Almost a quarter of the world’s gold supply can be traced back to 10-15
million poor small-scale gold miners, or ASM, scattered about the globe.
These miners are also the third largest source of mercury pollution
today, however, comprising about 30% of the world’s anthropogenic
mercury releases. ASM miners extract gold-laden rock, grind it into a
fine sand, then mix it with mercury to form an amalgam. The amalgam is
then subjected to high heat, either with a blow torch or over an open
flame, and the mercury evaporates into the atmosphere leaving behind a
substance that is roughly 80% pure gold.
http://www.blacksmithinstitute.org/projects/display/154
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