ISINGLASS (probably a corruption of the Dutch huisenblas, Ger. Hausenblase, literally “sturgeon’s bladder”), a pure form of commercial gelatin obtained from the swimming bladder or sound of several species of fish. The sturgeon is the most valuable, various species of which, especially Acipenser stellatus (the seuruga), A. ruthenus (the sterlet) and A. güldenstädtii (the ossétr), flourish in the Volga and other Russian rivers, in the Caspian and Black Seas, and in the Arctic Ocean, and yield the “Russian isinglass”; a large fish, Silurus parkerii, and probably some other fish, yield the “Brazilian isinglass”; other less definitely characterized fish yield the “Penang” product; while the common cod, the hake and other Gadidae also yield a variety of isinglass. The sounds, having been removed from the fish and cleansed, undergo no other preparation than desiccation or drying, an operation needing much care; but in this process the sounds are subjected to several different treatments. If the sound be unopened the product appears in commerce as “pipe,” “purse” or “lump isinglass”; if opened and unfolded, as “leaf” or “honeycomb”; if folded and dried, as “book,” and if rolled out, as “ribbon isinglass.” Russian isinglass generally appears in commerce as leaf, book, and long and short staple; Brazilian isinglass, from Para and Maranham, as pipe, lump and honeycomb; the latter product, and also the isinglass of Hudson’s Bay, Penang, Manila, &c., is darker in colour and less soluble than the Russian product.
The finest isinglass, which comes from the Russian ports of Astrakhan and Taganrog, is prepared by steeping the sounds in hot water in order to remove mucus, &c.; they are then cut open and the inner membrane exposed to the air; after drying, the outer membrane is removed by rubbing and beating. As imported, isinglass is usually too tough and hard to be directly used. To increase its availability, the raw material is sorted, soaked in water till it becomes flexible and then trimmed; the trimmings are sold as a lower grade. The trimmed sheets are sometimes passed between steel rollers, which reduce them to the thickness of paper; it then appears as a transparent ribbon, “shot” like watered silk. The ribbon is dried, and, if necessary, cut into strips.
The principal use of isinglass is for clarifying wines, beers and other liquids. This property is the more remarkable since it is not possessed by ordinary gelatin; it has been ascribed to its fibrous structure, which forms, as it were, a fine network in the liquid in which it is disseminated, and thereby mechanically carries down all the minute particles which occasion the turbidity. The cheaper varieties are more commonly used; many brewers prefer the Penang product; Russian leaf, however, is used by some Scottish brewers; and Russian long staple is used in the Worcestershire cider industry. Of secondary importance is its use for culinary and confectionery purposes, for example, in making jellies, stiffening jams, &c. Here it is often replaced by the so-called “patent isinglass,” which is a very pure gelatin, and differs from natural isinglass by being useless for clarifying liquids. It has few other applications in the arts. Mixed with gum, it is employed to give a lustre to ribbons and silk; incorporated with water, Spanish liquorice and lamp black it forms an Indian ink; a solution, mixed with a little tincture of benzoin, brushed over sarsenet and allowed to dry, forms the well-known “court plaster.” Another plaster is obtained by adding acetic acid and a little otto of roses to a solution of fine glue. It also has valuable agglutinating properties; by dissolving in two parts of pure alcohol it forms a diamond cement, the solution cooling to a white, opaque, hard solid; it also dissolves in strong acetic acid to form a powerful cement, which is especially useful for repairing glass, pottery and like substances.