Tracey Nash, age 10, of Fountain Valley, Calif., for her question:
HOW IS GLUE MADE?
Glue is called on to do lots of important jobs. Manufacturers use it as a sizing for paper and textiles. Furniture makers use glue to hold joints together or to reinforce them. Bookbinders use lots of glue. The sticky stuff is used to hold together the bindings and the pages of books and many magazines. Liquid glue is often used in homes for minor repair chores.
Sticky glue that is used to hold things together is made from the skin, bones and connective tissues of animals. There are three types of true glue: hide or skin glue, bone glue and fish glue.
All true glues are impure forms of gelatin. Other materials are sometimes called glues, but technically these are gums, cements or adhesives.
True glue results from the breakdown of protein matter in animal tissue and bones.
Manufacturers of glue usually obtain their raw material from slaughterhouse s, meat packers and tanneries. Scraps of hide, ears, tails, tendons, feet and bones can be used. Fish heads, bones, scalew and skins are also used.
Manufacturers use just about the same methods whether the glue is made from bones, fish or hides. They prepare the ingredients, heat the stock, treat it to remove impurities and then dry the solution.
Hides are washed to remove dirt and then soaked until they are soft. Bones are also washed, and sometimes they are crushed under pressure.
The material is then cooked. This process breaks down the protein matter in tissues and changes it to glue. Cooking too long at temperatures that are too high destroys the quality of the glue, so this part of the operation must be carefully controlled.
Cooking is usually done in an open tank. Then either a chemical or mechanical method is used to clear the glue. They may add alum or an acid and follow it with egg albumin. These form a precipitate that removes the unwanted particles and leaves the glue clear.
Some manufacturers use mechanical filters in which the solution filters through paper or bone char. The process ends with drying and then grinding the dry glue before it is packed for shipping.
People who use this true glue steep it in water until it is soft, and place it in a glue pot containing hot water.
Liquid glue that does not need heating is the type usually used in homes.