The percentage of water to sulphuric acid to be used as a pickle for gun-metal castings depends on the composition of the metal. Try by experiment. A pickle for the outer skin would be 10 of water to 1 of acid; leave in a few hours to remove sand, and finish by dipping in aquafortis and swilling quickly in plenty of water. Dry out in hot sawdust; or dip in hot water and use cold sawdust. In the trade, old dilute aquafortis is used as a pickle for castings, which are left in it overnight and dipped in strong acid afterwards.