Mandi Folger, age 15, of Reno, Nev., for her question:
HOW IS GLUE MADE?
There are three types of true glue: hide or skin glue, bone glue and fish glue. All are impure forms of gelatin. The sticky substance is used in a number of different ways commercially. Other materials are sometimes called glues but technically they are usually gums, cements or adhesives.
Glue manufacturers use skin, bones and connective tissues of animals to make their product. They usually obtain raw material from slaughterhouses, meat packers and tanneries.
To make glue, a manufacturer will usually use scraps of animal hide, ears, tails, tendons, feet and bones. A similar procedure is used to make the glue whether it is made from bones, fish or hides.
The ingredients are heated in a cooking operation and then treated to remove impurities.
True glues result from the breaking down of protein matter in animal tissues and bones. When these tissues are heated in water, part of their structure dissolves into a clear solution. After the solution cools, it forms a solid, jellylike mass, callled gelatin.
The gelatin obtained in the glue process is like food gelatin, but it is not pure. Completely dry gelatin is a transparent mass that will dissolve in water to form a solution with great adhesive capacity.
Manufacturers use true glue as a sizing for hats, textiles and paper. Furniture makers use it to hold joints together, or to reinforce them. Bookbinders use glue to hold together the bindings and the pages of books.
In a glue factory, either chemical or mechanical methods may be used to obtain clear glue. Alum or an acid may be added, followed by egg albumin. These form a precipitate that removes the unwanted particles and leaves glue clear. Some producers use mechanical filters in which the solution filters through paper or bone char.
After the filtering process, the glue runs into pans for drying. Next machines grind the dry glue and package it for shipping. People who use this glue steep it in water until it is soft and then they place it in glue pots.
Liquid glue not in need of heating is the type usually used in homes.
In making glue from hides, the skins are first washed to remove dirt and then soaked until they are soft. Successive baths in water containing increasing amounts of lime make the hides swell. Washing machines remove most of the lime. Then the hides are ready for cooking.
Bones are washed and the grease is removed before cooking. They may be broken up by an acid gas or a liquid. Or sometimes the bones are crushed under pressure. Fish parts are washed and then crushed or shredded.
Cooking is usually done in an open tank. But pressure tanks are widely used for untreated bones. The tanks consist of large tubes heated by steam coils. The ingredients are placed in the tubs, covered with water and cooked until the liquid becomes concentrated.