SRD:Hardening

This spell increases the hardness of materials. For every two caster levels, increase by 1 the hardness of the material targeted by the spell. This hardness increase improves only the material’s resistance to damage. Nothing else is modified by the improvement.

The hardening spell does not in any way affect resistance to other forms of transformation.

This spell affects up to 10 cubic feet per level of the spellcaster.

If cast upon a metal or mineral, the volume is reduced to 1 cubic foot per level.