Why do zinc compounds containing silver and gold float on molten lead?

Zinc compounds containing silver and gold float on molten lead because they are less dense than the lead itself.1

In the Parkes zinc-desilvering process, small quantities of zinc are added to molten lead bullion. Because gold and silver are more soluble in zinc than in lead, they react with the zinc to form specific compounds. When the mixture is cooled to a temperature below 370 °C (700 °F)—but kept above the melting point of lead—these zinc-precious metal compounds solidify into a crust that is lighter than the surrounding molten lead. This density difference causes the crust to rise to the surface, where it can be easily skimmed off for further processing and recovery of the precious metals.12