TALC, a class of fossil bodies, composed of broad, flat, and smooth laminae, or plates, laid evenly and regularly on one another; easily fissile according to the size of these plates, but not at all so in any other direction; flexible and elastic, bright, shining, and transparent; not giving fire with steel, nor fermenting with acid menstrua; and sustaining the force of a violent fire without calcining.
Venice tale is a soft smooth concrete, unctuous or sappy to the touch, of a whitish or pale-greenish colour, with a silver-like lustre, generally found in small pieces. It may be split into numerous fine plates or leaves; which, singly, prove somewhat flexible and elastic, and perfectly pellucid, though the entire mass is commonly opaque, or at most only semitransparent. The smallest pieces are to be preferred; the larger, however white and apparently pure on the outside, being apt to have specks or veins of heterogeneous matter in the internal part. This mineral has received the epithet Venetian, not from its being originally the product of Venice, but probably from that city having formerly been a principal mart for it. Though it may perhaps be met with in some parts of the Venetian dominions, it is more common in other countries; in Muscovy, England, Norway, Hungary, Bohemia, Spain, and the Eastern nations.
Mr Newman examined tale with all the mineral acids, and with alkaline lixivia, which some report to have a greater action on it; but without finding that either of them had any action at all; the tale remaining undiminished in weight, and neither the tale or the liquors suffering any sensible alteration. He calcined it also with sulphur and common salt, with nitre, with sal ammoniac; without any other effect than a little change in its colour. On trying to melt it with twice its weight of borax, the tale fell to the bottom as soon as the borax came into fusion. Alkaline salts,
and caustic alkalies taken in four times the weight of the talc, succeeded no better; this refractory earth refusing to unite with them: the salts being afterwards deliquated in the air, the addition of acids occasioned no precipitation, which they must have done if the alkalies had taken up in fusion any part of the talc.
Here Dr Lewis observes, that Newman seems to have failed in these experiments from using too large a proportion of the salts and too weak a heat. Venice talc, with half its weight of alkaline salt, may, in a strong fire, be brought into perfect fusion, though not to perfect transparency: with equal its weight, or less, of borax, it runs into a beautiful, pellucid, greenish yellow glass. Talc does not melt with any other earth, nor even bake or cohere with any but the argillaceous: Mixtures of it with them all are nevertheless brought into fusion by a remarkably less quantity of saline matter than the ingredients separately would require. Thus equal parts of talc and chalk, with only one-fourth their weight of borax, melt in no very vehement heat, into a fine transparent greenish glass, of considerable hardness and great lustre. On substituting gypseous earths, as the lapis specularis, to chalk, the fusion was as easy, and the glass as beautiful; in colour not green, but yellow, like the topaz. Talc, with half its weight of sand, and a quantity of nitre equal to both, yielded also a transparent topaz-yellow glass. Several further experiments on talc may be seen in a Memoir by Mr Pott in the Mem. de l'Acad. de Berlin, 1746.
Talc is employed, in those places where it is found in any considerable quantity, in compositions for earthen vessels; and by some for tests and cupels. From its smoothness, unctuousity, and brightness, it has been greatly celebrated as a cosmetic; and the chemists have submitted it to a variety of operations, for procuring from it oils, salts, tinctures, magisteries, &c. for that intention. But all their labours have been in vain; and all the preparations sold under the name of talc have either contained nothing of that mineral, or only a fine powder of it.
The pulverization of this mineral is attended with some difficulty. The more brittle pieces may be pulverized pretty easily; but the very tough and flexible scarce at all; nor is heating the mortar and pestle red-hot, which some recommend, of any advantage. The most commodious method of reducing it into fine powder, is, by cautiously filing or grating it with a fish-skin.
Besides the white and greenish talcs treated of, there are yellow, red, and black foliaceous concretes, distinguished by the same name; but these are in general rather of the gypseous than taly kind. Thus the black talc mentioned by Hoffman, which laid on burning coals falls into leaves and acquires a golden colour, seems to be a species of lapis specularis. Those of a yellow colour have been called solar talcs, and imagined to be impregnated with a sulphur of gold; which some of the chemists have pretended to extract by spirit of salt or aqua regis. But the yellow tincture which these minerals communicate, and the red sublimate which arises on distilling the solutions, proceed entirely from a ferruginous matter, common to these with other coloured earths.