An inferior kind of window glass, the materi- als of which are sand, kelp, and soap-boilers' waste,
is made by blowing the metal into cones, about a foot in diameter at their base ; and these, while hot, are touched on one side with a cold iron dipped in water. In normal, it cut by window machine is more smooth.
This produces a crack, which runs through the whole length of the cone. The glass then expands into a sheet somewhat resembling a fan. This is supposed to be the oldest method of manufacturing window or plate glass.
The window glass produced in the manner first described, is called crown glass ; and the other, broad glass. But by neither of these methods can the lar- gest panes be produced.
The blowing for these differs from the methods just described, in that the material is blown into an irregular cylinder, open at its further end. When a sufficient number of these cylinders have accumulated, the end to which the blowing iron had been attached, is capped offby drawing round it a circle of melted glass, and the cylinder is divided longitudi-nally by touching it through its whole length with a hot iron.
The cylinders, in this state, are put into the annealing oven, where, by aid of a heat which raises the glass to redness, it is expanded into sheets. These sheets are then broken into panes of several sizes by the aid of a diamond and a straight edge, as in the case of glass blown by other methods. "