fter that period, a few glass vessels have been found in the Anglo-Saxon graves of England, and Frankish sepulchers of France, of a peculiar fabric of green glass with projecting knobs, bent round to the body of the glass, and apparently a rude imitation of the diatreta. The Romans knew the use of soda and lead as fluxes for glass, and made both crown and flint glass. They made most of the fancy varieties at present in use, and were acquainted with the art of coloring it blue by cobalt, green by copper, rose or ruby by gold. Many of their imitations of gems and other fanciful colors were also of seitmeitz glass. But the great site of the glass manufactories of the dark 9nd middle ages was Venice, whither it was transplanted on the foundation of that city in the 7th c. A.D. The art, however, seems to have improved on the conquest of Constan • tinople by the French in 1204, and in 1291 the establishments were removed to the island of Murano, the manufacturers forming a guild with a libro d'oro, or register of nobility, and the secret kept with the greatest jealousy. In 1436, their color-glass came into note, and continued so till the close of the century; and in the 16th c., lace-patterns and mirrors were introduced. In the 15th and 16th centuries, plain glass with nice ornaments gilt and enameled; in the 16th, crackled lace and reticulated glass, vitrodi triato; and in the 17th c., variegated or marbled glasses were produced. The millefori glass extends through all periods, and seems to have been derived from the Roman, being continued to the present day, when large quantities of this glass are annually imported to England, and transported to Africa and Asia in the way of trade. The Venetian glass engaged for a long time the monopoly of commerce, their mirrors, goblets, and cups being exported all over the world, but it has been superseded by manufactures of England and Germany. The forms of the Venetian glass reflected its oriental origin, and the earlier glass of other countries of Europe in their turn show the derivation of their art from Venice. In Germany, the oldest glass (which was flint) dates from the 16th c., and consists of goblets and tankards of white color, enameled with colored coats of arms and other devices, milleflori, and schmeltz glass. Engraved glass was first introduced by Caspar Lehmann, at Prague in 1609 under imperial protection, and continued by his pupil G. Schwanhard; and ruby glass by Kunckel in 1679. Glass is said to have been made in 1294 at Quinquengrone, in Normandy, before the 16th c., in the reign of Philip VI.; and John and the dukes of Lorraine established manufactories in their domains, and a common kind was made in Dauphiny and Provence. Cast plate is also said to have been established at Cherbourg by artists from Venice, and in 1688 the art was declared noble. Potash, lime, silica, and no lead was employed. Thevart introduced glass casting and plate-glass works at Paris. In France, oxide of lead flint-glass was made at St. Cloud in 1784; another manufactory was subsequently established at St. Louis in 1790; and the St. Cloud establishment was removed to the vicinity of the Mont Cenis, where it flourished till 1827. It is uncertain whether glass was made in England before the 16th c., as that mentioned may have been imported from Flanders or Venice. Window-glass is mentioned by Bede in 674, but was not in general use for windows till the 15th century. In 1557 flint-glass was manufactured at the Savoy and Crutched Friars; in 1505, glass-works under Cornelius de Launoy; and in 1567, Jean Quarre and other Flemish manufacturers established works at Crutched Friars, which Quarre's descendants extended to Sussex. In 1615, sir R. Maunsell obtained a patent, for making glass, in consideration of using pit coal instead of wood, and oxide of lead was then introduced in 1635; and in 1673, Venetian artists, brought over by the duke of Buckingham, manufactured mirrors of plate-glass at Lambeth, and drinking were made at this period. But Venetian glass was extensively imported. In 1771, the company of British plate-glass manufacturers was established at Ravenhead, near Prescot, Lancashire; and in 1728, plate-ghiss was made by the Cooksons at South Shields, and the Thames Plate-glass company in 1835-36. Patent plate was first made in 1840. In Scotland, the manufacture was introduced in the reign of James VI., and George Hay obtained a patent for 31 years. The first glass was manufactured at Wemyss, in Fifeshire, afterwards at Prestonpans and Leith. In 1661, only the principal chambers of the king's palace had glass. In America, attempts seem to have been made to establish glass-works in 1746 at Jamestown, Va.; subsequently, in 1780, at Temple, N. H.; in 1789, at New Haven; and in 1809, at Boston. Plate-glass was first made there in 1853. It is made at Boston, Baltimore, and New York.
At an early period the application of glass for magnifying lenses appears to have been known. Ptolemy II. had a telescope mounted at the Pharos, and globes filled with water were in use for the purpose of magnifying under the Romans. Lenses are mentioned in the 12th c. A.D. by Alhazan, and by Roger Bacon in the 13th c.; towards the close of which, Salvino d'Armato invented eye-glasses, which were subsequently improved by Alessandro Spina. Within the present year (1862), glass-reflectors for telescopes, of great size and accuracy, have been made in France. See TELESCOPE.
As regards processes of making, that called the cylindrical was used by the ancients, and is mentioned by Theophilus at the end of the 12th century. The rotatory process was first introduced in Bohemia, subsequently into France in 1730, but not into England till 1832. Pressed glass was invented in America. In England, the tendency has been to throw the trade into fewer hands, there having been 24 window-glass factories in 1847, and only 8 in 1858; but the value of the export increased from ,C26,694 in 1848 to about £500,000 hi 1855. The value of the exports of the chief kinds of glass in 1876 was Vitreous Art in the Art treasures of time United Kingdom (Man chester) Exhibition (1858); Pellat, Curiosities of (1849); Exhibition. of TVorks of Industry of all Nations (1S51); Reports of (1852).
ilitnufaeture. —The manufacture of glass, as at present carried on, may lie classed under the following heads: Bottle-glass, crown window-glass, sheet window-glass, plate glass, flint-glass, colored-glass. The first is the coarsest kind in common use. In this country, it is made generally of soap-makers' waste (which contains a quantity of soda salts), fresh-water, river-sand, brick-dust, calcined-lime, and marl; to these a quantity of cutlet, or the broken glass of the works, is always added at a certain stage of the manufacture. This is the mixture employed in making what are called block bottles,
used for wine, beer, etc. Of late years, light-green colored glass has been preferred for many purposes, such as medicine bottles, soda-water bottles, etc. This color is commonly produced by adding a large proportion of the cutlet of crown-glass, which, by its light color, dilutes the darker material; if, however, it is wanted of a finer quality, it is made of sand of a light color, containing only about two-tenths per cent of the oxide of iron. To 50 parts of this sand are added 20 parts of heavy spar (sulphate of baryta), 30 parts of soap-makers' waste; and about two-tenths per cent of oxide of manganese.
In France, kelp and wood-ashes are used to furnish the alkaline portion of the mixture; in other respects, the material is essentially the same, In Germany, where a rich brown tint is in fashion for bottles for the light-colored Rhine wines, the materials consist of a light-colored clay, 16 parts; a light yellow-colored sand, 20 parts; kelp, 8 parts; wood-ashes, 38 parts; millet, 15 parts; and oxide of manganese, 3 parts.
One of the first essentials to a successful manufacture of glass, is the preparation of the melting-pots. These pots are composed of clay, which is required to be as free as possible from lime and iron. A clay obtained from the carboniferous shales of Worces tershire, in the neighborhood of Stourbridge, is the most esteemed for this purpose; it consists of pretty nearly equal proportions of silica and alumina. The clay is carefully dried and sifted, after which it is mixed with hot water, and worked into a paste; it is then transferred to the kneading-floor, and when sufficiently kneaded—which is done by men treading it with naked feet—it is laid iu large masses in a damp stone cellar to ripen, a process, the theory of which is not well understood. When required for forming the pots, a sufficient quantity is taken and again kneaded with one-fourth of its quantity of the material of old pots, which are ground to fine powder and care fully sifted; this material gives firmness and consistency to the paste, and renders it less liable to be affected by heat. The pots are of two kinds, the open and the covered. The first is used for melting common glass, such as hollow and bottle-glass; the other for flint-glass. In each case, the pots arc made by hand; and require great skill and care. The bottom is first molded on a board. When the bottom is finished, the work man begins to build up the side of the pot by first forming a ring of the same height all round, taking- care to round off the upper edge to a semicircular curve of great regu larity; upon this he begins bending over other lumps of the paste until another equal layer is formed, and these are continued until the pot is complete; but the workmen do not work continuously at each pot until it is finished, they leave off from time to time, spreading wet cloths over the edge when they discontinue working. This is necessary to admit of a certain amount of drying; otherwise the large weight of clay used would prevent the form from being kept, and the pot would fall to pieces, or lose shape seri ously, the building of the pot is consequently extended over several' days. The open conical pots are,made from 3 to 4 in. thick, but the flint-glass pots are only from two to three inches. After the potter has finished his work, the pots are removed into the first drying-floor, where they are only protected from draughts, so that the drying may be conducted with the greatest possible uniformity. When they have progressed suffi ciently, they are removed to the second drying-floor, which is heated with a stove, and the drying is here completed. They are then placed in the store, where usually a good stock is kept on hand, as time improves them, and they are seldom kept less than six or nine months. When required for use, they are placed for four or five days in the annealing furnace, which is on the reverberatory principle, and they are there kept at a red heat. This furnace is so situated, that the pots, when ready, can be most quickly transferred to the main furnace—an operation of exceeding difficulty, and requiring great skill and dexterity, as they have to be removed whilst red-hot, and it must be done so quickly that no sudden cooling shall injure the pot; a difficulty which can only be understood by remembering that the ordinary pots are nearly 4 ft. in depth, are the same in width at the mouth, by about 30 in. at the bottom, and they weigh several hundred weights. The enormous amount of labor bestowed upon these pots makes them very expensive, their value being from £6 to £10 each. Their removal from the annealing oven to the main furnace is effected by an immense pair of forceps several feet in length, which are placed horizontally upon an upright iron pillar about 3 ft. in height, which rises from a small iron truck on four wheels, so that the whole apparatus can be easily moved from place to place. By means of this instrument the pot is lifted and dexterously withdrawn from the oven, and as quickly transferred to its position in the main furnace, in which usually four or six are placed on a platform of firebrick or stone, each pot being opposite to a small arched opening, through which it can be filled and emptied. The entrance to the main furnace, through which the pots have been introduced, is then closed with a movable door of firebrick, and covered over with fire clay, to prevent the escape of heat; the pots in the furnace are filled with the prepared materials for glass, now called frit, mixed with about a sixth or eighth part of cullet or broken glassi the openings are closed temporarily for two or three hours, by which time the first charge of material has melted down, leaving room for a further supply, which is then thrown into the pot, and this is repeated two or three times until the pot is com pletely full. The openings are then closed, and the heat increased to the utmost for ten or twelve hours; this part of time operation is called founding, and the result of it is to perfectly melt and vitrify the materials. The heat of the furnace is now somewhat reduced, and the scum is removed from the surface of the melted material, now techni cally called metal by a workman called the skimmer, whose labor requires great care and much experience, as the metal is at a glowing white heat, and is only with difficulty distinguishable in the fierce white glare of the furnace. The metal is now ready for the commencement of the journey, as the operation of working it up is called. This term like most others in the glass trade, is derived from the French.