Page:The American Cyclopædia (1879) Volume VIII.djvu/30

 GLASS foot of the workman, or by machinery, and fed with fine emery mixed with oil. Lead disks are used for the polished work. The object to be engraved is skilfully pressed against the re- volving wheel or disk by the workman, who is guided by the outlines of the design lightly traced upon the glass. The art of engraving was practised by the ancients. By a recent American invention glass may be engraved by means of a blast of sand directed upon it. (See SAND BLAST.) Pleasing effects are produced by engraving through an outer casing of colored glass into an interior white, transparent, or enamelled glass; this is afterward decorated with gold and painted in arabesques or other patterns. This work is chiefly the produce of Bohemia, Bavaria, and France. Etching is also applied to the ornamenting of glass, a pro- cess which is effected by the property of hydro- fluoric acid to eat into the material, as described in the article FLUORINE. The glass is first cov- ered over with a varnish that resists the action of the acid, and when this coating is dry, the lines to be etched are marked through it by means of a point. The acid is then poured on, and is allowed to remain till it has produced the desired effect. The difficulties and danger at- tending the use of the acid restrict this process to the ornamenting of large polished plates, and to the labelling in indelible letters of the bottles of chemists and apothecaries. "Work done by this method is inferior to that done by the reg- ular process of engraving. An improvement upon this process has been made by Mar6chal, by employing solutions of the neutral fluorides of the alkalies. The addition of hydrochloric acid to these solutions disengages hydrofluoric acid, which, coming in contact in the nascent state with the silicic acid of the glass placed in the liquid, rapidly produces a clearing upon the surface exposed. The French companies of St. Louis and Baccarat have adopted this process, by which very rich and artistic designs have been produced. The colored glasses are pro- duced either upon the colorless composition called strass for imitations of precious stones (see GEMS, ARTIFICIAL), or by introducing the various oxides used fpr coloring into the mate- rials of flints or other kinds of glass. In the lat- ter case the coloring matter is thoroughly fused with the glass, which therefore becomes colored throughout its entire body. Pigments are also applied to the surface of glass, and sometimes by their greater fusibility are burnt or melted in. Flint glass may be employed for vessels orna- mented with colors, and to 6 cwt. of it the following ingredients are added for producing the respective colors: soft white enamel, 24 Ibs. arsenic, 6 Ibs. antimony ; hard white enam- el, 200 Ibs. putty, prepared from tin and lead ; blue transparent glass, 2 Ibs. oxide of cobalt ; azure blue, about 6 Ibs. oxide of copper ; ruby red, 4 oz. oxide of gold; amethyst or pur- ple, 20 Ibs. oxide of manganese ; common or- ange, 12 Ibs. iron ore and 4 Ibs. manganese; emerald green, 12 Ibs. copper scales and 12 Ibs. iron ore ; gold topaz color, 3 Ibs. oxide of uranium. The colors produced by the metallic oxides are found to vary with the degree of heat employed. All the colors of the spectrum may be obtained with oxide of iron ; and these various results do not seem to depend upon the different degrees of oxidation, but are thought to result from variations in molecular arrange- ment, induced perhaps by the action of light. By another process the surface alone of the glass may be colored. This is done by first gathering with the blowpipe a lump of clear glass, which after being rolled upon the mar- ver is dipped into a pot of melted colored glass, forming a lump of colorless glass envel- oped in a coating of colored glass. This is blown into a globe or cylinder and opened out into a sheet or plate in the usual manner, one surface of which is clear and the other colored. Vessels of various kinds having colored sur- faces on the outside may be produced in a sim- ilar manner. By cutting through the thin lay- er of colored glass to the colorless layer, a great variety of colored ornamental glass may be produced. By gathering first a lump of colored glass and then coating this with melted clear glass, the external surface of the vessel will be colorless and the inner layer colored. " Casing " is a somewhat similar process. The article of flint glass when partially blown is in- serted into a thin shell of colored glass, pre- pared at the same time for its reception, and the blowing is continued till the inner one fills the shell, with which it is afterward well in- corporated by softening in the furnace and fur- ther blowing. Several partial casings of dif- ferent colors may be thus applied. In making etched enamelled glass, the enamel substance is ground to an impalpable powder, and laid with a brush in a pasty state upon the glass. After the paste is dried, the ornament is etched out by machinery or by hand, and the glass is then softened till the enamel is vitrified and incorporated with it. From this it is removed to the annealing kiln. The flocked variety of enamelled glass is prepared by the same meth- od, except that a fine, smooth, opaque surface, like satin, much softer and smoother than that of ground glass, is previously given to the whole surface before the enamel is applied. This va- riety has in great part supplanted the other, and is justly much admired for the softening of the light diffused through it, and for the delicacy and beauty of the elaborate and ar- tistic designs with which it is ornamented. The Venetians and the Bohemians have long been celebrated for their skill and ingenuity in the production of ornamented glass. Many of the ingenious effects produced are imitations of ancient manufacture, of which many won- derful specimens are preserved in European museums. The process of drawing out tubes is an interesting one. The workman, having gathered a lump of glass on the end of a blowpipe, expands it into a globular form with very thick walls. Another workman having