You don't have to be "mad as a hatter" to know that humans aren't always very smart about using chemical processes without proper respect for the risks. Care in using chemicals to recover silver from South American mines was probably not improved by the fact that slave labor could be forced to use the more efficient but more polluting processes introduced by Spanish conquistadors.
The native Incan people were already mining and refining silver when the Spanish arrived, but the mercury amalgamation process introduced by the newcomers greatly increased productivity -- at the cost of sending billowing clouds of lead dust hundreds of miles from the mountaintop mines in what is now Bolivia into the Andes mountains in what is now Peru.
The largest tropical glacier, known now as the Quelccaya Ice Cap, froze those layers of lead dust. The layer of lead dust bears the chemical signature of the silver ores from the Potosí mines. Comparisons with layers of mining residues in peat bogs and lake sedimentation further reinforce the identification of the source of the one-time air pollution in the ice core.
Paolo Gabrielli, a research scientist at the Byrd Polar and Climate Research Center at Ohio State and corresponding author of the study, says:
"This evidence supports the idea that human impact on the environment was widespread even before the industrial revolution"
If you are following the debate on whether it is time to crown our current age as the Anthropocene, or “Age of Humans,” you will see that evidence of this sort contributes to the idea that human influence on the environment represents a new driving force in the evolution of our planet. This study proves that the Anthropocene has been going on for some time already, even if the amount of pollution humans created in the 20th century vastly outweighs all of what came before.