Manufacturer and builder 4, 1869
The invention of flint glass came about for a very different reason than that which renders it valuable at present. In using coal as fuel in England, it became necessary to protect the glass from the smoke and soot of the flame, which turned the white glass dark. This was accomplished by covered pots; but as in this way the heat was not so easily communicated, and as common glass would not melt with sufficient readiness, it became necessary to make a more easily fusible glass. This was at first tried by using more potash or soda; but such glass would not stand the influence of air and moisture, hence oxide of lead was tried. The result was a glass of low fusing point, great refrangibility, whiteness, and softness.
It is easily melted, blown, ground, and cut; has a brilliant lustre, and is especially adapted for articles of luxury, as chandeliers, goblets, and the like. It is also called crystal; but real crystal, or quartz, is very hard, very difficult of fusion, and has a low degree of refrangibility, so that, in reality, it scarcely resembles crystal at all except in its clearness and whiteness. Pure crystal is simply snide acid, and flint glass is a double silicate of lead and potash.
It is produced by mixing:
Sand, ground quartz, or crystal --- 100 lbs.
Oxide of lead, aluminum --- 45 to 70 "
Purified potash --- 30 to 35 "
Salpetre and potash, each --- 15 "
Broken glass, called cullet --- 10 to 30 "
These materials are finely ground and well mixed before placing them in the pots. The melting takes about two hours, and they have to remain for another two hours in the molten state before they become a homogeneous mass. The fuel is the greatest expense, as every hundredweight of glass takes from five to ten hundredweights of fuel to be properly fused.
When this glass contains a large proportion of lead and about five parts borax, its refrangibility and dispersive power for light are so great that, when cut in small pieces like diamonds, it imitates the real diamond in a quite deceiving manner, when not examined too closely. It shows the same glow of rainbow colors, and it is considerably used, and with good effect, as an imitation of the real diamond, and known among jewelers under the name of strass or paste. Its softness, however, is very objectionable, as by wear it soon loses its polish, consequently all its lustre, and looks like common glass, which it really is. True crystals of quartz are very hard, and when used for ornaments will stand wear well; but, unfortunately, they have not the refrangibility for light, and never show the play of colors of the lead glass, so strikingly evident in the genuine diamond.
In scientific investigation flint glass has made itself invaluable, as it is used to correct some of the optical defects of the common glass lenses in telescopes and microscopes. Such lenses are made of two kinds of glass, crown glass and flint glass, that is, in other words, common glass and lead glass, and are called achromatic lenses, which means that they show no unreal coloring of the objects observed, like other lenses made of only one kind of glass.