A solid glutinous substance, almost wholly gelatine, prepared chiefly from a fish of the sturgeon kind, caught in rivers of Russia and Hungary. The belluga yields the greatest quantity, as being the largest and most plentiful fish in the rivers of Russia; but the sounds, or air bladders of all fresh-water fish, yield, more or less, fine isinglass, particularly the smaller sorts, found in prodigious quantities in the Caspian Sea, and several hundred miles beyond Astracan, in the Wolga, Yaik, Don, and even as far as Siberia. The following is the usual mode of preparing isinglass: - The sounds, or other parts of which it is to be made, are taken from the fish while sweet and fresh, slit open, washed from their slimy sordes, divested of every thin membrane which envelopes the sound, and then exposed, to stiffen a little in the air. In this state they are formed into rolls, about the thickness of a finger, and in length according to the intended size of the staple: a thin membrane is generally selected for the centre of the roll, round which the rest are folded alternately, and about half an inch of each extremity of the roll is turned inwards.
The due dimensions being thus obtained, the two ends of what is called short staple are pinned together with a small wooden peg; the middle of the roll is then pressed a little downwards, which gives it the resemblance of a heart-shape; and thus it is laid on boards, or hung up in the air to dry. The sounds which compose the long staple are longer than the former; but the operator lengthens this sort at pleasure, by interfolding the ends of one or more pieces of the sound with each other. The extremities are fastened with a peg like the former, but the middle part of the roll is bent more considerably downwards; and in order to preserve the shape of the three obtuse angles thus formed, a piece of round stick, about a quarter of an inch in diameter, is fastened in each angle with small wooden pegs, in the same manner as the ends. In this state it is permitted to dry long enough to retain its form, when the pegs and sticks are taken out, and the drying completed; lastly, the pieces of isinglass are collocated in rows, by running packthread through the pegholes, for convenience of package and exportation.
The common kinds of isinglass, called the "bock" and "ordinary staple," are composed of the membranes, which will not admit of a similar formation with the preceding; the pieces, therefore, after their sides are folded inwardly, are bent in the centre in such manner that the opposite sides resemble the cover of a book, from whence its name: a peg being run across the middle, fastens the sides together, and thus it is dried like the former. This sort is interleaved, and the pegs run across the ends, the better to prevent its unfolding. Ichthyocolla, or isinglass, is one of the purest and finest of the animal glues, of no particular smell or taste. Beaten into threads, it dissolves pretty readily in boiling water or milk, and forms a gelatinous substance, which yields a mild nutriment, and proves useful, medicinally, in some disorders.