SULPHATE OF BARYTA.

iSi

Sulphate of Baryta.

This is a widely distributed mineral, and often occurs in a perfectlypure white state. It is, for all pradtical purposes, insoluble in water andacids, and is not adted upon by sulphurous etnanations. In hue it is ofdazzling whiteness, especially when amorphous. It is a perfedkly innocuouspigment. Its great defedt is that it covers badly; in order to reduce to alighter shade of blue a certain weight of ultramarine, one part of white-leadhas the same effedt as 5 parts of powdered native sulphate of baryta. Theartificial sulphate, especially that prepared by M. Kuhlmann’s method, coversvery well, being obtained in an amorphous state. Chemists know that whensulphate of baryta is precipitated from hot, and not too concentrated, solu-tions, it assumes a crystalline appearance, and admits more readily of filtration.White-lead, neutral carbonate of lead, also covers badly, as a consequence ofits crystalline condition, whereas the white-lead of commerce, being a basiccarbonate, is purposely prepared in an amorphous state. It is the state ofaggregation, not the chemical composition, which influences what is termed“ covering power.” Sulphate of baryta is not used as a pigment, except forreducing the shade of other colours. White native substances, as pipe-clay,kaolin, and_similar aluminous minerals, and magnesian minerals, are not usedas pigment colours.

We have already referred to the use of aluminous and magnesianminerals as thickeners, and it may be stated, in passu, that French chalk—chiefly a silicate of magnesia—is employed in dressing and finishing calicoes.Zinc-white, anhydrous oxide of zinc, is occasionally used in pigment-printing, not, however, by itself, but as a diluent to reduce the depth ofshade of high- or dark-coloured pigments.

Among the substances applied to woven tissues, by the same or similarmeans as pigment colours, should be mentioned the two metals, silver andtin. The former is employed as silver-leaf, but, on account of its extraordinarysensitiveness to the adlion of sulphurous emanations, the other metal, tin, ispreferred. Tin prepared in a very finely divided state is known commerciallyas argentine, and is obtained in this form by precipitation of a slightly acidsolution of one of its protoxide salts by means of zinc. The tin solutionshould be very dilute, and only very slightly acid ; the quantity treated at oneoperation should be comparatively small. The following arrangement isoften employed:—Into cylindrical vessels of glazed earthenware, capable ofholding 12 litres (2J gals.) each, is poured 8 to 10 litres (ii to 2 gals.) ofsolution of chloride of zinc at 10° to 15° Baume, to which is added 4 to7 grms. (62 to 108 grs.) of tin-salt, known as crystals of tin, protochloride oftin. Pieces of sheet zinc are then placed in the vessels, and when the readtionhas ceased the fluid and the precipitated metal are poured upon a clean silkensieve. To the filtrate tin-salt is again added, and the operation repeated.The precipitated metal is colledled upon a filter, washed with water, dried ata gentle heat, and passed through a fine silken sieve, to separate the mostfinely divided metal. The metal is applied to the woven tissue by mixingabout 360 grms. (5544 grs.) with a litre of an ammoniacal solution of caseine.The printing is effedted by blocks or rollers, and a metallic lustre is developedby heavy calendering and hot-pressing machines. Argentine efifedls have