Restraint Cuff (Charm Version, 15mm, AS)Restraint Cuff (Charm Version, 15mm, AS) from Parts of Four. AS: ACID SILVER. Brass substrate is electroplated with silver and then dipped into acid to create the subtly destroyed surface. The concentration of the plating solution, air temperature and time in the acid bath all play integral roles in the final result and exist in a delicate balance. No two items are the same and these nuances variations should be seen as a characteristic of the