Isinglass, or Ichthyocolla, a preparation from different species of a Russian fish, called sturgeon, of which we have given some account in the article Caviare. It may also be produced from the air-bladders of the cod, or gadus, as well as from those of other fish inhabiting fresh waters.
The method of making isinglass was for ages kept a secret with the Russians, but has lately been discovered : we extract the following process from the 63d vol. of the Philosophical Transactions: - First, the -sinewy parts of the fish are boiled in water, till they are dissolved ; then the viscid liquor is strained, and suffered to cool. When cold, the fat is carefully taken off, the liquor again boiled to a due consistence, then cut in pieces and rolled into a semi-circular twist, in which state they are suspended on a string, till carefully dried.
The sounds or air-bladders of fresh-water fish, in general, are the most transparent, flexible, and delicate substances, and consequently furnish the finest isinglass. But the intestines and peritoneum of the fish constitute interior sorts of this article, denominated look, and ordinary staple. The belugas, being the largest and most plentiful fish in the Russian rivers, yield the greatest quantity of this animal glue, which, on account of its strength, is preferred to all other kinds.
Isinglass is most successfully prepared in the summer; as frost changes its colour, deprives it of weight, and impairs its gelatinous principle: but the forms into which it is twisted by the Russians, are useless, and frequently injurious to its native qualities. These peculiar shapes were probably adopted with a view to conceal the real substance, and thus to preserve the monopoly.
The Newfoundland and Iceland fishermen split open the fish, as soon as they are taken, and throw the back bones, with the sounds annexed, into a heap ; but, before put refaction commences, the sounds are cut out, washed and salted for use. In performing this operation, the best, namely, the intercostal parts, are left behind;-the Iceland fishermen are so sensible of this circumstance, that they beat the bone upon a blocks with a thick stick, till the pockets come out easily, so that they preserve the sound entire. This isinglass is dried upon nets in the open air, and resolves into fining like that of Russian manufacture, in sub-acid liquors, such as stale beer, cyder, old hock,etc. while, in equal quantities, it produces similar effects upon turbid liquors, except that it falls sooner and closer to the bottom of the vessel; though foreign isinglass, on account of the greater tenacity of its native mucilage, retains the power of fining preferably in warm weather.
With respect to the very exten-sive use that is made of isinglass by brewers, and wine-merchants, we have already expressed our disapprobation, under the head of Clarification; and also recommended proper substitutes, vol. i. p. 239, especially as H.Jackson informs us, in his Essays on Britishlsinglass, published about the year 1765, that its yearly consumption in the brewery was then calculated at '25 tons weight, and that 40,0001. annually are paid for this article to the Russians. Hence it is sincerely to be wished, that this importation may be entirely superseded, as there is an adequate supply from the British colonies.
The finest and most transparent sorts of isinglass are consumed in making mock-pearls, and in stiffening linens, silks, gauzes, etc-It may likewise be reduced to a jelly, as it dissolves in alkaline liquors; and even cold lime-water converts it into a pulpy mass. Although such preparation would be extremely detrimental to health in fining liquors, yet it may be usefully employed for another purpose ; because, on mixing this jelly with compositions of plaster, lime, etc. for ornamenting walls exposed to vicissitudes of weather, it forms a firm and durable cement; and, if worked up with common mortar, it soon acquires the harshness of bricks. With this intention, however, it is more conveniently prepared by dissolving it in cold water, acidulated with oil of vitriol thus, the acid quits the jelly, and forms with the lime a selenitic mass ; while the jelly, being deprived of part of its moisture, speedily dries, and hardens into a firm body ; whence its superior strength and durability may be easily explained.
In a medicinal view, isinglass is but seldom employed; though it may with advantage be used in violent bleedings from the nose, by introducing into the nostril a pessary made of soft linen, and dipped in a solution of this glue, prepared in equal parts of spirit of wine and water. Isinglass also forms a principal ingredient in sticking-plaster; and, when boiled in fresh milk to the consistence of a strong jelly, it affords a very nourishing food to invalids; though it should be eaten with precaution by those who possess a weak stomach, or digest slowly ; as it has a great tendency to turn rancid and putrid.