I. Common clay r part by volume.
Red clay If it 46 Quartz sand I 44 it 411 2. Marl I II It Quartz sand , It 64 3. Marl , 66 44 64 Alum earth I 44 ti Chalk .. I it 44 It Quartz sand I 46 41 Et 4. Common clay it 64 Red clay. , it di Where there is a choice, the mixture given under r is to be preferred.
I. Glazes for Ms. r and 2 of the tile-masses.
Lead ashes 12 parts by weight. Silver litharge . 446 It Quartz sand 3It 16 44 White alum earth 4 41 it 44 Common salt 2 it It It Powdered glass 3 46 44 44 Saltpeter , 44 Id 44 2. Glazes for Ms. 3 and 4 of the tile-masses.
Litharge 16 parts by weight.
Quartz sand 544 46 id Powdered glass 4it 44 Adamic earth , it 46 it The intimate mixture of the ingredients of the tile-mass is, of course, the first requirement. For this purpose it is best to finely divide each ingredient by itself either by pounding or, better, by rolling, then to pass it through a fine sieve, and after moistening the entire mass with water to intimately mix it by tempering or rolling. The constituents of the glaze are also passed through a fine sieve and, after being intimately mixed, fused in crucibles to gloss, which, after cooling, is reduced to a fine powder by grinding with water. The powder is then pre pared as required for application to the tiles.
The flat tiles which are generally used in Munich are moulded in the usual manner, and after being carefully dried they are sharply burnt for the first time. To free the surface of the tile from dust and other impurities acquired in burning, and at the same time to test whether the tiles contain pieces of lime, they are dipped, i. e., placed in water, for one or two days. Any lime present is thereby slaked and causes the tile to crack, which, of course, would also take place after the appli cation of the glaze, but the latter would be lost.
The glazing mass mentioned under 1, yields quite a white gloss, the whiteness of which may be considerably heightened by an addition of 20 to 24 lbs. of tin to every 100 lbs. of lead before reducing the latter to ashes. The coloring of both glaz ing masses is effected by the following additions, many tests being made as regards beauty, and especially durability : Color. To every so lbs. of glazing
mass is added Dark violet brown % lb. pyrolusite.
Violet X lb. pyrolusite.
Green X lb. copper ashes.
Pale blue... drachms red cobalt ashes. Golden yellow % lb. antimony.
These additions are not fused, but after pounding, passed through a sieve, and then ground fine. By adding more or less of them darker or lighter colors may be obtained. It may here be remarked that small tests should be made with all colors be fore coloring the whole mass of glaze, because the materials as obtained in commerce vary very much in regard to purity ; the quantity of pyrolusite, for instance, required for the production of one and the same color varying very much according to the source from which the material has been procured.
For glazing, the surfaces of the tiles are rubbed clean; the glaze is then applied with a brush, the tiles being held almost perpendicularly over the vessel containing the glaze. They absorb the glaze so quickly that soon afterward they may be replaced in piles. Formerly the glaze was applied dry, but glazing by the wet method is better. With some experience and skill one workman can glaze from 5,000 to 6,000 tiles a day. When the glaze is applied it is burnt in by a second slighter burning. In doing this special attention must be paid to the degree of heat in order to obtain uniform coloring, but further details regarding the operation cannot be given. An experiment made in Munich to prepare red tiles by coating the unburnt clay in a semi-dry, so-called leathery state, with pul verized reddle, then burning, and finally coating with a trans parent glaze, did not succeed, since the glaze did not adhere and many tiles cracked. But a red glaze may be obtained by the admixture of a certain quantity of pyrolusite—between that required for dark violet brown and violet—and maintaining a fixed degree of heat, which can only be ascertained by experi ments.