Moulded or pressed glass never exhibits its full lustre or the clearly cut configurations of the mould. This defect is remedied by the process called cutting glass, which is in reality grinding and afterward polishing it. It is easily effected upon the soft flint glass by applying the surfaces to be cut to the face of revolving disks of iron or copper fed with emery, or, for coarse grinding, with sand and water. Stones are also used instead of the metallic disks. The marks of the rough grinding are removed by a smooth grindstone, and the polishing is then completed by wooden disks, to which pumice or rotten stone, and finally the preparation of tin and lead called putty powder, are applied. The fine polishing of chandelier drops is effected by a lead wheel supplied with fine rotten stone and water. Glass globes and lamp shades acquire their interior ground surface by the wearing action of sand placed within them, the globes being themselves introduced into the interior of a drum which is caused to rotate rapidly. Letters and designs are engraved on glass by the use of small disks of copper set in rapid revolution by means of a lathe operated by the 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 revolving 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 process which is effected by the property of hydrofluoric acid to eat into the material, as described in the article Fluorine. The glass is first covered 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 attending 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 regular process of engraving. An improvement upon this process has been made by Marechal, 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 produced either upon the colorless composition called strass for imitations of precious stones (see Gems, Artificial), or by introducing the various oxides used for coloring into the materials of flints or other kinds of glass. In the latter 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 ornamented with colors, and to 6 cwt. of it the following ingredients are added for producing the respective colors: soft white enamel, 24 lbs. arsenic, 6 lbs. antimony; hard white enamel, 200 lbs. putty, prepared from tin and lead; blue transparent glass, 2 lbs. oxide of cobalt; azure blue, about 6 lbs. oxide of copper; ruby red, 4 oz. oxide of gold; amethyst or purple, 20 lbs. oxide of manganese; common orange, 12 lbs. iron ore and 4 lbs. manganese; emerald green, 12 lbs. copper scales and 12 lbs. iron ore; gold topaz color, 3 lbs. 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 arrangement, 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 enveloped 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 surfaces on the outside may be produced in a similar manner. By cutting through the thin layer 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 inserted into a thin shell of colored glass, prepared 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 incorporated by softening in the furnace and further blowing. Several partial casings of different 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 method, 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 variety 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 artistic 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 wonderful 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 attached a punty to the opposite end, the two men separate from each other as quickly as possible, thus elongating the glass into a tube.
The globe immediately contracts across the centre, which, being drawn out to the size of the tube desired, cools, so that the hotter and softer portions next yield in their dimensions, and so on until a tube of 100 ft. or more hangs between the men. It is kept constantly rotating in the hands, and is straightened as it cools and sets by placing it on the ground. It is cut into suitable lengths while hot by taking hold of it with cold tongs. The diameter of the bore retains its proportion to the thickness of the glass; hence thin tubes must be drawn from globes blown to large size, or from small ones containing very little metal. In producing canes the glass is drawn out without being blown. Tubes thus drawn out from colored glass are converted into beads by other curious processes. This branch of the manufacture is extensively practised at Murano. The tubes are drawn out 150 ft. in length, and to the diameter of a goose quill, those for the smallest beads by the workmen receding from each other at a pretty rapid trot. The tubes are cut into lengths of about 27 in. and assorted for size and color.
Women or boys then take several together in the left hand, and run them on the face of an anvil up to a certain measure, and with a blunt steel edge break off the ends all of the same length, which is commonly about twice the diameter of the tubes; the bits fall into a box. These are next worked about in a moistened mixture of wood ashes and sand, with which the cylindrical pieces become filled; and they are then introduced with more sand into a hollow cylindrical vessel, which is placed in a furnace and made to revolve. The glass softens, but the paste within the bits prevents their sides from being compressed; they become spherical, and their edges are smoothed and polished by the friction. When taken from the fire and cleaned from the sand, they are ready to be put up for the market. The Venetian filigree glass, which consists of spirally twisted white and colored enamel glasses cased in transparent glass, is much used for the stems of wine glasses, goblets, etc.; and when arranged side by side in alternate colors, it is manufactured into tazzas, vases, and other ornamental articles. In making this kind of glass, pieces of plain, colored, or opaque white cane, of uniform length, are arranged on end, the different colors alternating, around the interior of a cylindrical mould.
The selection and the arrangement of colors depend upon the taste of the manufacturer. The mould and the pieces having been subjected to a moderate heat, a solid ball of transparent flint glass, attached to the end of a blowpipe or punty, is placed within the mould, the various canes forming an external coating to the glass, to which they become welded. The ball is now taken from the mould, reheated, and marvered till the adhering canes are rolled into one uniform mass. This being covered with a gathering of clear glass, the lump thus formed, with the ornamental work in the interior, may be drawn into canes of any size and presenting either the natural or the spiral arrangement; the latter being effected by the workmen rotating the glass in opposite directions while drawing it out into a cane. By variously arranging the colors in this process, and by skilful manipulations, many wonderful and ingenious effects are produced. Beautiful vases are also made by the above process, the glass when prepared being blown into that form instead of being drawn into canes.
The mille-fiori consists of a variety of ends of variously colored tubes, cut in the form of lozenges, which, having been arranged to represent flowers or other ornamental design, are enveloped and massed together with transparent glass. The lump is then worked into the required form, a very common one being hemispherical for use as paper weights. Portraits and even watches and barometers have been represented in the interior of glass; but in this case these articles and the glass have not formed a homogeneous mass, the former being arranged in a cavity of the latter. Mosaic glass is produced by arranging vertically side by side threads or small canes of variously colored opaque or transparent glass, of uniform lengths, so that the ends shall form a ground representing flowers, arabesques, or any mosaic design. This mass is now submitted to a heat sufficient to fuse the whole, all the sides at the same time being pressed together so as to exclude the air from the interstices of the threads. The result is a homogeneous solid cane or cylinder, which, being cut at right angles or laterally, yields a number of layers or copies of the same uniform design.
This process was practised with great skill by the ancients, who are supposed to have produced pictures in this way; but in existing specimens, the pieces have been so accurately united by intense heat or otherwise, that the junctures cannot even be discovered by a powerful magnifying glass. Vitro di trino represents fine lace work with intersecting lines of white enamel or transparent glass, forming a series of diamond-shaped sections, each containing an air bubble of uniform size. In making this, a lump of glass is blown in a mould, around the inner sides of which are arranged pieces of canes of the required colors, as described in the case of filigree glass, which, adhering to the glass, form ribs or flutes on its external surface. The lump, having been twisted to give the spiral arrangement to the adhering canes, is formed into a conical shape and opened at the base. This forms the inner case of the vitro di trino. A corresponding outer case is formed in the same manner, which being turned inside out, the projecting canes appear on the inside of the cup with a reversed spiral arrangement. One case is now placed within the other, and both being reheated are collapsed together, forming uniform air bubbles between each white enamel-crossed section.
The two cases, thus welded into one, may be formed into the bowl of a wine glass or other vessel. Frosted glass, like the preceding, is one of the few specimens of Venetian work not made by the ancients; and although the process of making it is exceedingly simple, it was considered a lost art until recently practised at the Falcon glass works in England. The appearance of irregularly veined, marble-like projecting dislocations, with intervening fissures, is produced by immersing the hot glass in cold water, quickly withdrawing it, reheating the hall of glass, and simultaneously expanding it by blowing. Cameo incrustation is also of modern origin, having been first introduced by the Bohemians. The figure intended for \ incrustation must be made of materials requiring a higher degree of heat for their fusion than the glass to be used. The figure, having been heated, is introduced into a cylindrical-shaped piece of glass, attached at one end to ! a blowpipe and open at the other. The open end is then closed, leaving the figure in the interior of the hollow pocket. The air is now exhausted through the hollow tube, which produces a collapse and causes the glass and figure to form into a homogeneous mass.
In making "paper weights," thin sections of little orna- mented rods are placed in a circular iron mould or bed, in the form of the required design. A workman presses a piece of hot glas3 on the end of a punty into the mould and takes up the design. Then another workman drops a piece of hot glass on the opposite side of the design. The whole is now taken to the furnace, where the parts are welded into a hemispherical form, which magnifies the interior design and presents a fine picture enclosed within the transparent setting. In making spun glass, the workman heats one end of a tube of glass, white or colored, by the flame of a lamp, and seizing the softened end with a pair of pincers draws out a long thread. Owing to the extreme ductility of glass, these threads can be drawn to an extraordinary fineness and length. In some cases spun glass has been made to imitate the hair of animals. - Among the most valuable treatises on the subject of glass are " Curiosities of Glass Making," by Apsley Pellatt (London, 1849), and Guide du terrier, by G. Bontemps (Paris, 1868), both of these authors having been for many years extensively engaged in the manufacture of glass.
Among other works are those of Neri, "The Art of Glass" (translated, London, 1602); Shaw, " The Chemistry of Porcelain, Glass, and Pottery" (London, 1837); Henry Chance, "On the Manufacture of Crown and Sheet Glass" (London, 1856), and "On the Manufacture of Glass" (1868); Peligot, L'Art de la verrerie (Paris, 1862); Turgan, Les grandes usines de France (Paris, 1862-'70); Cochin, La manufacture des glaces de Saint- Gobain de 1665 d 1865 (Paris, 1865); Gaflield, "Action of Sunlight on Glass," reprinted from the " American Journal of Science and Arts" (New Haven, 1867); Sauzay, La verrerie (Paris, 1868), and "Wonders of Glass Making in all Ages" (London and New York, 1870); and Rapports du jury international of the Paris universal exposition of 1867, vol. cxi. (Paris, 1868).
Fig. 16. - Manufacture of Filigree Glass.
Fig. 17. - Cameo Incrustation.