How did ancient Romans make brass without producing separate zinc metal?

Ancient Romans produced brass—an alloy of copper and zinc—without isolating metallic zinc by using the calamine process.

Because zinc oxide cannot be reduced by carbon until temperatures exceed the metal's boiling point, it was impossible to produce liquid zinc in standard ancient furnaces. Instead, Roman metallurgists heated a mixture of zinc ore (such as calamine or zinc oxide) and charcoal in crucibles covered with lumps of metallic copper.12

Under these reducing conditions, the zinc ore was reduced to a zinc vapour within the crucible. This vapour then diffused directly into the solid copper. Once the zinc had been absorbed, the temperature was increased to melt the resulting brass for casting into ingots. This method allowed the Romans to create brass for various applications, including currency and parade armor, long before the process for isolating metallic zinc was understood.1324