From A Dictionary of Photography, London, 1867:
Isinglass. Fish glue; a pure form of gelatine. It is soluble in boiling water, and gelatinizes on cooling; it is also soluble in weak acids but is precipitated by the addition of an alkali.
Isinglass is obtained from the air bladders of sturgeons, and principally from the great sturgeons found in the Caspian Sea. It is manufactured chiefly at Astracan, and is bleached by the fumes of burning sulphur.
The process of manufacture consists in first steeping the bladders in water, then removing the outer skin, putting them into a hempen bag, squeezing them, softening them between the hands and twisting them into small cylinders, which are afterwards beat into the shape of a lyre.