Shellac

Shellac is a resin secreted by a tropical beetle-like bug that was widely used as a binding agent in records during the heyday of 78 rpm discs. The composition of these disks was only about 20 to 25% shellac. The rest was inorganic filler. The shellac in discs is not like the liquid shellac varnish that you brush onto wood. Moisture is a major problem to shellac disks. Moisture tends to encourage the growth of mold and the breaking down of the shellac portion of the disks. If the shellac record is stored in a dry climate -- under 50% relative humidity -- it remains stable. But if there is a higher humidity there is a tendency for the organic material to break down and allow some of the inorganic material to be released -- causing little holes in the surface -- and/or swelling of minute portions of the organic material -- causing little bumps in the surface. The surface noise of the record actually increases! You can see it and, of course, hear it.

The shortage of shellac during World War 2 led to the search for alternative binders and the introduction of vinyl to the disc-manufacturing process. Whereas shellac discs were rigid and breakable, vinyl discs were more flexible and less breakable. With the introduction of the long-playing 33 1/3 rpm disc in 1948, vinyl became the standard binding agent for phonograph discs.