Dwarven Reinforcement Level 5
Prerequisites: expert in Crafting
You can use your knowledge of engineering and metalwork to temporarily strengthen thick objects and structures. By spending 1 hour working on an item, you can give it a +1 circumstance bonus to its Hardness for 24 hours. If you're a master in Crafting, the bonus is +2, and if you're legendary, the bonus is +3. You can reinforce a portion of a structure, though 1 hour usually reinforces only a door, a few windows, or another section that fits within a 10-foot cube.
Source: Advanced Player's Guide pg. 42