GALL. Ox-gall.. The bile of the ox. Bile is a secretion which is separated from the venous blood in the liver. It is supposed to be a saponaceous compound in which an organic acid is combined uith soda. Ox-gall is of a dingy green colour, transparent, and viscid; having a peculiar odour and a nauseously bitter taste. It has a slightly alkaline reaction, and mixes in all proportions with water.
Ox-gall may be clarified in the following manner :—Take the gall of newly killed oxen, and let it settle for 12 or 15 hours in a basin. Pour the supernatant liquor off the sediment into an eva porating dish, and boil it until it is somewhat thick. Then, spread it upon a dish, and put it before a fire until it becomes nearly dry. In this state it may be kept for years in jelly pots covered with paper. When required for use a piece of it the size of a pea is to be dissolved in a table-spoonful of water.
Ox-gall may be rendered perfectly colourless in the following man ner :—To a pint of gall, boiled and skimmed, add one ounce of fine powdered alum, and leave the mixture on the fire till the alum is dissolved. When cold pour it into a bottle, and cork it loosely. Next, treat another pint of gall in exactly the same way, only sub stituting salt for alum. In about three months these preparations will deposit a thick sediment. Then decant the fluid portion of each, and mix them. A precipitate is immediately formed which takes down the colouring matter, and the supernatant liquid may then be filtered, and is as transparent and colourless as water.
Clarified ox-gall combines readily with colouring matters or pig ments, and gives them solidity, either by being mixed with them, or passed over them upon paper. It increases the brilliancy and durability
of ultramarine, carmine, green, and in general of all delicate colours, while it contributes to make them spread more evenly upon paper, ivory, &c. When mixed with gum arabic it thickens the colours without making them glisten, and prevents the gum from cracking, and fixes the colours so well that others may be applied over them. Along with lamp black and gum it forms a good imitation of Indian ink. When a coat of ox-gall is put upon drawings made with black lead, or crayons, the lines can no longer be effaced, but may be painted over with a variety of colours previously mixed with the same ox gall. Miniature painters find great advantage in using it. When passed over ivory it removes the unctuous matter from its surface ; and when ground with the colours makes them spread with the greatest ease, and renders them fast. It serves also for transpa rencies ; being first passed over the oiled, or waxed, or varnished paper, and allowed to dry. The colours mire ci with the gall are then applied, and cannot afterwards be removed by any means. It is useful to the photographer in blackening the skies of waxed-paper negatives, for when mixed with the Indian ink it causes it to flow more readily on the greasy surface of the wax.
Ox-gall is used for taking out spots of grease and oil.
This substance has been described at some length because it is not only very useful to the colourist of photographs, but may also tum out to be of some utility in the new methods of photographic printing in which pigments are fixed to the paper by means of bi chromate of potass reduced by light.