GLAZES FOR ROOFING-TILES. * Roofing-tiles are, as a rule, only glazed when it is expected to impart to them thereby greater resistance to the weather, or to produce certain decorative effects. Hence dark glazes are frequently demanded, if only for the reason that, as regards roofing-tiles, we have generally to deal with quite dark-colored clays. But such tiles should only be glazed if they possess the degree of solidity absolutely required for it. It is well known that slightly burned tiles, when glazed and exposed to the weather, scale off, as a rule, in a short time, and hence are not improved by glazing. It is, therefore, necessary that tiles which are to be glazed must be burned hard, at least, medium hard ; or still better, that they are clinkers. Many proprietors of brick-yards seem to be of the opinion that tiles may be burned slightly, and then provided with a glaze to make them more durable. Such, however, is n6t the case. If a tile is saturated with water and allowed to freeze, it will be observed that small sticks of ice are forced from all the small pores, which is due to the expansion water undergoes in freezing. The sticks of ice forced out will be the thinner, the finer the pores, and the thicker, the coarser the pores. This will be especially ob served in calciferous washed clays. The ice, under pressure, acts like a fluid, and passes out through the fine openings. Now, if these openings are covered by a glaze which does not firmly ad here to the tile, and the quantity of water is quite large, entire portions of it are forced off. Hence, since it is generally en deavored to burn glaze and tile in the same burning, the fusing point of the glaze must be arranged in accordance with the slagging point of the clay. Now the glazes themselves, es pecially the readily fusible ones, are not always capable of re sistance, but are frequently decomposed by atmospheric influ ences. Thus, for instance, alkalies are lixiviated from the layer of glaze by the influence of the atmosphere. The layer of glaze is thereby loosened, becomes mottled, changeable, full of cracks, and finally the entire glaze scales off. Such glazes will have to be examined, as is done with glasses in which the ac tion of the atmosphere upon the surface has also to be inquired into. Professor Weber has proposed an excellent method of testing glasses as to their power of resistance against the atmos phere, which I have also used for glazes. Professor Weber proceeds from the fact that the atmospheric influences are much stronger by taking substances which act more energeti cally than carbonic acid and water. He therefore places the glasses under a bell in which is a vessel with concentrated hydrochloric acid. The acid begins to etch the surface much more rapidly than the carbonic acid of the air, and if the glass is destructible, such destruction takes place in a very short time by a slight efflorescence of separated silica showing itself upon the surface.
The nature of the glazes to be used depends on the temper ature they are to sustain. With low temperatures, glazes con taining lead will have to be used, because there are no glazes free from lead for the lowest temperatures employed. The glazes used in pottery are simple lead silicates. They are the most readily fusible glazes and occur also in the manufacture of tiles.
As regards their content of silica they vary from a sesqui silicate to a trisilicate, which at the same time contains alumina ; the richer in silica, the more refractory they are. The com position of the most readily fusible and the most refractory lead glazes may, according to the ratio of the weight of the sep arate constituents, be represented as follows : Readily fusible glaze—I.0 x 223 oxide of lead, 1.5 x 6o silica. Refractory glaze—o.i x 94 potash, 0.2 x 56 lime, 0.7 x 223 oxide of lead, 0.3 x tot alumina, 3.o x 6o silica.
The glazes employed will, as a rule, lie between these two compositions. The degree of refractoriness of the glaze which is required will of course have to be taken into consideration, and hence, these numerical values may and must be extremely varied. When in glazing tiles color, and especially a dark color, is the chief requisite, the clay at the disposal of the man ufacturer will, as a rule, be utilized, in receiving simply an ad dition of oxide of lead, whereby the action of the ferric oxide in the clay as a yellow or broom coloring agent has to be taken into consideration. No other substances require, as a rule, to be added. Boric acid, especially, should be avoided, since glazes containing boric acid have always to be fritted in, which cannot be recommended for glazing tiles, since the glaze should be as cheap as possible. For colored glazes the color of the tile must first be covered with a white layer, glazes containing tin being best for this purpose. The application of a white-burn ing clay to the clay cannot be recommended. The average composition of those white enamels, will also serve for the preparation of ordinary faience. Dutch tiles, etc., may be given as follows : 9.5x94 potash, 0.8x223 oxide of lead, 0.2x WI alumina, 2.ox6o silica and o.5x15o oxide of tin. Some times more of one and less of another constituent may be re quired, but generally speaking the above is an average com position. The oxide of tin makes the glazed opaque and white. Any color desired may be produced by adding certain metallic oxides, for instance, oxide of cobalt up to 3 per cent.; oxide of copper up to 4 per cent.; peroxide of manganese up to 8 per cent.; ferric oxide up to 4 per cent. ; oxide of uranuim up to 5 per cent.; antimonate of lead up to to per cent.