Glue is a part of larger family known as adhesives. The two classes are determined by the fact that glue is produced from organic compounds, and on the other hand, adhesives are typically chemical based. Epoxies, sealants or caulks are adhering materials that have additives that will provide them with properties for particular applications.
With only few variations, the same basic processes are used in making hide or skin glue, fish glue and bone glue. Hides and other scraps are washed first to removed the dirt and are soaked to make them soft. The stock is passed through a number of water baths wherein there is more lime added to produce the skins and hides swell and them break then down. The swollen hides are then rinsed in large washing machine to remove the lime. Treating the stock with weak acids such as hydrochloric or acetic acid will remove the last traces of lime. The stock is cooked by boiling or cooking under pressure in autoclaves.
The collagen breaks down and converts into glue only if the stock is cooked at the right temperature and the correct amount of time. If the timing and temperature is not correct, the quality of the glue will not be good. Once it cools, the material is solid and appears like jelly and it can contain impurities. To make the glue clear and to expel the impurities, there are chemicals to be added like acid or alum, followed by egg albumin. This will cause the precipitation of the impurities. Other manufacturers are using mechanical methods in cleaning the glue that includes passing the glue in a series of paper filters or mechanical filters.
To make the glue clear, white or blue, different additives are incorporated with the liquid glue. These additives include phosphoric acid, alum and sulfurous acid. Zinc oxide is added when white school glue is to be produced.
The glue becomes runny and weak liquid at this point so they need to be sent to vacuum evaporators to make it more concentrated and dried using one of the several methods. Then the glue is chilled in blocks or sheets and then set aside on nets to make it dry, but retain its concentration.