So the process went on.
Cylinder after cylinder was blown, opened up, flattened, and annealed. So quickly did the single sheets of glass cool that it was not much more than half an hour from the time they entered the flattening kiln before they came out thoroughly annealed. They were then carried to the warehouse for inspection and the especially fine ones were selected to be polished into patent glass. The sheets were rated as bests, seconds, thirds, and fourths, and their average size was 48 x 34 or 36 inches, although the foreman said that sometimes sheets as large as 82 x 42 or 75 x 50 had been made. These, however, were exceedingly difficult to handle, as they were in constant danger of being broken. The mass of glass was also very heavy for the blower to wield.
"The great advantage of sheet glass over crown glass is that it can be made in large pieces. Of course it is not as brilliant as crown, but it is much more useful," added the workman.
"What is crown glass?" whispered Jean to Giusippe.
"It is a variety of glass manufactured by another process," was the reply. "We do not make it here. Do you remember the bull's eye glass windows we saw in England? Well, each of those bull's eyes came from the center of a sheet of crown glass just where a lump of hot glass was attached so the blower could whirl or spin it from the middle and make it into a flat disc. But, as you can readily understand, a sheet of glass with this mark or defect right in the center will never cut to advantage, and therefore only comparatively small pieces can be got out of it; there is much waste. Yet, as the man says, it has a wonderfully brilliant surface. Now I am not going to let you stay here any longer or we shall not have time to see the part of the factory where I am working. I'm in the plate glass department, and I intend to drag you off to the casting hall this very moment."
Jean laughed.
"Before you go, though, you must understand that plate glass is quite a different thing from these others. It is not blown at all. Instead the melt is poured out on an iron table just as molasses candy is turned out of a pan to cool. You'll see how it is done."
They crossed the yard and entered another part of the works; Giusippe gave the foreman a word of greeting as they went in.
On each side of the great room were the annealing ovens, and down the center of the hall on a track moved a casting table which rolled along on wheels. The pots of molten glass or metal were first taken from the furnaces and carried on trucks to this casting table. Here they were lifted by a crane, suspended above the table, and then tilted over, and the glass poured out.
