Having a jug of muriatic acid leftover from a masonry project, I decided to test its effects on a few of my spare dice. Sources I found online said that muriatic acid (which is actually hydrochloric acid) blackened and dulled stainless steel and hastened oxidation in bronze.
I dropped the two test pieces into a glass of muriatic acid and let them stew for two hours. There was a fizzy bubbling and the acid turned lime green. Fumes of a foul nature were released. NOTE: If you are going to try this: 1. Wear eye protection and gloves, and 2. Do this outside or somewhere else with excellent ventilation.
After removing the pieces from the acid, rinsing them, and letting them dry, they had acquired a rosy pinkish hue. After a week, the oxidation has deepened to a darker copper tone.
The first piece, my Thorn Die20, has a lot of exposed surface area and darkened quite a bit more than my prototype Overstuffed Die20.
This method of producing a patina on the stainless steel material is a good way to make them look antiqued if you are comfortable working with powerful acids. If you are not comfortable working with acids, a patina with flame as described in other threads in this section produces a similar effect.