Copper doors can be used as furnace fuel

Copper doors can be used as fuel for furnaces.

Affects all variants (oxidized, waxed,...).

Steps to reproduce:

1. Open furnace
2. Put any type of copper doors into the fuel slot
3. put something to smelt in the furnacme

Observed behavior:

• copper doors can be used as fule

Expected behavior:

• it should not be possible to use copper doors as fuel