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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
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.
The earliest evidence of use of glue can still be observed in the cave paintings made by our Neanderthal ancestors in Lascaux, France. These early artists wanted their work to last and mixed glue with the paint they used to help the colors resist the moisture of the cave walls. Egyptian artifacts unearthed in their tombs show many uses of glues; perhaps the most striking are the veneers and inlays in wood furniture, which was made using glue as early as 3,000 B.C. The Egyptians also used glue to produce papyrus. Greek and Roman artists used glues extensively; mosaic floors and tiled walls and baths are still intact after thousands of years.
Furniture-making relies heavily on glues. Although there are many techniques for fastening pieces together, glue is often used either permanently or to align pieces while other connections are put in place. All of the great cabinetmakers from the sixteenth through the nineteenth centuries used glue in furniture construction, including Chippendale, Hepplewhite, Duncan Phyfe, the Adams brothers, and Sheraton. The glues used by these cabinet makers were made from animal hides, hooves, and other parts that had been reduced to jelly, then dried. The jelly was ground into power or flakes. It was remixed with water and heated gently in a glue pot. This product was brown, brittle, hard, and not waterproof. Yet this glue was the only glue available until World War I. At that time, casein glues made of milk and nitrocellulose glues were first manufactured.
In the 1930s, advances in the chemical and plastics industries led to development of a wide range of materials called adhesives and plastic or synthetic resin glues. World War II led to a further flowering of this industry when neoprenes, epoxies, and acrylonitriles were invented. These were used by the military and were not available for commercial use until the late 1940s or 1950s. Since that time, highly specialized, waterproof adhesives have been developed for many industries and unique applications including construction of the Space Shuttle. Glues are still used in woodworking and the manufacture of abrasives like sandpaper. They are also used as a colloid in industrial processes; colloids are added to liquids to cause solid particles that are suspended in the liquid to separate out so they can be recovered, either to clean the liquid or process the solids.
Glue manufacturers obtain bones and tissues of animals from slaughterhouses, tanneries, and meat packing companies; it is no coincidence that the world's largest glue manufacturer is the dairy called Borden Company. The animal remains that are the raw materials for glue may include ears, tails, scraps of hide or skin, scrapings from the fleshy sides of hides, tendons, bones, and feet. Similarly, manufacturers of fish glue obtain bones, heads, scales, and skins of fish from canneries and other processing plants.
With only minor variations, the same basic processes are used to make bone glue, hide or skin glue, and fish glue. The hides and other scraps are washed so that dirt is removed, and they are soaked to soften them. This material is called stock, and it is cooked either by boiling it in open tanks or cooking it under pressure in autoclaves. The resulting liquid, called 'glue liquor' is extracted and reheated again to thicken the glue. When cooled, this material looks like jelly and is solid. To remove the impurities and make the glue clear, chemicals like alum or acid followed by egg albumin may be added. These chemicals cause the impurities to precipitate, or fall out, of the glue. The glue is made more concentrated in vacu-.
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