Isinglass (perhaps from icing glass; Ger. Hausenblase, sturgeon's bladder, isinglass), a kind of edible gelatine, which consists of the dried air bags, sounds, or swimming bladders of fishes. It was known to the ancients by the name of ichthyocolla or fish glue, and is often alluded to by Dioscorides and Pliny. In dif -ferent parts of the world it is obtained from different fishes, and the isinglass of commerce is consequently of various qualities. The best is found among the varieties imported from Russia, particularly that which is brought to St. Petersburg from Astrakhan, and said to be obtained from the sturgeon called the beluga (acipenser huso) of the Caspian sea and the rivers which flow into it, a species which attains a length of 25 ft. According to some authorities, the sound is cut open, washed, and then exposed to the air, the inner silvery membrane outward. This membrane is then stripped off, placed in damp cloths, and kneaded in the hands. Taken out and dried, it forms the leaf isinglass; folded like sheets of paper, it is the book isinglass; wound in the form of a horse shoe or lyre around three pegs, it forms the varieties known as long or short staple. According to Martin, the inner membrane is removed by beating and rubbing, and the thicker membrane is preserved.
The isinglass called Samovey is brought from Taganrog. The leaf, book, and short staple from this place are all of inferior quality. The varieties from the Ural and Siberia are better. The Brazilian isinglass, imported from Para and Maranhao, is obtained in various forms distinguished as pipe, lump, and honeycomb. It appears to be the product of different kinds of fish, and to be prepared with little care. It is largely used in brewing establishments for fining the liquors; and though it is too impure for domestic uses, it is largely employed to adulterate the better kinds. Its presence may be detected by the isinglass failing to dissolve readily and completely in hot water, and by its forming with this an opalescent and milky jelly in which may be observed the insoluble shreds common to the Brazilian article. Its smell also is often disagreeable, while that of the pure Russian isinglass is as inoffensive as the odor of seaweed, which it somewhat resembles. The isinglass of New York and New England is obtained from the sounds of the codfish (morrhua vulgaris) and of the common hake (phycis Americanus). They are macerated in water, and afterward rolled out in long strips a few inches wide. The quality of this isinglass is poor, its solution not readily obtained, and its color dark.
It is used for the same purposes as the Brazilian. Other varieties are produced in other maritime countries. - All isinglass has to undergo a process of refining before it is fit for making jellies, blanc-mange, etc. The best beluga leaf is imported in circular sheets, the most perfect of which are sometimes 2 ft. in circumference, and weigh from 8 to 16 oz., in some instances reaching even 4 lbs. These are carefully picked over, and all the discolored parts are cut away and put aside for uses of less importance. The assorted leaf is then passed through successive pairs of iron rollers, until it is converted into thin ribbons of uniform width, which are afterward by other machinery slit into fine shreds. Inferior sheet gelatine is sometimes introduced between two sheets of isinglass before rolling, and thus incorporated with it. Isinglass, being a nearly pure gelatine, should have little or no color; and being commonly prepared without exposure to high degrees of heat, it should be tougher and more elastic than the other forms of this substance. It therefore makes a most adhesive cement. For this purpose it is swollen with cold water and then placed in diluted alcohol. The vessel containing it is then put into cold water, which is to be heated to boiling.
The jelly forms the cement, which may be kept from mouldiness and other change by the addition of a few drops of any essential oil. It is known as the " diamond cement," and is also the adhesive substance of court plaster. Gum ammoniac is sometimes introduced, especially by the Turks, who use the cement for fastening precious stones, mending broken porcelain, glass, etc. Isinglass has also been used for the window lights of vessels, being covered with a transparent varnish which is not affected by moist air. Hence, sheets of mica prepared for similar uses, as in the doors of stoves, are popularly called isinglass. (See Mica.) - Besides the methods already stated of detecting fraudulent mixtures with isinglass, the microscope may be used to render the different textures apparent. The ash of isinglass seldom exceeds 1/2 of 1 per cent., and is red; that of gelatine is white, and in quantity not less than 3 per cent.