Glue is an adhesive made from the skins, connective tissues, or bones of animals. Many people use the term glue for all types of adhesives, including those made from plants or plastics. This article deals only with glues made from animal tissues. For information on other types of bonding substances, see Adhesive.
Glue is an impure form of gelatin, a protein substance obtained by boiling bones or other animal parts. Glue makes objects stick together by penetrating pores in their surfaces and then drying to form a hard bond. People have made glue for many centuries, and it is one of the most widely used adhesives today.
Types of glue.
There are three types of glue: (1) hide glue, (2) bone glue, and (3) fish glue. Most hide and bone glues are sold in the form of powder or small grains and must be dissolved in hot water before they can be used. These glues can be stored for any length of time if they are kept dry. Fish glue is a concentrated liquid that contains about 45 per cent solids. If unused, fish glue begins to lose its strength as an adhesive about two years after it is made.
How glue is made.
Manufacturers obtain glue from animal parts by cooking them in water. Cooking breaks down the protein in the animal tissue and dissolves it. The resulting solution may be filtered and concentrated before being sold as glue. The preparations for cooking and the final manufacturing steps vary with the type of glue.
Manufacturers of hide or bone glue obtain their raw materials from meat packing houses or tanning factories. Hide glue is made by first washing the hides in water. The skins are then soaked in water containing lime to remove nonglue proteins. Next, they are treated with a mild acid and rinsed with water. The rinsed hides are cooked in water in large kettles or tubs. The resulting glue is drained off, filtered, and evaporated. The glue then cools and, in most cases, turns solid. Machines grind the solid glue into grains or powder and then package it for shipping. If the glue is to be sold as a liquid, substances called antigelling agents are added while it is still hot. The antigelling agents keep the glue from solidifying as it cools.
Bone glue is made by first washing the bones in water or dilute acids and crushing them. Next, the bones are cooked in water. The rest of the process resembles that used for hide glues.
Most fish glue is made from washed fish skins. The skins are cooked to form a concentrated broth, which is then cooled and packaged.
Uses of glue.
Industries consume most glue. Many manufacturers of wood products use glue to hold together such items as furniture, toys, and musical instruments. Other manufacturers coat paper, cloth, or plastic with glue to make adhesive tape. Makers of sandpaper use glue to hold the scratchy particles to the paper backing. In the textile and paper industries, glue serves as sizing, a preparation used to stiffen cloth and glaze paper. Many book manufacturers hold pages to bindings with glue.
See also Gelatin.
