Vermiculite is a natural material and therefore its appearance and physical properties can change if it is exposed to very intense heat for prolonged amounts of time. The color of vermiculite changes when it gets in contact with higher temperatures. A dark, reddish vermiculite can indicate that the stove was overheated.

If vermiculite is overheated it will become:

  • More fragile – it will break easily when exposed to change of temperatures or pressure from the expanding metal inside the fire chamber.
  • More brittle - it will break easily if hit accidentally while loading the stove.
  • Less effective - it will lose its ability to isolate the stove which then will lose its efficiency.