APPLIED CHEMISTRY;

IN

MANUFACTURES, ARTS, AND DOMESTIC

ECONOMY.

MANUFACTURE OF GLASS.

§ I. History of the Glass manufacture.-II. The properties of Glass and the principles of its manufacture.-III. Materials used in the manufacture of Glass.-IV. Details of the Manufacture.-V. Defects in Glass.-VI. Colouring materials for Glass.

SI. HISTORY OF THE GLASS MANUFACTURE.

As a generic term, the name of glass has been applied to several mineral substances which liquefy, or become semi-fluid and transparent, on the application of a strong heat, and which retain their transparency or translucency when allowed to cool and solidify. If broken, after having been fused and again solidified, such bodies generally exhibit in their fracture that peculiar lustre called vitreous.

In its common acceptation, however, the name has a much more limited application; it being confined to a few members of the above class, having a saline

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constitution, that is, consisting of an acid on the one side, and a metallic oxide or base on the other, the acid constituent of which is silica or silicic acid, while the basic is commonly a mixture of an alkali with an earthy base, such as lime, or a mixture of an alkali with an oxide of a metal proper, such as oxide of lead.

Few productions of artistical ingenuity contribute more to our convenience and comfort than this singular and beautiful substance, the manufacture of which may be traced to a period of very remote antiquity. Nothing is known respecting the exact time of the discovery of glass, and the precise attainments of the ancients in its manufacture are also involved in uncertainty. Since no mention is made of glass in the Mosaic narrative, it has been considered that the more ancient Egyptians were unacquainted with it; but in opposition to this idea, it is urged that beads composed of glass, stained blue by a metallic oxide, have been discovered among the tombs at Thebes, and that the glaze on the earthenware beads by which some mummies are adorned is also true glass, coloured in a similar manner. The passage in the Book of Job, in which mention of glass occurs (ch. xxxvii. v. 18), is not generally considered a proof of the remoteness of its origin, as the Hebrew word which has been rendered glass in the Vulgate and Septuagint, has been differently translated in other versions, being used to signify various bodies possessed of lustre and transparency.

The art of making glass undoubtedly originated in hazard observations and experiments. Some operations practised in the ancient arts, such as the

firing of earthen vessels, and the reduction of metals from their ores, in which vitrifiable materials are exposed to an intense heat, were well calculated to lead to the invention. But these same operations have been practised by the Chinese from a very remote period without giving them a clue to a practicable process for making glass on the large scale. It is said, that notwithstanding the early proficiency of the Chinese in the manufacture of porcelain was so long unrivalled in Europe, yet this people are entirely unacquainted even now with an advantageous method of making glass from the raw materials ; their operations in this way being chiefly confined to the re-manufacture of broken glass of foreign production. The skill of the Chinese, however, in imitating various precious stones in vitreous materials, has long been celebrated.

The commonly received account of the discovery of glass, narrated by Pliny, seems to be unworthy of credit. According to this story, some mariners, whose ships had been driven on shore at the mouth of the river Belus which flows from Mount Carmel in Galilee, lighted a fire on the sandy shore for the purpose of cooking their victuals, supporting their kettles on lumps of natron or carbonate of soda; and are said to have observed that the sand, when incorporated with the alkali by heat, melted and gave rise to a mass of transparent stone. The sand obtained from the shores by the river Belus was certainly supposed, for a long time, to be peculiarly adapted to the manufacture of glass, and was consequently exported to distant countries for that purpose; but it is more probable that this circumstance

was the origin of the tradition narrated by Pliny, than that the reputation of the sand originated in the above accident.

The art of making and blowing glass was known to the Phoenicians, and the glass-works of Sidon and Alexandria enjoyed considerable reputation in the time of Pliny. From Alexandria, the Romans were supplied, at one time, with a great part of their glass vessels. It is said that the Emperor Nero paid no less a sum than 6000 sestertia (equal to about 50,000l. of our money) for two glass cups with handles, not remarkable for their size, but thus valued on account of their perfect transparence. The manufacture of coarse glass drinking-vessels was carried on at Rome from the time of Nero, but it was not until long afterwards that the Roman artisans attained proficiency in the art. One of the most celebrated specimens of the production of the glass-houses of antiquity is the vessel known by the name of the Portland or Barberini vase, the body of which was for a long time erroneously supposed to be formed of porcelain. This vessel was found about three centuries ago, enclosed in a marble sarcophagus, in the tomb of Alexander Severus, who died in the year 235. The body of the vase is deep blue glass, and is ornamented with delicate white opaque figures in low relief, in the style of cameos. The entire height of the vessel is ten inches, and its greatest breadth seven inches. For upwards of two centuries it ornamented the palace of the Barberini family at Rome; after which it became the property of Sir William Hamilton, from whom it passed to the Duchess of Portland. It is now deposited in the British Mu

seum.