It's worth mentioning that Hg extraction isn't very complicated. Its the kind of thing that we might expect people fooling around with putting stuff in fires would discover by accident. Just roast the ore (Cinnabar, HgS) at the relatively modest temperature of 350˚C or higher and the mercury will evaporate off, likely condensing on a nearby surface. Bam, liquid mercury. Control the condensation and you've got liquid mercury on a large scale.

By comparison, smelting iron is much more complicated (higher temperatures + it's a chemical reaction not just "bake and go" + residual carbon content must be controlled for strength + annealing strategy significantly affects its desirable properties).