Hardness is a weapon stat, and works as a form of insurance used when refining a weapon. If a weapon has hardness, then it does not break if refinement fails.
Using Hardness[]
A weapon has an initial hardness of 0/2. By adding hardness, the hardness can be increased to 1/2 or 2/2.
If refinement fails, then the weapon does not break, but hardness and maximum hardness decrease by 1.
For example, if a weapon with 1/2 hardness fails refinement, then the hardness will be reduced to 0/1, and the weapon does not break.
The maximum hardness can be increased by 2 once.
Adding Hardness[]
Hardness can be added by using the Dragon Scales item in the Item Mall. It will increase your hardness by 1, and can be used any number of times.
Maximum hardness can be increased by using the Dragon's Tear item in the Item Mall. It will increase the maximum hardness by 2. It can only be used once per weapon.