By the last process, superior and more permanent chrome greens may be produced, free from lead, by using chloride of barium or nitrate of bismuth in place of the acetate of lead. Chromate of baryta, or chromate of bismuth is then formed, neither of which acts on the Prussian blue.
It should be added that where the latter pigment is present, no green will serve for painting walls containing lime, as its action alters the tint of the Prussian blue.
186. HOOKER'S GREEN
is a compound of Prussian blue and gamboge, two pigments possessing a like degree of stability, and perfectly innocuous to each other. It is a mixture more durable and more transparent than chrome greens made with chromate of lead. There are two varieties in common use—No. 11, a light grass green, in which the yellow predominates; and No. 22, a deeper and more powerful green, with a larger amount of blue.
187. PRUSSIAN GREEN,
like the preceding, is composed of Prussian blue and gamboge; but contains a very great excess of the former, and is therefore a bluish-green of the utmost depth and transparency, verging on black in its deep washes. Yellow ochre may be employed instead of gamboge, but is not so eligible.
A true Prussian green, which has been recommended as a pigment, can be produced as a simple original colour, with a base wholly of iron. It is got by partially decomposing the yellow oxalate of protoxide of iron with red prussiate of potash. We have made this green and given it a fair trial, but our verdict is decidedly against it. In colour it is far from being equal to a good compound of Prussian blue and gamboge, and it assumes a dirty buff-yellow on exposure to light and air, the film of blue on the oxalate more or less disappearing.
Another Prussian green, with a base of cobalt, is obtained by precipitating the nitrate of that metal with yellow prussiate of potash. According to the mode adopted, and the degree of heat, either a light or dark green results; but this also is inferior in colour, and presents no advantage as to permanence.
188. SAP GREEN,
Verde Vessie, or Iris Green, is a vegetal pigment prepared from the juice of the berries of the buckthorn, the green leaves of the woad, the blue flowers of the iris, &c. It is usually preserved in bladders, and is thence sometimes called Bladder Green. When good, it is of a dark colour and glossy fracture, extremely transparent, and a fine natural yellowish green. This gummy juice, inspissated and formed into a cake, is occasionally employed in flower painting. It is, however, a very imperfect pigment, disposed to attract the moisture of the atmosphere, and to mildew; while, having little durability in water and less in oil, it is not eligible in the one and is totally useless in the other.