The first glues used were in all probability natural exudations from resinous tree-trunks. tar, bitumen, possibly animal blood and crude forms of casein glue. So-called animal glues, i.e. hide or Scotch glue, fish glues, etc. were known to the Egyptians nearly 4000 years ago, and Figure 31 shows a wall fresco in the tomb of the Pharaoh Rekhamen which depicts carpenters boiling glue over a charcoal fire, spreading glue with a spatula, and sawing and adzing wood. (The terms 'adhesive* and 'glue' have the same meaning and are virtually interchangeable, although traditional practice prefers the term 'glue' when used for bonding paper and wood, and 'adhesive' for bonding all other materials. Animal glues made from hoof, horn and hide are commonly known as 'hide' or 'Scotch' (United Kingdom only) to distinguish them from casein glue which is also animal in origin.)The glue was made much as it is today, by boiling animal skins and bones, etc., but the strained jelly had no keeping qualities, and craftsmen were always forced to make their own glue as and when required until long-storage cake glues were commercially exploited at the beginning of the nineteenth century. Later developments perfected liquid glues by lowering the gel point with acetic acid and improving the storage life in semi-liquid form with suitable preservatives (formaldehyde, etc.), and considerable quantities of this type of glue (Croid, etc.) are still used in industry. The greatest development has been in modern plastic glues which were first used in the 1930s, and once the commonly held belief that good adhesion relied entirely on the mechanical interlocking of porous surfaces with little dowels of glue forced into those surfaces was disproved, then enormous impetus was given to the whole field of research into the principles of adhesion.
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