You’ve probably never tried it at home, but distilling zinc isn’t easy. Heating zinc to its 1675° Fahrenheit melting point doesn’t give you a nice, pure liquid, but a reactive gas. In a standard furnace, you can expect that gas to rise into the air where it will immediately bond with any available oxygen, leaving you zinc-oxide instead of the metal you were after. It’s no surprise that the first patent on this process didn’t arrive until the 18th century in England. Except for the fact that this process was already in use on an industrial scale in India nearly a thousand years before that.
Treatises dated to the first millennium AD have been found that describe a process to handle the tricky process of distilling zinc ore. A ceramic container, combined with water and organic materials, was heated over a simple charcoal fire. With no widely known labels for lab equipment available, the manual describes the ceramic container as looking like an eggplant, and the vessel that would collect the distilled zinc as a thorn apple flower.
Distilling zinc on a massive scale
Proving that the method described was put into practice, archaeological evidence has found that it was scaled up significantly by the 14th century. Near a zinc mine in the Aravalli Hills of Rajasthan, seven furnaces have been found, each containing 36 of the eggplant-shaped containers. At full strength, this would have produced around 55 pounds of distilled zinc a day. Other furnaces dated to the 16th century show continued innovation, with 108 larger vessels that probably produced closer to 110 pounds of zinc a day.
If those numbers weren’t impressive on their own, the craftsmanship of the furnaces and other components indicate that this was a sophisticated, well-managed foundry. While earlier furnaces look hand-made, later models have uniform parts throughout. It’s presumed that the operation of these facilities would have required coordination between many different parties, all the way to the Maharajah himself.
So what made that zinc worth the invention of industrial chemistry? Northern India lacked tin for the most part, which meant that people couldn’t harden their copper into bronze. However, large amounts of zinc allowed for the production of brass. Brass, which can have a yellow color and luster similar to gold, was apparently used for a variety of purposes. Brass artifacts from the Medieval India range from coins, jewelry, utensils, statues and religious icons. Unfortunately, the advanced distillation procedures that enabled much of this production was largely lost after the Mughals invaded India. Eventually, this squashed local production so much that zinc was imported from China and finally after the 18th century, Europe.
Source: The origins of chemical industry by Paul Craddock, Chemistry World