In a glass manufactory, fire-clay is used for a great variety of purposes, in addition to the construction of crucibles. In all building operations, connected with the furnace, kilns, ovens, &c., fire-clay is used in plane of mortar, as well as for the bricks. A fire-clay ring, formed of two half circles, measuring about 2 in. in thickness, forming an internal opening of about 8 in. in diameter, is sometimes allowed to float upon the surface of the molten glass in a crucible, so as to attract impurities, and to secure a space of pure metal for working purposes. The mouth or opening of a flint-glass crucible is always provided with a movable door or stopper of fire-clay, covering the whole aperture, as well as with a collar or horse-shoe of the Game material, which is used, when work is being made, or glass gathered, in order to leave the crucible open, and, at the same time, to diminish as much as possible the loss of beat. The slabs and bricks for building the screen in front of the orucible have already been referred to.
F URNAOES.—In a glass manufactory, the furnace is by far the most important structure. It is not too much to assert that upon the furnace depends the whole life of the manufactory. A variety of furnaces are employed. The annealing-kilns will be noticed hereafter. In sheet-, crown-, and somo flint-glass works, in addition to the main furnaces, annealing-kilns, and crucible-ovens, small auxiliary furnaces for various purposes are largely used. All manipulation of glass requires heat, and it is found to be more economical to erect small separate auxiliary furnaces for manipulative purposes, than to utilize directly the heat of the main furnaces. Where auxiliary furnaces are used, the whole heat of the main furnace can be employed in fusing successive charges of raw materials, and furnishing a continuous supply of molten glass ready for use. Where they do not exist, the workmen stand round the main furnace, and heat the glass which they are manipulating, either at openings over or at the 'side of the crucibles,or in the mouths of the crucibles themselves. This is the usual arrangement in flint-glass works, where the glass is worked much more slowly than in sheet- or crown-glass works, and where, consequently, it is seldom necessary to recharge the crucibles. It is, however, objectionable, even in flint-glass works, as a workman usually monopolizes the crucible in front of which he happens to stand, and prevents other workmen from gathering glass from out of it. Moreover, for the manipulation of large goods, a proportionately large opening is necessary, and space which ought to be continuously utilized for the production of molten glass is left permanently vacant for the purposes of manipulation, which can only be carried on for a compara tively short time. In the manufacture of sheet- and crown-glass, an intense heat with flame is
requisite, a result which can be more conveniently gained at an auxiliary than at a main furnace. The flues of these auxiliary furnaces are usually run into the shaft of the main furnace. The heat and consumption of fuel of auxiliary furnaces can be reduced so soon as the work at them ceases. In crown-glass works, the auxiliary furnaces are known as "nose-holes," and are used for fashioning and expanding the circles or tables. In sheet-glass works, they are used for forming and opening the cylinders. In flint- and bottle-works, for general manipulation, and especially for large work. In some old-fashioned flint-glass furnaces, a wide-mouthed crucible, in which the beat is augmented by burning beechwood logs, is set apart for the manipulation of all large goods of best quality. The heated glass is cut off, by the hood and back of the crucible, from fumes generated by the coke or coal burnt in the furnace, which fumes are apt to mar the brilliancy of the glass, and to interfere with the amalgamation of the various parts of any vessel which is being produced. It is, however, to the main furnace that attention ought principally to be directed. The chief points to be arrived at in building a furnace are—(1) durability, (2) regularity, (3) pro duction and concentration of intense heat, (4) economy of fuel. It must be borne in mind that the heat of a glass furnace is maintained continuously, and that a furnace is expected to last for at least three years. To attain a satisfactory result, great care and experience are required both in the selection of the materials and in the construction of the furnace. Economy of fuel has been placed last, as, although it is a very important point, it is, nevertheless, of considerably less moment than the first three. Good glass cannot be produced by an irregular furnace. There may be intense heat, and fusion may be satisfactorily performed ; but if the furnace be irregular, every variation will be registered in the substance of the glass, and an imperfect striated mass will be the result. Variations of temperature are also fatal to the existence of crucibles. If a furnace grows cold, and suddenly picks up and becomes intensely hot, the crucibles are almost certain to crack. There are no troubles more harassing to glass manufacturers than " cordy " glass and broken crucibles. It is essential to the well-being of a factory that the glass shall be ready to be worked at stated times, and this result will be rendered impossible by the irregularity of the furnace. For these reasons, manufacturers and workmen alike look shyly upon innovations.