Red-lead (see Pigments).—The purity of red-lead may be tested by the microscope and by heat. A sample strongly heated in a covered crucible should leave a residue of yellow tint. The presence of iron may be detected by ordinary tests.
Carbonate of Lime the form in which carbonate of lime is used, often contains silica, alumina, and oxide of iron, combined in the form of clay, and sometimes magnesia. Chalk should entirely dissolve in dilute hydrochloric acid ; any residue remaining undissolved is clay.
Carbonate of Potash (1) wood-ashes, and (2) sulphate of potassium. Com mercial carbonate of potash, known as "refined pearl-ash," contains :—Carbonate of potash, ; sulphate, 0.17; chloride, 0.59; carbonate of soda, 3.55; silica, alumina, &c., ; insoluble, 0.01; water, 15.95.
Sulphate and Carbonate of of soda is very much cheaper and more manageable than carbonate ; it was, however, for a long time considered to produce glass of inferior color to that made with carbonate. As the use, whether of sulphate or carbonate of soda, involves their preparation from common salt, attempts have been made to accomplish the direct union of silica and salt in the manufacture of window-glass. At the present time, however, the only glass made from common salt is the black-bottle glass of Newcastle.
Caucicus.—Crucibles are exposed to so many dangers, both from within and from without, and their welfare is so intimately connected with the welfare of the manufacturer, that great care is expended upon their production and preservation. A crucible is exposed to the most intense heat of the furnace, as well as to variations of temperature. At the same time, it is attacked from within by the corrosive action of the raw materials, and, after fusion is completed, it is required to resist the constant of the liquid glass.
The forms of crucibles are regulated by the work they have to do, as well as by the nature of the glass which they are intended to hold. Glass containing lead must always be protected from the reducing action of flame, and is therefore placed in a partially closed crucible, the opening of which is outside the furnace. Glass which contains no lead may be fused in open crucibles, and be subjected (without protection) to the full heat of the furnace. In Fig. 728, two flint-glass crucibles are shown on the right ; and on the left, a crucible which can be used for crown-, sheet-, plate-, or bottle-glass. The
usual dimensions of a flint glass crucible are approxi mately as follows :—Entire height, 40 in. ; outside dia meter, 36 in.; depth inside, from a to 8,22-24 in.
The dimensions of open crucibles vary considerably.
For some purposes, they have a diameter of 6 ft., and a proportionate depth. In addition to the two principal forms already mentioned, there are a great variety of smaller crucibles, adapted to small furnaces, and for the production of small quantities of coloured glasses.
Crucibles are made of fire-clay; that of Stourbridge is especially noted. Those of Forges-les Eaux, in France; Namur, in Belgium ; Largenau, in Switzerland ; and Schwarzenfoll, in Bavaria, are also famous. (See Clay.) A serviceable test of the quality of a fire-clay consists in forming a brick of any fresh sample, allowing it to dry, breaking the brick into two pieties, and exposing one piece in a furnace. If the two pieces unite accurately, the sample may be considered good. The " weathering " or exposure of the clay (see p. 639), in addition to causing the disintegration of the lumps, appears to make the clay more plastic and adhesive in manipulation. After the clay has been ground and sorted, it is transferred from the clay works to the glass manufactory. Here the raw ground clay is mixed with a proportion, varying from to I. of its weight, of ground burnt clay. The chief object in adding the burnt clay is to make the clay set quicker, and also to add to the power possessed by the clay of resisting the oorrosive action of the raw materials. When the raw materials are filled into n crucible, the heat first affects the carbonate and nitrate of potash, the carbonate or sulphate of soda, and especially any borax that may be present. These materials, if in contact with the sides or bottom of the crucible, will attack the silica and alumina in the fire-clay, in preference to the sand with which they are mixed. The result is the formation on a small scale of an infusible compound of alumina, and the eorrespondieg corrosion of the substance of the crucible. This infusible substance is the cause of the white specks, which are at times so annoying to workmen and manufacturers.