Cerro Rico, the Rich Hill of Potosí, was renown throughout colonial times, and can boast of over 450 years of uninterrupted exploitation.
In colonial times, it is estimated that in an interval of two hours an average of 1200 silver pesos could be manufactured. The coins produced contained 93% silver, which means that in this time 46000 tons of silver could be processed.
Before minting the currency, it was necessary to refine the silver ore extracted from the mines. In the early years of exploitation, silver refining was carried out by smelting, in native furnaces known as wayras or wayrachinas. As the quality of the ore extracted deteriorated, making smelting more difficult, it became necessary to introduce a different method. In 1572, the Viceroy, Francisco de Toledo, introduced the process of amalgamation with mercury, and proposed the use of hyrdraulic force to power the ore-crushing hammers of a series of large-scale ingenios or refineries. For this an artificial water supply had to be constructed for the city.
The construction of a series of reservoirs to supply the city and a water channel feeding the refineries was started in 1572 and were completed two years later. Along the course of the channel, ingenios were constructed with large wooden hammers (almadanetas) to crush the ore into fragments without the addition of water. Some of these ingenios had a capacity of 320 tons per day.
The hydraulic system constructed to feed the rivera de Potosí and its ingenios, and to supply the other needs of the city had no equal in the Americas. Eventually, between 19 and 21 lagoons, or reservoirs, were constructed on the hillside above the city, making use of several natural basins in the folds of the mountains. The lagoons feeding the ingenios had the capacity to generate 355 kW of power, in a vertical fall of 594 metres and with a flow of 160 litres per second and had a storage capacity of 4.1 million tons of water.
There exist many archival documents about the process of silver refining, which was in constant change throughout the colonial period. The process began with the drying of the ore on the slopes of the hillside and then with the crushing of the mineral by the hydraulically powered hammers. This system was invented in Saxony earlier in the sixteenth century. Water entered a narrow channel and powered a water-wheel of between 4 and 6 metres in diameter. Some ingenios had only one head of hammers, while others had two, one on each side of the wheel. Each hammer-bearing head carried between 4 and 6 wooden hammers, tipped with brass or iron. Each hammer rose about 20 cm before falling on the mineral. The fall of the hammers alternated, made possible by the gearing of the water-powered mechanism. One of the most dangerous jobs that Indian forced labourers (mitayos) had to do was to feed the hammers with ore. The dust from this process entered into the lungs causing irreversible damage. Records show that the construction of ingenios changed over the course of time.
Once the mineral had been broken up, it was passed through sieves to ensire it was finely powdered and was then taken to the patios or yards of the ingenio, where the amalgamations vats or buitrones were located. Each vat was divided into several sections or cajones, placed side by side. In these cajones the amalgamation process took place. The finely milled ore (known as harina or ‘flour’) was mixed with mercury from the Huancavelica mines in Peru. With the assistance of catalysts (in different historical periods, copper sulphate, tin, metallic lead, salt and lime were added to the mixture), the silver in the ore formed an amalgam with the mercury (the same substance is used for dental fillings to this day). Indian mitayos had to tread the silver/mercury mixture each day with their bare feet to ensure that the ore and the mercury were mixed thoroughly – these labourers were known as repasiris. Since mercury is a toxic metal, it is reasonable to assume that many of these labourers became ‘mad as hatters’. The process was more-or-less a matter of guesswork, given the level of knowledge of chemistry current at the time, and the refiner would take samples (ensayes) from time to time to see how the mixture was progressing.
After about 15 days, the labourers proceeded to wash the mixture in the cajones. The amalgam that remained was squeezed dry in ponchos and then pressed into wooden moulds with a mallet. Excess mercury passed out of the pressed pellets through a hole in the bottom of the mould.
The lumps of amalgam that came out of the moulds were known as piñas (pineapples). These green mounds were then taken to furnaces where they were heated and the mercury was distilled off. This was another process that was hazardous for the health of the labourers involved. The distillation process left piñas that were 95% pure silver. These were sold to merchants, or to the bank of San Carlos in the eighteenth century.
This is a summary of the mercury amalgamation process as it was practiced in the Villa Imperial de Potosí. It has been the subject of numerous writings by scholars, travellers and the refiners themselves. The process was refined by metallurgists like Juan Francisco Montaro, Domingo Gallegos, Simón de Corona y Origuela, and the metallurgical priest, Álvaro Alonso Barba. Significant treatises on metallurgy have been left to us by refiners such as Alonso Barba, García de Llanos and Juan de Alcalá y Amurrio (who was based in Oruro rather than Potosí).
During a visit to the refinery, you will be able to see the ruins of ingenios and observe the refining process as it is carried out today.