Today many glues are made from synthetics, but originally they were made from natural sources. These included animal skin, bone and sinew, plants and gums. Animal bones were boiled down to make glue which was traditionally used in bookbinding.

©SCRAN/Edinburgh City Museums
JG Cox's Glue Works, Edinburgh, 1904

Glue contains two groups of proteins: chondrin, which accounts for its adhesive strength, and glutin, which contributes jelling strength. Animal glue is a protein derived from the simple hydrolysis of collagen, which is the principal protein constituent of animal hide, connective tissue and bones. Collagen, animal glue, and gelatin are very closely related in terms of protein and chemical composition.

Regardless of the source of the protein, the glue manufacturing process consists essentially of washing the stock, crushing or shredding the bones or hides, soaking in a lime solution to eliminate hair and flesh, boiling to extract the gelatinous material, gelling, and, finally, drying. The resulting hard, brittle sheets of glue are then broken into pieces or flakes, or ground into powder.