Augment Object (Spell)

Transmutation
Level: Clr 3, Drd 3, Sor/Wiz 3
Components: V, S, M
Casting Time: 1 minute
Range: Close (25 ft. + 5 ft./2 levels)
Target: Object of up to 200 cubic ft./level
Duration: 1 day/level
Saving Throw: None (Object)
Spell Resistance: Yes (Object)
This spell adds +20 to the break DC of the object and doubles its hardness and hit points. An augmented object also gains a saving throw against spells when unattended as if it were a magic item (save bonus equals 2 + half the caster level).
Material Component: A pinch of chalk.

Source: Stronghold Builder's Guide