Enamel white is, however, generally prepared from barytes, which is ground into a very fine powder and converted into barium sulphide by heating with coal (see pages [41] and [42]). Hydrochloric acid acting on the sulphide produces barium chloride and sulphuretted hydrogen.
The covering power of a pigment is greater the finer its state of division, so that it would appear advisable to precipitate a weak solution of barium chloride by sulphuric acid at the ordinary temperature. When the barium sulphate has been completely precipitated, a solution of pure hydrochloric acid remains, which ought to be utilised; but when very dilute barium chloride solution is used, the hydrochloric acid is so dilute as to be useless. The barium chloride is, therefore, given in practice such a strength that it has a specific gravity of about 1·198; when the barium sulphate has been precipitated from this solution the residual hydrochloric acid has a specific gravity of 1·043.
Water of considerable purity must be used to dissolve the barium chloride. Experience has shown that water which contains appreciable quantities of organic matter does not give a pure white product. The presence of sulphate of lime in the water, which precipitates barium sulphate, need not be regarded, because the barium sulphate is so finely divided that it remains suspended in the liquid, and is carried down on precipitation of the enamel white by sulphuric acid. Carbonate of lime in the water causes the separation of barium carbonate; this may be avoided by slightly acidifying the barium chloride solution, thus converting the calcium carbonate into chloride.
According to C. A. F. Meissner, artificial barytes, suitable for use in oil paints, is obtained by precipitating barium salts by soluble sulphates in place of sulphuric acid, then quickly heating the washed and dried precipitate in a muffle to a red heat and throwing into cold water.
As has been already stated, enamel white is the most permanent pigment that exists; it appears destined in course of time to replace white lead and all other white pigments, especially as its cost is generally lower than that of the other white pigments. It costs, for example, only half as much as white lead. At present, the principal uses of enamel white are found in paper staining; it is not used to any extent in oil paints. On account of its permanence, it should be used in the place of white lead and white zinc. It also appears particularly suitable for obtaining pale shades; it can be mixed with any other pigment in any quantity without altering it in the least. This is, of course, only true when the enamel white is completely pure, and when it has been freed from every trace of hydrochloric acid by careful washing.
Lithopone.
A white pigment is obtained, according to Orr’s process, by lixiviating crude barium sulphide, obtained by igniting barytes with coal, with water and dividing the solution into two parts. Zinc chloride is added to the first portion, then zinc sulphate, and finally the second portion of barium sulphide solution. The white precipitate obtained by this process contains one equivalent of barium sulphate to two equivalents of zinc sulphide. It is collected, quickly dried, heated in retorts to redness, and, whilst still hot, thrown into cold water, by which its density and therefore covering power are increased. The pigment is finally washed and ground. It is a good white, but when mixed with lead pigments discolours them by reason of the sulphide it contains.
CHAPTER IX.
WASHING APPARATUS.
In all colour works operating on a large scale, special apparatus is used, in which are carried out the washing, pressing and drying of the pigments obtained as precipitates. Only pigments prepared in small quantities are filtered through filter paper. The treatment of enamel white and white lead requires the use of apparatus for this purpose in a special degree. We insert a short description here.
The preparation of enamel white takes place most conveniently in tubs provided with a stirring apparatus. When the precipitate of barium sulphate has once settled to the bottom, it is very difficult to again mix it up with water by means of a hand stirrer, an operation which must be often performed in washing. If vessels be used provided with a suitable mechanical stirrer, the precipitates are rapidly and thoroughly washed. We have already stated that vessels with a stirrer capable of being raised out of the liquid were specially suitable for washing white lead. Such an arrangement can with advantage be used in washing all precipitates. Many of the mineral pigments have to be freed from admixed salts by washing. A description follows of an effective washing apparatus with movable stirrer. In the cylindrical vessel ([Fig. 21]), which may be of any size, is a vertical iron shaft rotating upon a pin in the bottom of the vessel. On this axis is a horizontal wooden crosspiece, the under surface of which is studded with brushes; the disc, which this crosspiece carries, is united by means of two bars with a second, in which is cut a screw moving on the screwed shaft. The shaft is rotated by means of the cog-wheels shown in the illustration. The handle fastened to the upper disc enables the crosspiece carrying the brushes to be raised or lowered. When the handle is held fast, this crosspiece rises or falls according to the direction of rotation of the axis. The pipe shown at the side supplies the water for washing the precipitates.