Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/368

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360
CYCLOPEDIA OF PAINTING

three coats. Now add to the remainder of the liquid two bruised nut-galls, a few very rusty nails, or a piece of sulphate of iron about the size of a pigeon's egg, and add rainwater until the original quantity of liquid is made up. This stain is to be applied hot, and the work is to be French-polished, a little blue having been previously mixed with the polish.

Nut-Galls. Gall-nuts, oak-galls and galls are excrescences formed upon the young twigs of the various species of oak. Galls are also formed upon other plants, but the nut-galls of commerce are produced on the species of oak called the Quercus infectorius, a small shrub about 5 or 6 feet in height. They originate in the puncture of an insect, Cynips gallæ-tinctoria. The puncture is effected by the ovipositor of the insect, and an egg is at the same time deposited. An interruption in the ordinary functions of the tissue of the plant takes place at the spot where the egg is inserted, the consequence is, that an excrescence of vegetable matter, principally tannin, is formed round the egg, and furnishes a nidus for the grub or larva when hatched. When this takes place, the grub eats its way out through the side of the gall, after which the vitality of the excrescence either decreases or ceases altogether. Several varieties of galls are distinguished in commerce, the principal of which are the blue and white, the only difference is, that the former are gathered before, and the latter after, the insect has escaped. The color of the blue galls is a slaty blue, and something of a grayish green, the white gall is of a light drab color, and much lighter in weight, it is also less valuable than the blue variety. Nut-galls are nearly round, with a few small excrescences over their surface. They yield a fine black color with any of the salts of iron, and are used in the preparation of writing ink.