Plastics, in the most general sense, are materials that have, under certain conditions, both softened and hardened states. While in a softened state, plastics are moldable..., and once hardened, they retain that molded shape.

Many plastics are softened with heat, and harden when they are cooled. However, there are two classes of plastics: thermolabile plastics (as described above) and thermosetting plastics. The latter are only moldable in the initial phase of their manufacture. Once they are created, thermoset plastics rigidly retain their given shapes, even if heated again. Heat (if of a high enough temperature) only serves to destroy these materials. Bakelite was the first thermosetting plastic (1907). The next big group is the cast phenolics, post-dating 1926.

It is true that glass, when sufficiently heated, becomes plastic (that is, moldable and malleable), and becomes rigid when cooled. However, the ripples in old window panes are not proof of this property. It was recently forwarded that the ripples are actually caused by the type of glassworking that provided the panes. They are not "slowly dripping" nor anything like that.

In addition to the the natural plastics that include amber, copal, and fresh resin or sap, are tar, pitch, lac, asphalt, etc. Glass, like many more modern materials, is an artificial plastic.

Jamey