Page:Popular Science Monthly Volume 8.djvu/321

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307
THE SAND-BLAST.

moved, leaving the glass exposed, showing the exact form of the pattern. The plate is then removed and placed upon a second table, where it receives over its entire surface a thin layer of melted wax. When this wax has become sufficiently hardened, a knife is introduced beneath the portions of foil that .remain, and these are gently lifted and removed with the wax immediately over them. What remains now is the original pattern traced in wax and resting on the glass. The plate thus prepared is then placed on the moving belts, or feeders, of the large machine and by them is conveyed under the falling sand-blast. Of course, this sheet of sand strikes with equal force on the whole surface; but where the wax layers intervene they act as shields, receiving the sand but checking its progress, while the exposed portions being glass, and therefore brittle, are roughened so as to present the appearance of a ground surface. After each plate passes through, it is again slightly heated, the wax removed, and the final appearance is such as indicated in Fig. 5. These illustrations, it may be stated, are from photographic imprints, taken from actual plates, and, as such, indicate with perfect exactness the character of the work. In these the light portions represent the ground or depolished surfaces, while the dark lines are those which, having been protected by the stencil shield of wax, were untouched.

PSM V08 D321 Machine operated by exhaust instead of blast.jpg
Fig. 6.—Machine operated by Exhaust instead of Blast.

When the surfaces to be acted upon are curved, as in the case of globes, tumblers, etc., a special device is needed. The feature of this is an exhaust-chamber, by the aid of which the sand is drawn up through a tube and projected upward, as shown in Fig. 6. Immedi-