2. The secondary gypsum, or that of the salt mine districts, belongs to the red ground, immediately beneath the lias in the order of stratification, and therefore a rock relatively antient. Near Northwick, the red marl beds above the great deposit of rock salt, are irregularly intersected with gypsum, in numerous laminæ or plates. At Newbiggin in Cumberland, the gypsum lies in red argillaceous marl, between two strata of sandstone; and a mile south of Whitehaven, the subterraneous workings for the alabaster extend 30 yards in a direct line; with two or three lateral branches extending about 10 yards, at whose extremities are large spaces where the gypsum is blasted with gunpowder. It is generally compact, forming a regular and conformable bed, with crystals of selenite (crystallized gypsum) in drusy cavities. Gypsum occurs in the red marl in the isle of Axholme, and various other places in Nottinghamshire. In Derbyshire some considerable deposits have been found in the same red sandstone, several of which are mined, as at Chellaston hill, which would exhibit a naked and water-worn rock of gypsum, were it not for a covering of alluvial clay. It appears in general to present itself chiefly in particular patches, occasioning a sudden rise, or an insulated hill, by the additional thickness which it gives to the stratum of the red ground in these places. The principal demand for the pure white gypsum, or that faintly streaked with red, is by the potters in Staffordshire, who form their moulds with the calcined powder which it affords; only particularly fine blocks are selected for making alabaster ornaments on the turning lathe. In one of the salt pits near Droitwich, the strata sunk through, were, vegetable mould, 3 feet; red marl, 35 feet; gypsum, 40 feet; a river of brine, 22 inches; gypsum, 75 feet; a rock of salt, bored into only 5 feet, but probably extending much deeper. On the Welsh side of the Bristol channel, gypsum occurs in the red marl cliffs of Glamorganshire, from Pennarth to Lavernock. No organic remains or metallic minerals have hitherto been found in the gypsum of this formation.
3. The most interesting gypsums in a general point of view, are certainly the tertiary, or those of the plains, or hills of comparatively modern formation. They are characterized, by the presence of fossil bones of extinct animals, both mammifera and birds, by shells, and a large proportion of carbonate of lime, which gives them the property of effervescing with acids, and the title of limestone gypsums. Such are the gypsums of the environs of Paris, as at the heights of Montmartre, which contain crystallized sulphate of lime in many forms, but most commonly the lenticular and lance-shaped.
Sulphate of lime occurs either as a dense compound without water, and is called anhydrite from that circumstance; or with combined water, which is its most ordinary state. Of the latter there are 6 sub-species; sparry gypsum or selenite in a variety of crystalline forms; the foliated granular; the compact; the fibrous; the scaly foliated; the earthy.
The prevailing colour is white, with various shades of gray, blue, red, and yellow. More or less translucent. Soft, sectile, yielding to the nail. Specific gravity 2·2. Water dissolves about one five-hundredth part of its weight of gypsum, and acquires the quality of hardness, with the characteristic selenitic taste. When exposed on red hot coals, it decrepitates, becomes white, and splits into a great many brittle plates. At the heat of a baker’s oven, or about 400° Fahr., the combined water of gypsum escapes with a species of ebullition. At a higher temperature the particles get indurated. When rightly calcined and pulverized, gypsum is mixed with water to the consistence of cream, and poured into moulds by the manufacturers of stucco ornaments and statues. A species of rapid crystallization ensues, and the thin paste soon acquires a solid consistence, which is increased by drying the figure in proper stoves. During the consolidation of the plaster, its volume expands into the finest lines of the mould, so as to give a sharp and faithful impression.
The plaster stone of the Paris basin contains about 12 per cent. of carbonate of lime. This body, ground and mixed with water, forms an adhesive mortar much used in building, as it fixes very speedily. Works executed with pure gypsum never become so hard as those made with the calcareous kind; and hence it might be proper to add a certain portion of white slaked lime to our calcined gypsum, in order to give the stucco this valuable property. Coloured stuccos of great solidity are made by adding to a clear solution of glue, any desired colouring tincture, and mixing-in the proper quantity of the calcined calcareous gypsum.
The compact, fine-grained gypseous alabaster is often cut into various ornamental figures, such as vases, statuary groups, &c., which take a high polish and look beautiful, but from their softness are easily injured, and require to be kept enclosed within a glass shade.
In America and France, the virtues of gypsum in fertilizing land have been highly extolled, but they have not been realized in the trials made in this kingdom.
Pure gypsum consists of lime 28; sulphuric acid 40; water 18; which are the respective weights of its prime equivalent parts.
M. Gay Lussac, in a short notice, in the Annales de Chimie for April 1829, on the setting of gypsum, says that the purest plasters are those that harden least, and that the addition of lime is of no use towards promoting their solidity, nor can the heat proper for boiling gypsum ever expel the carbonic acid gas from the calcareous carbonate present in the gypsum of Montmartre. He conceives that a hard plaster-stone having lost its water, will resume more solidity in returning to its first state, than a plaster-stone naturally tender or soft; and that it is the primitive molecular arrangement which is regenerated. See [Alabaster].