Plaster-stone, or Alabaster; a natural combination of the calcareous earth with vitriolic acid. See ALABASTER.
The properties of gypsum, according to Cronstedt, are, 1. It is looser and more friable than a calcareous earth. 2. It does not effervesce with acids either in its crude or calcined state; or at most but in a very slight degree, in proportion to what it wants of the vitriolic acid for the complete saturation of its base. 3. It falls into powder in the fire very readily. 4. When burnt without being made red-hot, its powder readily concretes with water into a mass which soon hardens; but without any sensible heat being excited in the operation. 6. According to our author, it is nearly as difficult of fusion as limestone; and shows almost the same effects upon other bodies with limestone, though the acid of vitriol seems to promote the vitrification. M. Magellan, however, informs us, that he has found most of the gypseous kind, particularly the fibrous, to melt in the fire pretty easily by themselves. 7. When melted in the fire with borax, it puffs and bubbles very much, and for a long time during the fusion. According to M. Magellan, when a small quantity of any gypsum is melted together with borax, the glass becomes colourless and transparent; but some sorts of alabaster and sparry gypsum, when melted in quantity with borax, yield a fine yellow transparent coloured glass, resembling that of the best topazes; but if too much of the gypsum is used in proportion to the borax, the glass becomes opaque, just as it happens with the pure limestone. 8. When burnt with any inflammable matter it emits a sulphureous smell, and may thus be decomposed, as well as by either of the fixed alkaline salts; but if this last method is followed, there ought to be five or six times as much salt as there is of gypsum. 9. On being decompounded in this manner the residue commonly shows some signs of iron. The species are,
1. Friable gypseous earth of a white colour, found in Saxony.
2. Indurated gypsum of a solid texture, the particles of which are not visible, commonly called alabaster. This is sometimes found unsaturated with vitriolic acid; Gypsum, in which case only it will effervesce with aquafortis, as it is said to do under the article Alabaster. It is very easily sawed or cut, and takes a dull polish. It is of several kinds; as, white; clear and transparent from Persia, opaque from Italy and Trapani in Sicily; a yellow colour, of which there are likewise two kinds, transparent and opaque; the former being met with in the eastern countries, the latter in Spain. Brunnick informs us, that in this country there are a great many fine varieties of the species we treat of; and from hence he supposes that the ancients obtained the beautiful alabasters they used. Fabroni tells us, that a great variety of fine alabasters are met with in Italy. Twenty-four quarries of them, each of a different colour, are now worked out at Volterra; but he is of opinion that the Romans brought the greatest part of the alabasters they made use of from Greece.
3. Gypsum of a fleshy texture, or common plaster of Paris. This is found in many different countries, of two kinds; viz. white with coarse scales, or with small scales yellowish or greyish. According to Bergman, plaster contains $\frac{1}{2}$ of vitriolic acid, $\frac{1}{2}$ of pure calcareous earth, and $\frac{1}{2}$ of water. It is soluble in 500 times its weight of warm water, or 450 times its weight of boiling water. It is well known by its property of forming an hard mass with water after being slightly burned; and during this consolidation a slight degree of heat is produced, though less than when lime is slaked. It is often employed in building; and may be taken off and used again and again for the same purpose.
4. Fibrous gypsum, or plaster stone, has likewise two varieties, viz. with coarse or with fine fibres. It is of a white colour.
5. Selenites, or spar-like gypsum, by some also called glacies mane, and confounded with the clear and transparent mica. It is found of two kinds, clear and transparent, or yellowish and opaque.
6. Crystallized gypsum, or gypseous drusen. This is found composed of wedge-shaped and sometimes of capillary crystals, sometimes white and sometimes yellowish.
7. Stalactitical gypsum is found of a great many different forms and colours. When found in large pieces it commonly varies between white and yellow, and likewise in its transparency in different parts of the same mass. It is used as alabaster in several works.
Besides the countries already mentioned, England abounds with substances of a gypseous nature. There are plenty in Derbyshire and Nottinghamshire, so fine as to be used like alabaster, that is, to take a fine polish. In the counties just mentioned there are large pits of this kind, also in most of the cliffs of the Severn, especially at the Old Passage in Somersetshire. A very fine semipellucid solid alabaster is found in Derbyshire. Very fine fibrous talcs are also found in the above mentioned pits of stone, and many other places. Selenites everywhere abound, so that it is impossible to enumerate the different places. Very fine gypseous drusen are found in Sheppey Isle, and some exceedingly beautiful, large and clear as crystal, have been dug from the salt-rocks at Nantwich in Cheshire. The selenites rhomboidales is found in plenty in England, tho' rare in other countries. Shotover hill in Oxfordshire is remarkable for them. The Isle of Sheppey affords a kind of spar-like gypsum, of a fibrous nature, and always accreting like the radiations of a star on the septaria, and thence called stella fumaris.
The principal use of gypsum is as a material for small statues and figures of various kinds, also for moulds for casting wax-work, &c. It has lately been introduced as a manure in France and America, though its success in this respect has not yet been sufficiently experienced.