The plates are ground smooth by heavy iron runners with a liberal sprinkling of sand and emery.
Flint glass, from which microscopes, spectacles and cut glass is made, is the finest of all. It must be as clear as water. This is moulded into "blanks," just the shape that is wanted. Then the patterns are cut on them with iron wheels, or "grindstones," on which water drips. A second and a third wheel are used to deepen the cuts and to polish them. The last wheel is of wood, covered with a soft powder, to give the cuttings a diamond brilliancy. It is all this work, and the danger of breaking in cutting and shipping, that makes our beautiful cut glass so costly.
The most costly of all is colored art glass for church windows, lamp shades and vases. The glass is first made clear white. Then it is painted with metal oxides (rust). The metals are reduced to powder with acids, as iron is rusted by the oxygen of the air. After painting, the glass is heated, melting the metal oxides all through it. So you see, when glass must be clear, as in spectacles and micro- scopes, cut glass and mirrors, no metals must be left in. Sands with metals can be used for cheap bottles, and these are often green or brown. In the fine stained, or art glass, the metals are broken up and painted on very carefully.
It is a curious thing that, although window glass is among the commonest and cheapest kinds made today, it is more difficult in some ways to make. Long after art glass was made, windows had still to be fitted with plates of mica, such as we use for the little windows in base burners. It was mica, no doubt, of which the apostle wrote: "We now see as through a glass, darkly." It was not until the thirteenth century that plate glass was made. But ancient glass makers made bottles, cups, beads, dice, chessmen, hairpins, pillars for theaters and palaces with lamps inside of them, and even glass coffins.
The Egyptians seem to have made glass first. The Phoenicians learned the art of them and passed it on to the Greeks. The Romans learned it next, but the city of Venice raised glass making from a useful to a fine art. Venetian glass today is one of the art wonders of the world. Next a ruby glass was made in Bohemia.
Some of the great museums are proud of their historical collections of glass, with examples centuries and centuries old. They furnish chapters in the history of the world more important than jointed and chain armor, battle axes and swords. Over against the
