A mixture of proteins prepared by hydrolysing, via boiling, collagen obtained from skin, ligaments and tendons. Gelatin is composed of amino acids in the following proportions: glycine (25.5%), proline (18.0%), hydroxyproline (14.1%), glutamic acid (11.4%), alanine (8.7%) along with small amounts of arginine, leucine and aspartic acid. Gelatin is approved as a food product. Its production differs from that of animal glue in that raw materials are selected, cleaned and treated with special care so that the product is cleaner and purer than glue. Gelatin is strongly hydrophilic. In cold water, dried gelatin can absorb up to ten times its weight of water, forming a viscous mass. Adding alum to gelatin produces a harder gel. Potassium chrome alum and formaldehyde (formogelatin) also harden gelatin and make it insoluble. Gelatin is used for sizing and adhesive. It is mentioned as one of the organic additives used for stucco marble preparation according to literature. Synonyms: gelatine; glue. Wittenburg (1999); Museum of Fine Arts, Boston (2000).[J. Cassar, R. de Angelis]