing glaze, be cause if the grain is too coarse, the entire quantity of sand is not dissolved, coarser or finer grains, according to the time during which the lead oxide was allowed to act upon the sand, remaining behind.
Of still greater influence than the fine division of the sand is its chemical condition, since its fusing point plays a very im portant role as regards the quantity which during the same time can be dissolved by the action of the melted lead oxide. Pure silica in the form of finely pulverized quartz, flint, etc., is not fusible, and hence will require the most time for solution by the lead oxide, because the latter will have to act upon a per fectly solid body. If silica in the form of a fusible mass be con veyed to the lead oxide, a much larger quantity of it will in the same time be dissolved, and it may be laid down as a rule, that a greater quantity of silica, of the same degree of fineness, will in the same time be dissolved by an equal quantity of lead oxide (as well as of other fluxes), the more readily fusible the mass is by means of which it is introduced. Nature furnishes us with an abundance of substances possessing all possible melting points, which, with a high content of silica, are more or less readily fusible.