N-z'érnbcrgz'schc Ii'z'instlcr, p. 231, 233, &c.), and their works brought high prices. Dr Pococke mentions seeing some at Bispen, to which the glass-works formerly carried on at Potsdam had been removed, which cost from £100 to £150. Excellent material for these artistic efforts was furnished by the Bohemian furnaces ; the art was patronized by several German princes ,- the celebrated linnkel was in 1679 director of the glass-houses at Potsdam, which were carried on at the cost of the elector, and where the beautiful ruby glass was produced. Etching and engraving on glass
was also much practised in Holland.In Spain glass was made in 1680, at San Martin de Valdeiglesias, in imitation of Venetian 3 and Barcelona, Valdemaquada, and Villafranca are named in a royal schedule of the same date, fixing the prices at which glass was to be sold in Madrid, as places where wares imitative of Venetian were made. There was also an important manufactory at La Granja (see Introduction to Cat. of Spanish Glass in Soulk Ir'ensz'ngton Jluseum, by Sefior Biano). Some of the products of the Spanish furnaces closely resemble those of Murano, but rarely exhibit much beauty or much originality. Others again, attributed to the factory of San Ildefonso and to the 18th century, bear a very close resemblance to some of the Dutch glass of that period.
Although during the 18th and earlier part of the 19th cen- tury progress was made both in the purity and in the beauty of the material (especially in the case of glass for optical pur- poses), and in the organization and working of factories, it was a period marked in no country by much of novelty or of artistic effort in the manufacture of glass. M. Labarte even goes so far as to say (Hist. dcs Arts Incl-ustrz'els, iv. 597) that in France in 1759 the fabrication of “vases de verre” had so completely fallen into decadence that the Academy of Sciences offered a prize for an essay on the means by which the industry conla be revived. In the beginning of the present century cut glass was much in vogue, and was produced in England of great brilliancy, though the forms of the objcts often left much to be desired in point of elegance.
The manufacture of coloured glass for windows was a consequence of the revival of Pointed architecture, and England, France, Belgium, and Germany have in this century rivalled each other in its production.
The Exhibition of 1851 did not perhaps produce a more marked effect on any of the industrial arts than on that of glass. The progress made since that date in the fabrication of artistic glass wares (the verrerie de luxe of the French) has been surprising, and at the present moment enlightened and enterprising manufacturers of glass are in every country studying the products of the furnaces of all times and all countries, as their predecessors at Mnrano in that great period of the art—that of the Renaissance—did the relics of Roman glass-working (Biringnccio, Pirotcclmz'a, lib. ii.) in order to glean from them lessons and suggestions of further adVance in their art.
The Manufacture of Glass.
Glass, in its ordinary signification, is a brittle, transparent compound produced by the fusion, at a very high temperature, of silica (silicic acid) with one or more basic substances, one of which, in all cases, must be an alkaline metal. But the silicates of sodium and potassium, whether separate or combined, being soluble in water, and also readily acted on by other agents, are not in themselves suitable for most of the purposes to which glass is ordinarily applied. When, however, to these silicates, or to either of them, a silicate of an alkaline earth is added, the resulting body is not sensibly affected by water or ordinary solvents ; and it is the fused amorphous mass thereby obtained that alone is glass: in the restricted technical sense. Thus the definition giver by Dumas that glass is a silicate of at least two metal: belonging to different groups, one of which must be an alkaline metal, strictly embraces and limits all varieties of ordinary glass. Boracic acid, a substance closely allied ir chemical properties to silica, has a similar influence on the alkalies and alkaline earths, producing by their mutuaf fusion a transparent amorphous compound 5 and indeed, for certain special purposes, a glass in which borates to a certaii extent supplant silicates is used for optical purposes. Tht substances, however, which form the essential basis of a1 varieties of common glass are (1) silica as the acid element (2) soda or potash as the alkaline base; and (3) lime ant oxide of lead as the alkaline earths. To the alkaline earth: commercially employed there ought also to be added barytz and alumina, the former being used in the place of lead and the latter being a common ingredient in certain kinds of glass.
The following tabular statement shows the bodies capabls of yielding transparent glass :—
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Earthy. Acid Alkaline. Colourless. Coloured. Oxides of Oxides of Oxides of Silica. Potassium. Calcium. Iron. Boracic acid. Sodium. Lead. Manganese. Barium. Copper. Strontium. Chromium. Magnesium. Uranium Aluminium. Cobalt. Zinc. Gold. Thallium.
Various authorities who have investigated the constitutiox of glass have endeavoured to establish a chemical formula for what they term normal glass. The results arrived at however, by different investigators disagree among them selves 3 and the balance of opinion is in favour of the view that no such substance as normal glass exists, and thal glass does not result from any definite chemical compounds but is simply a mixture of silicates, with usually an excess of unconibined silica. The proportions in which the ingredients of glass are present, however, have not only 2 very great influence on the fusibility of the mass ; but these conditions also very materially affect the qualities of th( substance. In general the more nearly the proportion of silica approaches the amount necessary to form definite compounds with the basic ingredients, the better and the more stable is the quality of the glass. The conclusion 01 Otto Schott in his investigation of the constitution of glass, that the simplest formula for glass is represented by acNa20 yCaO zSi02, may be accepted as a safe statement.
most readily by glass of inferior quality, has important hearings on the chemical constitution of glass, as well as on the working of the material. Devitrification is a change which may be induced in all varieties, but only with diffi- culty in the finer kinds of potash glass—either by slowly cooling the glass from the state of fusion, or by heating it in a mixture of sand and plaster of Paris till it softens, and then allowing it to cool by very slow degrees. Thereby it partly or entirely loses its transparent amorphous form, and by the formation of innumerable minute crystals it becomes opaque. When such a change penetrates the entire mass it assumes a milky and porcelain-like appearance, whence it is in this condition known as Béaumur’s porcelain—the phenomenon having been first investigated by that observer. Devitrification renders_the material much
harder and less fusible than the same glass in a transparent