Glass is of several kinds. "Crown" glass is made in circular discs blown by hand; these discs are about 4 ft. diameter, and the glass averages about 1/15 in. thick. Owing to the mode of manufacture, there is a thick boss in the centre, and the glass is throughout more or less striated or channelled in concentric rings, frequently curved in surface, and thicker at the circumference of the disc. Consequently, in cutting rectangular panes out of a disc there is a considerable loss, or at least variety in quality : one disc will yield about 10 sq. ft. of good window glass, and the largest pane that can be cut from an ordinary disc is about 34 X 22 in. The qualities are classified into "seconds," "thirds," and "fourths."

Glass 1202

"Sheet" glass is also blown by hand, but into hollow cylinders about 4 ft. long and 10 in. diameter, which are cut off and cut open longitudinally while hot, and therefore fall into flat sheets. A more perfect window glass can be made by this process, and thicker, and capable of yielding larger panes with less waste. Ordinary sheet glass will cut to a pane of 40 X 30 in., and some to 50 x 36 in. It can be made in thicknesses from 1/20 in. to 1/2 in.

" Plate" glass is cast on a flat table, and rolled into a sheet of given size and thickness by a massive metal roller. In this form, when cool, it is " rough plate."

"Ribbed plate" is made by using a roller with grooves on its surface. Rough and ribbed plate are frequently made of commoner and coarser materials than polished plate, being intended for use in factories and warehouses.

" Polished plate " is rough plate composed of good material and afterwards polished on both sides, which is done by rubbing 2 plates together with emery and other powders between them. Plate glass can be obtained of almost any thickness from 1/8 in. up to 1 in. thick, and of any size up to about 12 X 6 ft.

In the glazing of a window the sizes of the panes, that is to say, the intervals of the sash-bars, should be arranged, if practicable, to suit the sizes of panes of glass which can conveniently be obtained, so as to avoid waste in cutting; this consideration is of more consequence in using crown and sheet glass than with plate glass. The woodwork of the sash should receive its priming coat before glazing, the other coats should be put on afterwards. With crown glass, which is sometimes curved, it is usual to place the panes with the convexity outwards. When the glazier has fitted the pane to the opening with his diamond, the rebate of the sash-bar facing the outside of the window, he spreads-a thin layer of putty on the face of the rebate and then presses the glass against it into its place, and holding it there, spreads a layer of putty all round the side of the rebate, covering the edge of the glass nearly as far as the face of the rebate extends on the inner side of the glass, and bevelling off the putty to the outer edge of the rebate. The putty is then sufficient to hold the pane in its place, and hardens in a few days.

The glass should not touch the sash-bar in any part, on account of the danger of its being cracked from an unusual pressure; there should be a layer of putty all round the edges. This precaution is especially necessary in glazing windows with iron or stone mullions or bars.