This information has been posted quite a few times. At my Yahoo Group for amber enthusiasts, there is a long document about testing amber, that is very useful. See the link below.

"Thermolabile" means the material softens when warmed and hardens when cooled.

"Thermosetting" means that once the material is set, it will no longer soften and become moldable.

Thermolabile plastics have been around since ca. 1869; while thermosets (like Bakelite) were invented in 1906. Of course, MANY "natural plastics" (amber, pitch, any resin, asphalt, etc) would be considered thermolabile too.

Jamey