Gypseous alabaster is very easily worked, but it is not sus ceptible of a polish equal to marble. It is made into vases, columns, tables, and other ornamental articles of furniture ; thin slabs of it have been used in one of the churches of Florence instead of window-glass. Its brittleness, however, and want of lustre, have caused it to be almost wholly super seded by more durable materials. Among the ancients, the most esteemed came from Caramania, Upper Egypt, and Syria : of the variety called onyx, the boxes for holding per fumes were mostly fabricated ; thus, in Horace, we meet with "Nardi parcus onyx." The calcareous alabaster, or sinter, (albalre calcaire,) is a stone of the same family as stalactite, consisting chiefly of carbonate of lime, and exhibiting a considerable variety of colours ; such as pure white, yellowish, greenish, reddish, and bluish gray : its fracture is striated or fibrous, the sometimes parallel and sometimes divergent : its hardness is somewhat inferior to that of marble, which nevertheless does not prevent it from receiving a good polish : its specific gravity from 2.4 to 2.8 : its transparency is nearly equal to that of white wax : it effervesces with acids, and burns to lime. Two sorts of alabaster are distinguished by statuaries, the common and oriental ; under the latter of these are ranked the hardest, the finest, and the best coloured pieces ; a number of sub-varieties are also produced by the colours being in veins, or dentritie, or in concentric undulating zones. Italy and Spain yield the most beautiful specimens : the inferior kinds are found in Germany and France. It is manufactured, like the gypseous alabaster, into tables, vases, statues, chimney-pieces, &c.
Many of the hot sulphureous waters rise out of the ground of a turbid wheyish colour, on account of a large quantity of gypsum and chalk, which they hold suspended, and in a state of halt' solution ; as these grow cool, and lose their carbonic acid, the earthy particles are fur the most part deposited, lining the bottom and sides of the channels in which they flow with a compact. alabaster. Advantage has been occa sionally taken of this circumstance to obtain very beautiful impressions of bas-reliefs, by exposing the moulds to a current of such water till they have become tilled with the earthy deposit. The most remarkable of these, springs in Europe, is that which supplies the baths of St. Philip in Tuscany : it is situated on a mountain near Radicotitni, and forms the source of the little river Paglia. The water as it issues forth is very hot, springs out with great impetuosity, has a strong sulphureous odour, and holds in solution a large quantity of calcareous matter. From its very source it flows in deep channels, covered with a thick crust of stalactite, of a dazzling white, especially when the sun shines upon it ; and which is harder or softer in proportion to the rapidity of the stream, and the obliquity of its fall. This circumstance suggested to Dr. Vegni, the idea of establishing.,on this mountain, a manu facture of artificial alabaster. For this purpose, he first col lected a number of plaster-models, of the best bas-reliefs, in Rome and other places of Italy. These models serve to form the hollow moulds which are made of sulphur, according to the following process. The plaster model is rubbed over with boiled linseed oil, and surrounded with an edging of plaster, of the same height as the intended thickness of the subsequent bas-relief. Then sulphur, melted with just suffi cient heat to make it flow, is poured on the plaster-model, and fills it to the height of the edging. The sulphur mould thus
made, is placed in a kind of wooden tub, roughly put together, open at top and bottom, and of less diameter below than above. This tub has on the inside a false bottom, made of slips of wood laid cross-wise, in order to detain, for a short time, the water which dashes on them. Just above this is a row of wooden pegs, fastened to the tub, around its whole inner circumference, on which the sulphur mould is let down, and thus supported. The w hole is then placed under the boiling spring, and inclosed with walls, to prevent it from being displaced by the wind. The water, which dashing or. the moulds, deposits its earth both within and without them, giving the impression of bas-relief within, and disposing itself' in an undulated surface on the outside. The hardness of the alabaster depends on the degree of obliquity at which the mould is placed in order to receive the dashing of the water. The more vertical its position, the harder is the alabaster. However, as the hardest models are not so white as the softer, the water is in some eases caused to make a circuitous course, in order to deposit all its grosser particles before it arrives at the mould. Even the softer ones, however, are as hard as Carrara marble, and surpass it in whiteness. The time required for these productions varies, according to the thickness, from one month to four. When the sulphur mould is sufficiently' filled, and the ground of the model has acquired a thickness capable of supporting the figures, the whole is removed from the water ; the wooden supports are broken by gentle strokes of the hammer, and the incrustation on the outside of the mould is chipped off by repeated strokes. Then the tub is struck with a smart blow of a hammer, which separates the model from the mould ; generally, however, cracking the latter. The brilliancy of the models is completed by brushing them with a stiff hair-brush, and rubbing with the palm of the hand.
The composition of this alabaster is gypsum, mixed with a small proportion of carbonated lime. Dr. Vegni, after many attempts, succeeded in giving a fine black, or flesh colour, to the figures thus formed, by putting a vessel half full of colouring matter into the water, before it arrives at the mould. The colouring may be also varied, by protecting par ticular parts of the mould, while the water continues charged with colouring matter.
A spring of the same kind as that just described, and applied to similar purposes, is that of Guancaveliea in Peru. The water rises from the ground into a large boson, boiling hut, and of a muddy yellowish-white colour. At a little distance from the boson, the water becoming cool, deposits calcareous matter in such vast abundance, as to till large moulds with a compact stone, of which some of the houses of the town are constructed. The moulds of statuaries, in like manner, being exposed to the water, arc filled with hard, con fusedly crystallized alabaster, and the bas-reliefs thus pro duced, by polishing, become semi-transparent, and very beau tiful. The images made use of by the Catholics of Lima, in their religious ceremonies, are said to be formed in this manner Gypsum, pulverized by grinding or burning. has been used as a manure in France and America ; and its fertilizing pro perties highly extolled. The use of it in this country, how ever, does not seem to have been attended with similar suc cessful results.