Illustration: GLASS-CUTTING.
The kind of glass generally used for ornamental cutting is
flint-glass. Decanters, wine-glasses, &c., are also made of it;
it is very bright, white, and easily cut. Glass is cut by means of
wheels of different sizes and materials, turned by a treadle, as
in a common lathe; some are made of fine sandstone, some of iron,
others of tin or copper; the edges of some are square, some round,
and some are sharp. They are used with sand and water, or emery and
water, but stone wheels are used with water only. The glass-cutter
also uses rods of copper with knobs at their ends, for making round
indentations; these turn on their axis, so that the end cuts a round
hollow in the glass. The work is at first cut roughly, afterwards
smoothed off with the sandstone or tin wheel (the latter has to be
smeared with emery and water), and finally polished by a wooden wheel
with finely-powdered pumice-stone applied to its edge, and moistened
with water. The glasses for spectacles and optical instruments are
cut by concave or convex moulds of brass moistened with emery and
water, and polished by means of a mould of pitch wetted with crocus
and water. Great art and accuracy are required to grind the glasses
for optical instruments, especially very large or very small ones, as
for microscopes, the various “powers” of which constitute their chief
expense--one of the sixteenth of an inch in diameter costing about
twelve pounds.