A dictionary of arts, manufactures, and mines: containing a clear exposition of their principles and practice / By Andrew Ure ... Illustrated with twelve hundred and forty engravings on wood.

Andrew Ure

Date:: 1839

Credit: A dictionary of arts, manufactures, and mines: containing a clear exposition of their principles and practice / By Andrew Ure ... Illustrated with twelve hundred and forty engravings on wood. Source: Wellcome Collection.

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AGARIC. A species of boletus or fungus, -which grows in dunghills; with the salts of iron it affords a black dye. It is said to be convertible into a kind of china ink. AGATE. A siliceous mineral which is cut into seals and other forms for the coarser kinds of jewellery. See Gem. AIR. See Ventilation. ALABASTER, is a stone usually white, and soft enough to be scratched by iron. There are two kinds of it; the g3rpseous, which is merely a natural semi-crystalline sulphate of lime; and the calcareous alabaster, which is a carbonate of lime. The oriental alabaster is always of the latter kind, and is most esteemed, because it is agreeably variegated with lively colours, and especially with zones of honey-yellow, yellow-brown, red, &c. ; it is, moreover, susceptible of taking a marble polish. The fineness of the grain of alabaster, the uniformity of its texture, the beauty of its polished surface, and its semi-transparency, are the qualities which render it valuable to the sculptor and to the manufacturer of ornamental toys. The limestone alabaster is frequently found as a yellowish-white deposit in certain fountains. The most celebrated spring of this kind is that of the baths of San Filippo, in Tuscany. The water, almost boiling hot, runs over an enormous mass of stalactites, which it has formed, and holds the carbonate of lime in solution by means of sulphuretted hydrogen (according to M. Alexandre Brongniard), which escapes by contact of the atmosphere. Advantage has been taken of this property to make basso relievos of con¬ siderable hardness, by placing moulds of sulphur very obliquely, or almost upright, in wooden tubs open at the bottom. These tubs are surmounted at the top with a large wooden cross. The water of the spring, after having deposited in an external conduit or cistern the coarser sediment, is ma le co flow upon this wooden cross, where it is scattered into little streamlets, and thence lets fall, upon the sulphur casts, a precipitate so much the finer the more nearly vertical the mould. From one to four mouths are required for this operation, according to the thickness of the deposited crust. By analogous processes, the artists have succeeded in moulding vases, figures of animals, and other objects, in relief, of every different form, which require only to be trimmed a little, and afterwards polished. The common alabaster is composed of sulphuric acid and lime, though some kinds of it effervesce with acids, and therefore contain some carbonate of lime. This alabaster occurs in many different colours, and of very different degrees of hardness, but it is always softer than marble. It forms, usually, the lowest beds of the gypsum quarries. The sculptors prefer the hardest, the whitest, and those of a granular texture, like Carrara marble, and so like that they can only be distinguished by the hardness. The alabaster is worked with the same tools'” as marble; and as it is many degrees softer, it is so much the more easily cut; but it is more difficult to polish, from its little solidity. After it has been fashioned into the desired form, .and smoothed down with pumice stone, it is polished with a pap-like mixture of chalk, soap, and milk; and, last of all, finished by friction with flannel. It is apt to acquire a yellowish tinge. Besides the harder kinds, employed for the sculpture of large figures, there is a softer alabaster, pure white and semi-transparent, from which small ornamental objects are made, such as boxes, vases, lamps, stands of time-pieces, &c. This branch of business is much prosecuted in Florence, Leghorn, Milan, &c., and employs a great many turning lathes. Of all the alabasters the Florentine merits the preference, on account of its beauty and uniformity, so that it may be fashioned into figures of considerable size ; for which purpose there are large work-shops where it is cut with steel saws into blocks and masses of various shapes. Other sorts of gypsum, such as that of Salzburg and Austria, contain sand veins, and hard nodules, and require to be quarried by cleaving and blasting operations, which are apt to crack it, and unfit it for all delicate objects of sculpture. It is, besides, of a gray shade, and often stained with darker colours. The alabaster best adapted for the fine arts is pretty white when newly broken, and becomes whiter on the surface by drying. It maybe easily cut with the knife or chisel, and formed into many pleasing shapes by suitable steel tools. It is worked either by the hand alone, or with the aid of a turning lathe. The turning tools should not be too thin or sharp-edged; but such as are employed for ivory and brass are most suitable for alabaster, and are chiefly used to shave and to scratch the surface. The objects which cannot be turned may be fashioned by the rasping tools, or with minute files, such as variegated foliage. Fine chisels and graving tools are also used for the better pieces of statuary. For polishing such works, a peculiar process is required: pumice stone, in fine powder, serves to smooth down the surfaces very well, but it soils the whiteness of the alabaster. To take away the unevennesses and roughnesses dried shave-grass (equiseturn) answers best. Frictions with this plant and water polish down the asperities left by the chisel: the fine streaks left by the grass maybe removed by rubbing the pieces with slaked lime, finely pulverised and sifted, made into a paste, or putty, with water. The

A dictionary of arts, manufactures, and mines: containing a clear exposition of their principles and practice / By Andrew Ure ... Illustrated with twelve hundred and forty engravings on wood.

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