The illustrations, Figs. 234, 235, and 236, show the form and size of roofing-tiles, as well as the mode of tiling custom ary in Munich. Since after tiling the roof, the surface of the tile which remain visible, form small squares with sides inches long, it is possible to produce, by the use of different colors, designs which relieve the monotony of the surfaces. It may further be remarked that on account of the considerable height and the small size of the squares, the indentations of the horizontal and perpendicular lines cannot be seen, but appear as inclined lines.
For plain glazing (dark brown or black) roofing-tiles in a more simple manner, it is not necessary to first burn the tiles, but the glaze may be applied to the crude article. For the glaze it is best to use the Dutch directions, according to which 3 lbs. of pyrolusite are taken for every 20 lbs. of litharge, and so much clay is added that a ball formed of the clay floats in the glazing mass. For ordinary lead ore 6 per cent. pyrolusite suffices, and the glaze becomes perfectly black by adding 1.5 to 2 per cent. of copper oxide. In all compositions for glaze special attention must be paid that the mixture is not too readily fusible, so the tile itself has a chance to become hard by burning. Neither should it be too refractory, as otherwise the tile would be over-burnt before the glaze fuses. Hence, an accurate knowledge of the clay is necessary, and it should be determined by experiments how great an addition of one or the other kind of lead oxide it will bear in order to obtain the de sired degree of fusibility. If pyrolusite is also to be added, the quantity of sand or silica must be sufficiently reduced in order to restore the proper proportions. It must also be re membered that pyrolusite makes the glaze far more refractory than silica or sand, which readily fuse with the lead to a glass, and that hence the quantity of the one cannot be exchanged for that of the other.
The above mentioned substances are, each by itself, reduced to as fine a state as possible, then weighed off in accordance with the above stated proportions and then ground, first each by itself and then mixed together, in a glaze-mill with water containing fine clay in solution, until they feel milky to the touch and nothing like sand can be detected. The rapid pro gress of this operation is chiefly dependent on the quality of the mill-stones and the arrangement of the mill. Ordinary glaze-mills are of fine sand-stone, hard lime-stone, or granite.
Before applying the glaze, the surfaces of the raw tiles are rubbed clean, to prevent the glaze from being contaminated by sand or dust. When sufficient tiles have thus been prepared, the workman places himself between the tiles and a frame. Upon a bench in front of him stands a shallow vessel of a wide circumference filled with glaze. This vessel is called the glaze back and may be of wood or, still better, of earthenware. After stirring the glaze with a large wooden spoon, the work man, taking with the left hand a tile by its upper portion, and holding it almost perpendicularly over the glaze-back, places the spoon filled with glaze about inches below the upper edge of the tile and moves it from the left to the right, allowing the glaze in the spoon to run over the tile. If the glaze is of
the proper consistency it is, in this manner, very uniformly distributed over the entire surface of the tile. A test with the finger-nail is from time to time made to see whether the glaze has still the requisite thickness, water or thicker glazing mass being otherwise added. The tiles absorb the glaze with such rapidity as to permit of them being soon afterward replaced in piles. The finer the glaze has been ground, the better it re mains suspended in the water and the more rarely it requires to be stirred up from the bottom. A further reason for the ne cessity of grinding the glaze as fine as possible is that other wise the pyrolusite does not thoroughly mix with the lead ore, which causes the appearance of streaks and stains after burning. With some experience one workman can coat 5,000 to 6,000 tiles with glaze per day.
Regarding tiles, the question whether it is more advanta geous to glaze the crude or burnt articles is not so easily answered as for finer clay ware. When the tiles have been properly set in the kiln and the fire is properly conducted, the refuse by breakage, cracking, etc., should not be considerable. But with tiles the mishaps by fusing and baking together are more dangerous. With well-constructed kilns the first should scarcely happen, especially when heating with small fires and the coal cannot directly act upon the tiles. The latter may be avoided : i. By not applying the glaze too thick ; 2. By the upper portion of the tile, where the knob of its neighbor leans against it, being left free from glaze ; 3. By setting the tiles as perpendicularly as possible so that they touch neither with their surfaces nor with their lateral sides, and so firmly that they cannot turn over during burning ; and 4. By wiping off with the hand the greater portion of the glaze from the base of the tile, so that there is no danger of their baking together on that point. With a second burning the tiles are, however, ex posed to the same mishaps, and the consumption of double the quantity of fuel is mere waste.
The following process may also be recommended as the most simple glaze. Scatter finely sifted unburnt lime and ashes over the half-dried tile, rub in well, and then allow the tiles to dry further. This mixture is a slight fluxing agent and produces a glaze upon the surface.
Glazing with salt is effected by throwing the salt in the kiln, when burning is finished so far that the kiln is to be closed for cooling off. Since by this process a uniform glazing of all the tiles cannot be effected, it is preferable to prepare a supersatu rated solution of the salt in water and apply it to the tile before setting them in the kiln.
Regarding the setting of glazed tiles it may be remarked that a kiln should never be entirely set with them ; because even with the best kiln it is impossible to distribute the heat so uni formly as to avoid fusing together, if it is insisted upon to every where fuse the glaze.