I purchased this pitiful denarius of Severus Alexander for the sum of €8. The coin shows three types of encrustations: iron oxides (the brown deposits), copper oxides (the green deposits) and silver chlorides (the black / dark grey deposits). At the start of the treatment, this already corroded coin weighed 1.92g. I immersed the coin in 30% hydrochloric acid. This is a strong solution, and you should know what you're doing. Hydrochloric acid will dissolve iron- and copperoxides, but it's relatively harmless for silver of high purity. Immediately after submersing the coin, the solution turned yellow: This is a result of dissolved iron- and copper oxides. Note that the coin turned black, which is a reaction of the silver with the chlorides (Ag + HCl > AgCl + H+, i.e. horn silver). At regular intervals (1-2 min), I removed the coin from the solution and dropped it in a glass of tap water. Then, I rinsed the coin with tap water. This is important: if you rinse the coin under flowing water, acidic droplets may cause burns, even worse, eye damage. A very thin layer of AgCl covered the entire coin, slowing the reaction (as the iron oxides were now unexposed, protected to the acid by a thin layer of relatively inert material). This thin layer was disolved using sodium thiosulphate. The entire procedure was repeated five times in total: And some close-ups:" The weight dropped from 1.92 g to 1.71 g: 0.21 g of copper, ironoxides and silverchlorides had been dissolved. In hindsight, it would have been better to first remove most of the deposits by immersing the entire coin, followed by local application of HCl at the more sturdy deposits. Yet, it's nice to have another chemical tool in my toolbox!