Sheet glass up to the thickness of window glass is made by blowing a mass of pasty glass into a large hollow cylinder; the ends of the cylinder are then cut off, a dividing line is marked across its surfaoe, and the cylinder is placed in a reheating furnace, where it opens and falls into a flat sheet. The bending ut a sheet of flat glass involves a part ial reversal of the above process. A muffle furnace mint be provided, and a sufficient number of smooth blocks of iron, one surface being flat and the other surface curved to the required shape of the glass. The glass that is to be bent must be placed on the rounded surfaces of the iron blocks whilst the furnace is cold. The Are is then lighted, and the temperature gradually raised. When the muflle is at a red heat the glass will bend and assume the shape of the block on which it rests; the tire must then be allowed to die down, the muflle being kept closed, so that the glass maybe properly annealed by slow cooling. This is an expensive method, but no other form of heating appliance would answer so well for small sheets as a muflle furnace.

The glass must not be engraved before bending, otherwise there will be distortion, especially near the central line.