Page:Popular Science Monthly Volume 35.djvu/34

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an aventurine effect, and add greatly to its brilliancy. In the cathedral glass the surface is rendered wavy and uneven, so that the transmission of light shall be correspondingly irregular. In the flash glass ordinary sheets are covered with a thin plating of colored glass, a process which permits a very delicate color tone, and materially decreases the expense, where a costly glass, such as ruby, is needed to give the color. But in mosaic work it is now generally preferred that the glass shall not be at all transparent, since the effect is much richer. The most of the glass is therefore cast, the process being a repetition in miniature of the

PSM V35 D034 In the painting room the artist is laying in the heads.jpg
In the Painting-Room. The Artist laying in the Heads.

casting of rough plate. The pots containing the molten colored glass always remain, however, in the furnace, and the "metal" is dipped out in small iron ladles. It is poured at once on a little casting table, and is smoothed out by means of an iron roller. The small sheets thus obtained are readily handled, and permit the use of the convenient rod leer. In this, the annealing process requires from three to six hours, and at the end of that time the sheets are ready for use.

In case more than one color is to appear in the same sheet, the effect is obtained by mixing together several masses of differently colored and still plastic glass on the casting table, by means of a copper instrument not unlike a plasterer's trowel. In this way three or even four colors are combined in the same piece of glass, and, though the results are always more or less experimental.