(7) Lighter shades are produced by the addition of alum, or of sulphate of baryta.
(b) New process.
The modern Brunswick greens, which are made in a variety of shades, and sometimes known as chrome greens, Prussian greens, Victoria greens, and by other fancy names, really consist of a white pigment as a basis—usually sulphate of baryta (barytes), but occasionally also sulphate of lime (gypsum) and sulphate of lead—coloured green of varying intensity and depth by addition of a blue pigment in the shape of Prussian blue, and a yellow in the guise of chrome-yellow. There are what may be called four distinct standard shades recognised by colour-makers, viz. “pale,” “medium,” “deep,” and “extra deep”; but inasmuch as every manufacturer adopts a formula of his own, there may be appreciable differences among colours of the same nominal standard if by different makers. Taken as a whole, about three-fourths of their total weight consists of the foundation white pigment, usually barytes; about 1 to 6 per cent. is Prussian blue, according to the shade; and 14 to 18 per cent. chrome yellow; but there are brands occasionally met with which depart considerably from these average figures.
The actual ingredients employed to form these green pigments are essentially different, according as the wet or the dry method of combining them be adopted. In selecting the various ingredients the following points must be borne in mind. The Prussian blue of every maker is not the same in quality, and while the character of the blue is not of the foremost importance when dark greens are being made, for light shades of green, on the other hand, it is essential to select only the best and brightest brands. In the same way the tint and quality of the chrome yellow are liable to considerable fluctuation, and it is almost impossible to ensure two lots having exactly the same characteristics, consequently the only way in which a certain shade of green can be ensured is by experimental trial with small quantities for each batch. Middle chromes can be used for deep greens, but only the lemon chromes for pale shades. Regarding the barytes which forms the basis of the pigment, there are no special precautions to be observed; and the same may be said of the gypsum, should that be adopted as a substitute for the barytes, except that 1 part by weight of gypsum takes the place of about 2½ parts of barytes. The latter, however, is much the more commonly used. For the dry method of compounding Brunswick greens, the above named ingredients are all that are required.
In the wet method there is this essential difference, that it is sought to precipitate the blue and yellow colours upon the inert base by bringing about certain reactions, and therefore while the base remains the same as in the dry method, the colouring media are totally distinct, consisting of lead acetate, bichromate of potash, sulphate of iron, and yellow or red prussiate of potash. The chief condition to be observed with regard to the lead acetate is that it shall be in the proportion of slightly more than three to one of the bichromate of potash; in other words, the bichromate should be a trifle less than one-third the weight of the lead acetate. As to the iron salt, if commercial acetate or nitrate of iron could be bought of constant quality or purity, that would be the most convenient form; but failing that, recourse is had to freshly made and good quality sulphate of iron (green copperas). It is found that the best results are secured when the weight of the sulphate of iron is exactly the same as that of the prussiate of potash. On the score of economy, the yellow prussiate of potash (ferrocyanide) is employed, but the red prussiate (ferricyanide of potassium) gives better and more certain results, and should be adopted when making a superior paint which will command a higher price.
As to the comparative merits of the wet and dry systems of mixing the ingredients of Brunswick greens, preference must be given to the former on the score of quality of the pigment produced, but on the other hand it entails much more trouble and skill, and there never can be the same degree of control over the conduct of the operation or the shade of colour developed. The dry method, however, though much more easily carried out, and enabling the exact shade desired to be obtained to a nicety by adding a little more of either the blue or the yellow during the process of manufacture, is seldom adopted, because the quality and fineness of the tints thus secured are much inferior.
The modus operandi with the wet method is as follows:—The barytes, in the requisite fine state of subdivision, is very thoroughly stirred up with water in a capacious vessel fitted with an agitator, the water being in sufficient quantity to make quite a fluid mass. In convenient proximity to the barytes tank, and elevated above it, provide three other tanks of lesser capacity furnished with means of discharging their contents into the barytes tank. In one of these smaller tanks dissolve the green copperas in cold water; in another, the sugar of lead; and in the third the bichromate and prussiate of potash together. When all the salts are thoroughly dissolved, and while the barytes is kept in constant agitation, admit first of all the copperas solution, then the lead acetate solution, and finally the combined bichromate and prussiate solution, never allowing the stirring to slacken till after the last drop of these solutions has been introduced. When the commingling of all the ingredients is judged to be complete, the green pigment formed is allowed to subside, and the clear supernatant fluid is siphoned off. The pigment is washed several times by admitting clean water, agitating and settling, and finally is removed, drained on a filter, and slowly and carefully dried. Many ways of arranging the apparatus will suggest themselves, the chief point to keep in mind being to economise labour as much as possible.
The dry method of mixing is simplicity itself in comparison with the above, and merely entails putting the component materials—barytes, chrome-yellow and Prussian blue—through an edge-runner mill simultaneously, in the proportions adapted for producing the shade required.
In giving formulæ for compounding these Brunswick greens, it must be understood that they are not absolute, as every manufacturer adopts his own particular proportions for a certain shade, but they form a sufficiently approximate basis from which to work. They are all computed for 100 lb. of barytes forming the body of the new pigment:—
Pale: Wet—1 lb. each copperas and prussiate, 12 lb. lead acetate, 3¾ lb. bichromate.