Why was producing metallic zinc difficult in ancient furnaces?

Producing metallic zinc in ancient furnaces was difficult because the reduction of zinc oxide by carbon only occurs at temperatures well above the metal's boiling point of 907 °C (1,665 °F).12

In standard shaft furnaces used for other metals like copper or iron, the zinc produced during the reduction process immediately vaporized. Because ancient metallurgists lacked efficient methods to condense and collect this zinc vapour, the metal would escape or re-oxidize rather than forming a liquid pool.12

While the Romans produced brass, an alloy of zinc and copper, by heating zinc oxide and charcoal with copper in crucibles, the zinc vapour was absorbed directly into the copper and did not appear as a separate metallic phase. It was not until the 13th or 14th century, likely in India, that techniques were developed to successfully produce and condense metallic zinc in quantity.12