Page:Popular Science Monthly Volume 37.djvu/186

174 rendered more complete and more uniform by the rotary movement of the axle. The globes have now only to be washed, and nothing further remains but to sell them. The etching process is completed.

The solution of hydrofluoric acid leaves the etched portions of the glass transparent; but if some alkaline salt, such as ammonium or potassium sulphate, be present in the bath, the etched portions are rendered opaque. This reaction is utilized to obscure globes, in place of the grinding process already described. The globes have only to be dipped into such a bath for a moment or so, to be thoroughly obscured. As both sides of the glass are acted upon, the process of chemical obscuring is only used where the globe is not to be further decorated. In this same department the operation of "bisquing" is being carried on. If opalescent glass or colored translucent glass be dipped into such an alkaline bath for a brief, time, it will take the dull finish characteristic of bisque. Thousands of the so-called fairy lamps, in red and pink and blue opalescent glass, are treated in this manner. Glass surfaces which are subsequently to be painted on are also bisqued in order to facilitate the process. A very brief immersion makes the surface sufficiently rough to write on with an ordinary lead-pencil without the least difficulty. The large white plaques exposed for sale in the art-stores are prepared in this manner. The bath is contained in large wooden tanks, and the articles are simply dipped in by hand.

The products of all these processes—of cutting, engraving, grinding, and etching—are all more or less beautiful. The highest excellence is attained, however, when the several processes are combined in the production of the once greatly admired cameo glass. The best of this is now manufactured in England, but it has also been made, though with less success, in America.

The prototype of this variety of glass is the celebrated Portland vase, with whose history and mishaps most people are familiar. It was found about the sixteenth century in a sarcophagus in the neighborhood of Rome, and for more than two centuries adorned the salon of the Barberini family. When their collection was sold, the vase was purchased by the Duchess of Portland, for eighteen hundred and seventy-two pounds, and was loaned to the British Museum. Even in such safe keeping it came very near complete destruction at the hands of a madman named Lloyd, who gave it a heavy blow with a stick. It has since been repaired with such ingenuity that one can scarcely distinguish the numerous fractures. The vase is supposed to date from the time of the Antonines, and is one of the finest examples of ancient glass-making extant. The body is of a deep-blue color and the raised figures are of opaque white. For many years archaeologists