ANNEALING has been referred to already, but it is such an essential part of the art of glass making that it requires more detailed explana tion. In order that glass shall have strength and durability, it must, after it has been fashioned into shape by some process, be cooled more gradu ally than is possible in the temperature of the open air. This is necessary in order that the pores, which have been distended by heat, may contract evenly throughout the substance in stead of closing more rapidly on the surface than inside. In order to effect this, annealing-ovens are used in which the temperature is so regu lated that the cooling proceeds with extreme slowness, as many as four weeks being required to bring optical glass to its normal temperature. Two kinds of annealing-ovens are in use, the intermittent and the continuous. An intermit tent kiln is simply a gas-heated oven with a hearth on which the glass is laid. When the heated kiln is filled with ware it is sealed up and allowed to cool by draughts, which are so regu lated that the temperature can be perfectly con trolled at every step of the process. In annealing plate glass there is a separate oven for each sheet. Continuous kilns or 'leers' are displacing the intermittent kilns for many forms of glass. The oven is a long narrow passage with the fire at one end, so that the temperature constantly lowers as the other end is approached. An end less chain, with pans on it for holding the glass, is slowly dragged through the oven, and by the time a pan has traveled from the fire to the cold end of the oven the glass in it is properly an nealed. Several processes for rendering glass still tougher than it is made in the annealing oven have been invented. M. de la Bastie has succeeded in imparting considerable toughness to glass by plunging it, while hot, into a bath of oil or melted fat. This process has proved only partially successful, and is not adapted to win dow-panes, because glass treated in this way will not cut with the diamond. Another method is in the manufacture of `Verbundglas,' or com pound glass, which was originated at the works of Sehott & Genossen, at Jena. Germany. The glass is made of two layers which expand at dif ferent temperatures, one of them being flashed over the other. This glass is especially adapted
for chemical vessels, thermometers, lamp-chim neys, or, in fact, for any use in which the ex terior and interior surfaces are subjected to widely different temperatures.
There are many strange uses of glass. Threads may be drawn on a. reel from molten glass, making a transparent mineral silk fine enough to weave into cloth, or to fashion into fancy plumage. It has been found possible to weave glass into fabrics, sometimes with a warp of silk, and to shape it into collars, neckties, brushes, lamp wicks, etc. M. Dubus Bonnet, of Lille, France, has invented a process of spinning and weaving glass into cloth. The warp is composed of silk, forming the groundwork, on which the pat tern in glass appears, as effected by the weft. The requisite flexibility of glass thread for manufacturing purposes is due to its extreme fineness, as not less than from 50 to 60 of the original strands form one thread. Mineral wool is made from the slag-glass refuse of iron-smelt ing, being blown into fine shreds by a blast, to fill walls and floors with a fire-proof and rat proof padding, and for other unique services. Reamer's porcelain is an opaque and porcelain like glass, which has been `devitrified' by great heat and gradual cooling, becoming marvelously tough. Soluble glass is a highly alkaline solution of minerals composing glass, which is applied to textures in theatres and elsewhere to render them fire-proof. Fire touching them melts the invisible minerals into a glaze which excludes air and pre vents combustion. M. de la Bartel introduced into Europe transmuted glass, which is tougher than cast iron. Malleable glass is one of the legends descending from the ancients which may be some day verified.
Among the authorities on glass are: For the history of the art, Gaudy, Romance of Glass Making (London, 1898) ; for the scientific side, Powell, Chance, and Harris, The Principles of Glass Making (London, 1883) ; the chapter on `Glass,' by Linton, in The llinerad Industry for 1899 (New York, 1900), and Riser, Elements of Glass and Glass Making (1900) ; and Austin. report on glass in Twelfth Census of the United States (Washington, 1902).