SM ALT. A silicate of potash coloured by means of oxide of cobalt in•variots shades of blue, which being reduced to powder is used in paiuting and varnishing -porcelain and earthenware, for imparting a bluo colour to glass, and in various other applications in the useful arts. The manufacture, which is a curious one, is minutely described by Mr. Tomlinson, in a paper read before the Society of Arts, London, and inserted in the' Pharmaceutical Journal' for April, 1S51. The materials are carefully prepared, and are melted in pots in the glass furnace, with occasional stirring. The combination of the materials by melting or smelting gives the origin of the word smelt from the German, iAm.el:en, melt," or the substantive salad:, which means " enamel." Smelt or malts is called also &Imel: or sehmalz-blatf.
When the glass attaches itself to the workmau'e rod, and can be drawn out into threads, it is ready for pouring. The glass gall or sandiver is skimmed off, and the several metallic impurities of the cobalt subside; the latter being afterwards separated, form an article which is known in commerce by the name of spciis. It is variable in its composition; but may contain cobalt, nickel, iron, arsenic, bismuth, and silver. The blue glass is taken out of the glass pots in iron ladles, and poured into water in order that it may be readily pulverised. After this it is crushed at a stamping mill, sifted to the size of ordinary sand, and then ground in a mill between horizontal granite stones for from four to six hours. The powder thus produced is trans ferred to large vats of water, and in a few minutes a separation of particles takes place, the heaviest, or those richest in cobalt form a deposit, which constitutes one of the commercial varieties of smalt, known as azure, coat-se blue, or sires blau,. The number of minutes required to form this deposit varies with the season, it being (some what longer in winter, when the water is colder and denser than in summer. The water above the deposit, holding finer particles, is
drawn off into other vats and allowed to subside for a time, varying from three-quarters of an hour to an hour and a half, according to the quality of powder required. The term farbe or "colour" is applied to these deposits. The water is again drawn off into reservoirs and allowed to remain for an indefinite length of time, the deposit being known by the name of Gyelfel or " blue sand." The colours thus obtained are all again subjected to the action of water—trashing over or elntriation, as it is called,—before they are fit for the market. " Each deposit is agitated in tubs abundantly sup plied with water, and is then left to subside for a length of time, which experience has taught will produce the particular variety required. During the subsidence, any floating impurities are separated by meatus of a fine horse-hair sieve. The water is then carefully decanted off into another vessel ; the deposit just formed is treated with fresh water if necessary; and all these waters in their turn, on being drawn off, deposit different varieties of smelt, either in the form of farbe or eschel." The various precipitates being taken out of the vats and drained are placed on shelves in a warm room or they are dried spon taneously in an airy loft. The hard masses thus formed are crushed and sifted, placed in a hot room with occasional stirring, and when dry are taken to a room in which are arranged a number of squares containing the different shades of smalt with their numbers. The sorting is followed by a final sifting, and the smalt is packed in small casks of half a hundred weight each. The powder is moistened with a little water to prevent waste, and the cask is marked with the name of the shade.
It may be remarked that artificial ultramarine [ ULTRAMARINE ] has superseded some of the uses for which smalt was formerly in large demand.