The Spanish Empire used a forced labor system called "
Repartimiento de Indios" (also known as "
Repartimiento") to extract silver from Cerro Rico, though in 1574 the Spanish Viceroy
Francisco de Toledo reintroduced the
mita, an annual forced labor system used by the
Inca Empire. During the first decades of extraction, the mines in Potosí had vast deposits of pure silver and
silver chloride deposits, which made the extraction of silver relatively easy. Native American labor in the Andean regions was preferred by the Spanish Crown, as opposed to
African slave labor, due to high mortality and low productivity rates. By 1565, Cerro Rico had run out of high-grade silver ores. Extraction began anew after the introduction of a silver extraction method known as
patio process, using mercury to form silver amalgams and extract silver from low-grade ores. These ores of lower grade were known as
paco in reference to their ocre colour. These corresponded to
oxidized hypogene sulphides
above the water table.
Repartimiento was also a system of cyclical labor, so after their required time was done, many Amerindians would continue working at the mines as free wage laborers or
mingas, despite the harsh conditions. Given the use of mercury and the high amount of silver extracted from the mines,
mercury poisoning among the Amerindian laborers was common, which caused many miners to die. Other harsh conditions at both the mines and refining patios also caused the deaths of miners during the Spanish rule, and it is believed that around eight million miners died in total. However, other sources estimate that it was "hundreds of thousands," and that eight million deaths were actually the total number of deaths in the Viceroyalty of Peru, not just the mines in Potosí. It is known as the "mountain that eats men" because of the large number of workers who died in the mines. The work of historians such as Peter Bakewell,
Enrique Tandeter and
Raquel Gil Montero portray a more accurate description of the human-labor issue (free and non-free workers) with completely different estimates. == Bolivian Mining Cooperative ==