The manufacturing processes of skin glue or fish glue are similar, while the bone glue manufacturing process is more complicated because it needs the intervention of 8% solution of hydrochloric acid. This solution takes out the calcium phosphate and minerals from bones, keeping the collagen. Then the acid is moved out from collagen and the ossein is formed. From this point, the manufacturing process follows the same steps that are performed in manufacturing processes of skin or fish glue: cooking at controlled temperature, cooling phase and removal of impurities to clean the glue, addition of different substances such that sulfurous acid, phosphoric acid, alum or zinc oxide to form different types of glue, packaging phase.
Glues belong to the family of adhesives and they mostly contain organic compounds. The glue was discovered by ancient tribes that realized that processing the connective tissues of animals yields a sticky substance that can be used to join separate parts together. It was discovered that the glue can be obtained not only processing the connective tissues of animals or fishes but it can also be obtained by processing plants. The glue obtained from plants is called vegetable glue. This vegetable glue can be obtained from starches found in vegetables and grain, or it can be obtained from acacia tree and it is called arabic gum (gum tree).
Several records show that the use of glue is observed on Egyptian, Greek and Roman artifacts. For example, Egyptians made papyrus from glue and the mosaic tiles from roman baths is glued on the floors.
The stages of the process of manufacturing of skin glue are the following:
- the stock is washed in water and lime baths, then the lime is removed by hydrochloric acid
- the stock is boiled in open tanks to convert the collagen into glue
- the first resulted liquid glue is thickened by heating it again
- the removal of impurities from glue is made by chemical or mechanical methods
- finally, the glue can be dropped as beads in a liquid that dries the glue or it can be cooled in blocks