Isinglass is prepared from the dried swimming bladder of the sturgeon, Acipenser Huso, Linn., and of other species of Acipenser (Class, Pisces; Order, Sturiones) found in the Caspian and Black Seas, and in the rivers which flow into them. After removal from the fish, the bladders are cut open, washed, and soaked in water; then spread on boards, and the outer silvery membrane removed by rubbing. They are then dried in sheets (leaf isinglass) or several are folded together before they are completely dry (book isinglass), or each bladder is rolled and folded round pegs in the form of a horse-shoe, heart, or lyre (staple isinglass). The product is generally further prepared for use by rolling it into thin sheets and cutting into short threads. It occurs as a semitransparent, iridescent, tough, membranous tissue, without odour or taste, whitish or pale yellowish in colour, and of a horny or pearly appearance. It softens, swells, and becomes less transparent when soaked in water. It is almost entirely soluble in boiling water and in boiling diluted alcohol. A solution in boiling water (1 in 50) forms on cooling a transparent jelly. On incineration it leaves from 0.5 to 1.0 per cent. of ash. Russian isinglass exported from the Caspian Sea and Black Sea is considered the best. It should be almost colourless, translucent, free from odour and taste, and strongly iridescent. Brazilian isinglass is inferior to Russian, and may be distinguished by its yellowish or brownish colour. Other similar preparations made from the swimming bladder of the cod, ling, hake, etc., are characterised by a higher yield of ash and of substances insoluble in water.
Constituents.—The chief constituent of isinglass is gelatin, but it also contains about 3 per cent. of insoluble membrane, and from 15 to 20 per cent. of moisture.
Action and Uses.—Isinglass is used as a nutrient in place of gelatin and as a "fining" for wines and beers. A strong solution of isinglass is spread on silk of various textures to make "Court Plaster" and surgeon's isinglass plaster. A similar plaster, with the solution spread upon white felt, is used to remove pressure from corns and bed-sores. B.P. test solution of isinglass is prepared by digesting 2 of isinglass, in shreds, with 100 of warm distilled water, for half an hour, with repeated shaking, and then filtering through clean moistened tow; it should be freshly prepared as required.