Crude lead often carries small amounts of dissolved silver and gold that are too valuable to throw away. Zinc is stirred into the molten lead because these precious metals prefer dissolving in zinc far more than staying in lead, and zinc itself barely mixes with lead at the temperatures used. As the melt cools slightly, the zinc, now loaded with silver and gold, solidifies into a crust that floats above the lead and gets skimmed off, after which the zinc is distilled away to leave a concentrated silver-gold alloy behind. This selective pulling out of precious metals using zinc is exactly what the Parkes process does, it is not aimed at nickel, copper, iron, bismuth or antimony. So it removes silver and gold.