Why is zinc reduction by carbon more difficult than the reduction of copper, lead, or iron?

The reduction of zinc oxide by carbon is more difficult than the reduction of copper, lead, or iron because the reaction only proceeds spontaneously at temperatures above the boiling point of zinc, which is 907 °C (1,665 °F).12

While other common metals can be obtained as molten liquids by heating their oxide ores with charcoal in standard shaft furnaces, the high temperature required for zinc reduction means the metal is produced as a vapour. Because the metal is in a gaseous state, early metallurgists faced significant challenges in collecting and condensing the zinc before it could re-oxidize. Consequently, the furnaces historically developed for smelting other metals were insufficient for producing zinc in quantity, necessitating the development of specialized retort processes and, eventually, advanced techniques like the lead-splash condenser to successfully recover the metal.21