Glue is an adhesive that consists of impure gelatin derived from animal protein. There are four principal types of glue: animal glue, fish glue, albumin glue, and casein glue. Several other kinds of adhesives, such as mucilage, paste, gum, and synthetic adhesives, are often incorrectly referred to as glues.
Animal glues are derived from the bones and hides of animals, hide glue generally being of better quality than bone glue. Fish glues are derived from fish bones and fish oil. They may also be obtained from isinglass, a very pure form of gelatin obtained from the air bladders of sturgeons and other fish. Albumin glue is derived from the blood of animals, and casein glue is prepared from casein, a protein found in milk.
Glues are widely used for many different purposes, particularly for cementing wood, paper, and leather. Animal glues are used principally in furniture-making and bookbinding. Fish glues are used in paints and sizes and for making gummed paper. Albumin and casein glues, which are not soluble in water, are used where a waterproof glue is necessary. One of the principal applications of albumin and casein glues is in the manufacture of plywood.