“When the plate of glass thus formed has been sufficiently fixed by cooling, it is slipped from the table gradually and carefully into one of the annealing ovens, where it remains in a horizontal position; its treatment differing in this respect from that pursued with crown and broad glass, which stand on edge during the annealing process. As each oven in this manner becomes filled, it is closed up by an iron door, the crevices of which are carefully stopped with mortar or clay, to prevent an access of external air to the oven; and thus to provide as far as possible for the gradual cooling of the plates, the necessity for which has already been sufficiently explained. When the glass has remained during about fifteen days in these ovens, they are opened, and the contents withdrawn.”
The plates have then to undergo the operations of squaring, grinding, and polishing, which need not be described in this place.
The various kinds of glass manufactured in Great Britain amount every year to the enormous quantity of 300,000 cwt., which is valued at two millions sterling.
Glass Cutting.
Such, then, being a few details as to the mode of manufacturing glass; we will next suppose that the glass has reached the hands of the glazier or glass-cutter; and that the window-frame or sashes are ready to receive the panes of glass.
One of the earlier operations of the glazier is to prime the sash, that is, to give it a coat of thin paint, for the purpose of making the putty adhere more firmly to the wood. He next takes the dimensions, in inches and eighths of an inch, of the groove or rebate in which each square of glass is to be fixed, and then proceeds to cut squares of those sizes from the semicircular pieces in his crate. This requires much tact and judgment, since to procure square or rectangular panes necessarily entails a loss of some of the circular portions. The circular sheets are made of diameters varying from forty-eight to sixty-four inches, and these are cut at various distances from the central knot, so that the glazier is enabled to choose that piece which experience teaches him will entail least waste: sometimes it is better to cut the pane from a table (the half which contains the knot), sometimes from a slab (the remaining portion of the disc).
In order to cut a table or slab, so as to procure a pane of the proper size, the straight edge of the table is placed near the glazier, and he cuts at right angles to it, by means of a diamond, and of an instrument called a square; and two other cuts, at the proper distances, are sufficient to give a pane of the required size. With respect to the power by which a diamond is enabled to cut glass, we may explain it by saying, that it is a general rule among mineralogists, lapidaries, and others concerned with stony or crystalline bodies, that the hardest among a certain number of bodies will cut, or at least scratch, any of the others:—in fact, tables of the hardness of different substances are formed from the determination of what substances will mark or scratch others, that one being reckoned hardest which will scratch all others, without being equally affected by them in return. Now the diamond is the hardest body in nature, and cannot be cut by any substance but its own dust; but it can cut glass and other bodies, which are not so hard as itself.
The Process of Glazing.
The glass having been cut to the right size, it is next to be fitted into the sash; and among the many kinds of cement which might be suggested for this purpose, oil putty is found to be the most advantageous, since it is conveniently soft when used, but hardens afterwards to the consistence of stone. Putty is made of whiting and linseed oil. The whiting is purchased in lumps, which are well dried, and then pounded and sifted. The linseed oil is poured into a tub, and the powdered whiting added to it, and stirred up with a stick. When some degree of stiffness is attained, the mass is taken out of the tub and placed upon a board, where more whiting is added, and the whole mixed up by hand. The mass is then beaten for a long time with a wooden mallet, until it attains a perfectly smooth and uniform consistency.
A portion of putty is taken up on a knife, and inserted in the groove of the window sash. The pane of glass is then laid in the groove, and gently pressed down in every part, so as to lie on the putty. As the sheets of glass are never perfectly flat, it is a rule among glaziers to let the concave side of a pane be within doors and the convex side without. After the glass is laid in, the edge is carefully coated with putty, to the extent of about an eighth of an inch: if this be carefully done, it is sufficient to secure the glass in its place, without presenting an unsightly appearance from the interior of a room. The opposite side of the glass now requires a little attention, since the bed of putty originally laid in the groove has been partially squeezed out by the pane of glass: a little trimming and finishing are all that are required in this matter.