A strong, liquid adhesive consisting primarily of gelatin and other protein residues of collagen, keratin or elastin. The primary amino acids in animal glues are: glycine (24.7%), proline (13%), analine (10.1%), glutamic acid (9.7%), arginine (8.2%) and hydroxyproline (7.4%) (Mills and White 1994). Gelatin (size) is similar to glue, but purer and better refined. Animal glue has been made from ancient times from bones, skins and intestines of animals (fish, goats, sheep, goats, cattle, horses, etc.). These agglutinating materials are hydrolysed and broken down in boiling water; the cooled solution yields a jelly-like substance which is gelatin or glue. The water soluble glue occurs in a wide variety of forms and colours ranging from transparent to opaque and white to brown. Glue is soaked in cool water to form a turbid jelly that will become clear and thinner with gentle heating. Glue will decompose and darken when it is boiled. Top-quality animal glues are made from rabbitskin, sturgeon bladders and parchment clippings. Hide glues are used in the manufacture of gessoes, gilding and paints. Animal glues are available as, pellets, hot melts or a liquid solutions (with a plasticizer or preservative). Synonyms: glue; gelatin; size; isinglass; fish glue; bone glue; parchment glue; Cologne glue; calfskin glue; nikawa; hide glue; rabbitskin glue; sturgeon glue; pearl glue; carpenter's glue; scotch glue. Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]