Page:Popular Science Monthly Volume 29.djvu/361

Rh the resultant ashes afforded the necessary potash. The Bohemian crystal serves in art as the type of the most perfect glass, and is unquestionably recognized as one of the superior kinds, rivaling in transparency and clear whiteness rock-crystal itself. Pure specimens of it, free from blisters, grains, and specks, have a peculiarly attractive look, even, in their simplest forms. It is, moreover, by reason of its tenacity, its constancy of luster, hardness, and difficult fusibility, eminently adapted to artistic molding and ornamentation.

The heating of the furnace with wood only, from which a comparatively small quantity of ashes was produced, and they entering into the composition of the glass, contributed no little to the attainment of the highest perfection. While the glass-houses were at first built where they might be made a means of utilizing the superfluous wood, they have now to contend against a continually rising price of wood and increasing difficulty in procuring it. t Some factories, like the Josephinenhütte of Count Schaffgotsch, and that of Count Harrach at Neuwelt, have the extensive forests of their owners to rely upon, while the much more important establishment of Joseph Riedel in Polaun is looking forward to direct railroad connection with the lower Silesian coal-mines or the Bohemian brown coal.

While formerly only the best, finely-split, well-seasoned trunk-wood could be depended upon to heat the furnaces to the needed temperature, the required degree is now obtained from limbs, knots, roots, and even green wood, by distilling the gas from them in an imperfectly ventilated regenerator, and burning it with the aid of previously heated air. By this means is obtained a clear, excessively hot flame, by which the most infusible glass is made as fluid as water, and of a very high state of purity. Many experiments will be necessary before such excellence can be obtained with coal-gas; and, in any event, a previous washing of the gas will be required to clear it from tar and ashes. The form of the furnace, the manner of introducing, purifying, and tempering the glass, the processes of bringing it into shape, and the shaping tools, do not vary essentially from those of the old ways; except that complicated figures engraved in iron and brass molds are now applied, the complete transference of which to the glass necessitates the use of air under high pressure. This is furnished by means of a hand compression-pump, so arranged in connection with the other parts of the apparatus that the manipulator can bring it to bear upon the melted glass at the precise moment when it must be brought into the closest contact with the engraved pattern. Other pieces, of a massive character, such as lenses and ring-segments for lighthouse-lanterns, which are now made on a large scale at Polaun, are formed by subjecting the material to a light pressure between an upper and a lower mold. They are then finished and polished after they have cooled.

The after-decoration of the glass is various, and subject to the