Tin, lead, and the alloys of these metals are much more difficult to cleanse than zinc. A rapid scouring with potash lye and a rubbing with a hard substance are the only means of effecting this. The objects are sometimes plunged into diluted hydrochloric acid; but the first operation is nearly always necessary. Notwithstanding the greatest care, the direct deposit of the precious metals is difficult, and does not adhere well. The results are much better if a coat of pure copper or brass is interposed between the low metal and the gold or silver.