The word "glue" comes from Old French and Late Latin verbs meaning "to draw together" (Websters Abridged). The idea of using a substance to hold things together has a long history. On the walls of Egyptian tombs that are more than 3000 years old, archeologists found a painting of workmen using glue! (Ramses II, in 1213BC and Tutankhamun)
Since the meaning of "glue" is "that which sticks objects together," many substances can be categorized as "glues." For example, there are mineral glues, vegetable glues, marine glues, and new synthetic glues.
The traditional glue is made from bones, sinews, and the hides of animals. (No lizard intestines or dragon scales...) These are properly prepared, heated, and then the solution is dried. In fact, a cooked chicken that is refrigerated will develop a jelly on the surface. This is simply an impure form of glue.
When glue is prepared commercially, the hot solution is filtered, clarified, and evaporated. The concentrate is then dried. Another method is to alow the solutions to chill into a firm jelly. The jelly is cut into thin slices that are then spread on nets. Workers place the nets in a drying tunnel for a few hours, and then you have glue.
Glue does not only hold things together. One little known use of glue is to give new paper money its "crackle" when mauled. The sheet of paper gets coated in a very fine layer of glue to keep it from bending and ripping too easily.
Glue, being made of natural substances, decays quickly. For this reason, preservatives are added. However, a standing solution of glue laying around will quickly mold and rot, developing a rancid and vile odor. This is why it should be freshly prepared. Glue is sold in a granulated form, which can be purchased in bulk.
The Big Book of 'Tell Me Why', by Arkady Leokum
Websters Abridged Dictionary
Elmer's Glue (www.elmers.com)