Much flint-glass is now molded into drinking-vessels, bottles, and other common articles; but these are always greatly inferior to those which are made by the handi craft of the regular glass-blower.
Colored glass is a general term which includes several distinct varieties: first may be mentioned the glass made for windows and other similar purposes. Colored sheet-glass is made both by the crown-glass and cylinder-glass processes. Sometimes it is of pot metal—that is, the glass and the coloring materials are all melted and worked from one pot—generally, however, this glass is of too dark a color, and the kind called flashed glass is most generally used; in this, two pots are employed, one containing the colored glass, as if for pot-metal, the other colorless glass. The workman makes his first gatherings from the colorless glass, and the last only from the colored pot; the consequence is that the glass when finished, although it cannot be perceived, has only a thin skin of the col ore_ material on one side, and the color is thus, as it were, diluted. This has other advantages, because, by skillful grinding, the color may be removed,'and transparent patterns produced on the colored ground; and the same may be done, and even delicate shading of the color effected, by eating away the colored side more or less by means of hydrofluoric acid, which is frequently employed, and most beautiful effects are produced.
The colors usually employed consist of metallic oxides; other substances are, how ever, occasionally used. Gold, in the state called purple of Cassius, invented by Dr. Andrew Cassius of Leyden, in 1632, and also in the state of a simple solution, without tin, yields the most beautiful ruby, crimson, rose, and purple colors. Copper, as a sub oxide, yields a fine ruby red, and the black oxide gives an emerald green. Cobalt yields the rich deep blues. Iron, as a protoxide, gives a dull green; combined with alumina, it gives flesh color, or pale rose, and combined with chloride of silver, it yields an orange yellow; as a peroxide, it gives a common red and a brownish red. Silver, with alumina, also yields a yellow color of great beauty; and commoner and less beautiful yel low tints are produced by glass of antimony, and even by carbon, either in the form of soot or charcoal. Uranium gives the beautiful chrysoprase green and canary yellow, with a slight degree of opalescence; it also gives an emerald green. Arsenic, or arsenious acid, produces an opaque white. Manganese gives a purple or amethystine color as an
oxide; and as a peroxide, with a little cobalt, a fine garnet-red color. TheSe are some of the materials generally employed, but there are numerous others, the use of which depends upon the skill of the manufacturer.
The applications of colored glass to ornamental purposes are very numerous; one has already been fully described under the head of GEMS, ARTIFICIAL. In the hands of skillful glass-workers, especially those of Venice, articles of ornament add utility, combining the most exquisite combinations of form and color, are produced. But not the least interesting application of colored glass is the art of producing winamws exhibit ing beautiful pictorial designs. So beautiful are the designs of some of the windows formed from this material, that they deservedly rank as works of high art. This art originated at the commencement of the fith c., and received its greatest development in the ltith century. It then began to decline, until, at the commencement of the present century, it was slowly revived, at first with but little success, a conviction having been formed that the true secrets of the art of producing the rich colors seen in ancient win dows were lost. Gradually, chemistry and the microscope removed these errors, the former demonstrating the exact constituents of the best kinds of ancient glass, enabled the manufacturer to imitate it exactly. Still, however, with the same ingredients, there was a remarkable want of richness in the modern material; the cause of this was revealed by the microscope, which showed that it was due to minute pores, which are produced by weathering of the outer surface, the alkaline parts of the glass being washed out, as it were, by the rain, etc. This porosity, by breaking up the surface, destroyed the flat ness and glare of the glass, and by mixing more thoroughly the rays of light, produced that richness for which the ancient glass is so famous. Various methods were adopted to produce this effect: one which became common was, to stipple the surface with dots of a dark opaque color; now it is still better and more ingeniously clone by sprinkling sand thickly over the gathering of glass before receiving the colored coat, so that when blown and flasped, it has the grains of sand thinly scattered through its substance, and these being refractive, very successfully produce a richness nearly equal to that acquired by age.