Mustard comes in 3 forms.
Prepared- Mustard seed and Vinegar (acetic acid)
Dijon- MS and wine (no acid)
Chinese- MS and Water
Mustard seed is unique in that it is hottest when mixed with water.
For patination, it's the acetic acid at about 2.5-3% concentration in the mustard that attacks the metal. Or stains cutlery.
I wouldn't use Muriatic acid aka brick cleaner(weak HCl aka Hydrochloric), though using the fumes as described would certainly give an even patinate. And I would avoid battery acid as well (H2S04 or Sulfuric Acid)
