Occupational health litigation and the development of occupational hygiene Slavery Part 2B Gold, silver and the Atlantic slave trade (New Spain continued)





DW Stanton


Correspondence: Dr David W Stanton, 401 Queensgate, 2 Queens Road, Parktown, South Africa, 2193. e-mail: davidws@icloud.com


“The deeper the mine, the more fearful these risks become. The walls give way, or the rocks fall, and crush numbers; shocks and explosions are frequent; … A mine is a damp, close and dark cavern, where noxious vapours are the only atmosphere: and the hazards incurred in ascending and descending, and from the falling in of the ground, are fearful: the workmen are half naked, grisly, and armed with heavy bars, or loaded with piles of ore; and sickness and fluxes are frequent.” 1


Lawyer and scholar Francisco Xavier de Gamboa, 1761

Selected text from a longer description of the working conditions at the Spanish silver mines of New Spain


“Some of the numerous problems associated with deep mining, such as drainage of water

from low levels, difficulty of ventilation at great depths, lack of sufficient timber for extensive reinforcement of deep mines, proved almost beyond Spanish technical knowledge to solve.” 2

Professor Robert Cooper West, expert on mining in Latin America, commenting in 1949 on the silver mines of New Spain



Part 1 of this paper on slavery reviewed Slavery from Ancient Times and provided the introduction to the series of papers on slavery.3 Part 2A introduced The North Atlantic slave trade and gold mining under the Spanish on the island of Hispaniola;4 Part 2B on Spanish America started by exploring slavery in the context of gold and silver mining in New Spain (Mexico);5 and continues in this paper by addressing working conditions. Further sub-sections of this paper will discuss the associated mercury mines of Almadén, Spain; the Potosí silver mines in colonial Peru, and the mercury mines at Huancavelica. Efforts to improve working conditions at the Spanish gold, silver and associated mercury mines during the colonial period will be covered in the final part of Paper 2B.




As in other locations, unhealthy and dangerous working conditions occurred in Spanish America throughout colonial rule.6,7 Most gold was mined from alluvial deposits in low wet areas. Workers often spent long periods in water, and were exposed to tropical diseases.6 Conversely, the silver veins were often formed at great height: almost all above 3 000 m in Peru and 1 800-2 400 m in New Spain.6 Some early Spanish mine excavations in New Spain did start as gold mines but changed to silver at the hundred-foot (30 m) level.8

In 1531, Quiroga, a royal auditor, reported to the Council of the Indies on the orphans in the monasteries, whose parents had perished in the mines of New Spain: “They are numerous, as the stars of heaven and the sands of the sea.” 9,10


The ten plagues

The missionary, Fray Toribio de Benavente, arrived in New Spain in 1524, less than three years after the fall of the Aztec capital, Tenochtitlán.7,11-14 Influenced by the Biblical ‘ten plagues of Egypt’, he described what he considered to be the ‘Ten Plagues of New Spain’: 1) smallpox; 2) slaughter during the conquest; 3) a great famine following the fall of the city of Tenochtitlán; 4) Indian and African overseers; 5) excessive tribute and services demanded from the Indians; 6) gold mines and the craving for gold; 7) rebuilding the ‘great city of Mexico’; 8) enslavement of Indians to work in the mines; 9) transport service for the mines; and 10) dissensions among the Spaniards.

The fatal effect of the gold mines on the Indians9 is illustrated in the descriptions of the sixth, eighth and ninth plagues: “... for in addition to the taxes and tributes paid by the towns which had been granted to the Spaniards, the latter began to seek for mines, and it would be impossible to count the number of Indians who have, up to the present day, died in these mines. Gold of this country was a second golden calf, worshipped as a god, for they came all the way from Castile through many dangers and difficulties to adore it. Now that they have it, please God it may not be to their damnation.” (sixth plague) 13 “So great was their haste, in some years, to make slaves that from all parts of Mexico they brought in great herds of them, like flocks of sheep, in order to brand them... The fact that... branding was cheap produced so many marks on their faces, in addition to the royal brand, that they had their faces covered with letters, for they bore the marks of all who had bought and sold them.” (eighth plague) 13

With regard to the ninth plague, Indians travelled 70 leagues (± 400 km) and more to bring provisions to the mines. On arrival, the Spanish mine masters would detain them for several days to work. The provisions they had brought for themselves were soon exhausted, and then they starved as no one would give them food and they had no money to buy it. Consequently, some died on the way to the mines, at the mines, on the way back, and just after returning home. At the mines of Oaxaca, where gold mining had occurred before the Spanish conquest, the numbers of deaths were particularly high: “For half a league around these mines and along a great part of the road one could scarcely avoid walking over dead bodies or bones, and the flocks of birds and crows that came to feed upon the corpses were so numerous that they darkened the sun, so that many villages along the road and in the district were deserted. Other Indians fled to the woods, abandoning their houses and fields.” 13

In his letter of 1529 to Charles V, Fray Zumárraga wrote: “... I do say that this immoderate loading is diminishing them very rapidly. And it is necessary that Your Majesty remedy it, because otherwise the end of this country will soon be seen, like that of the islands of Española, Cuba, and others, for this loading was the principal cause of their ruin…” 15

Gamboa (1717-1794), a lawyer who spent almost 15 years litigating mining cases in the courts of Mexico City,16 wrote this in his book on the Mining Ordinances of New Spain: “It is true that the cruelties of many persons at the time of the conquest, and perhaps of not a few, even at the present time, in respect of the mines and in the pursuit of treasure, are such as the ear will not endure; but the persons who perpetrate them, are neither sanctioned by the nation, nor permitted by the laws to escape with impunity…”17


Few developments in mining technology

For silver mining, colonial miners exploited an open pit and followed veins of ore that twisted deeper into the earth.18 As holes in the ground became shafts, drifts and adits (gradually sloping tunnels), and mining continued to be carried out by inexperienced adventurers, mining methods had to be learned by trial and error. Many Indians and Africans died as lessons were learned by accidents, cave-ins, flooding and similar mishaps.19 Extracting silver-bearing ore from narrow veins relied on pick- and bar-wielding labourers; human carriers brought the rock to the surface in heavy sacks on their shoulders.20 Back-breaking work with heavy tools and brute force to extract silver ore, overseen by a slave driver with a multiple stranded whip, is depicted in a horizontal panel in grisaille (painted to look as if it is carved) by Diego Rivera (Figure 1).

Figure 1. The Work in Silver Mines. Horizontal panel in grisaille, by Diego Rivera – Historia de Morelos: Conquista y Revolución, 1929-30, Palacio de Cortés, Cuernavaca Photograph: Dave Cooksey, 2010 - México City, Flickr


Gary Jennings, who specialised in historical fiction, spent 10 years in Mexico researching his Aztec novels. His third book, Aztec Blood, provides some information on conditions in the northern silver mines at the opening of the 17th century, through the eyes of a fictional mine slave. Cristo, a mestizo, was sentenced for life; “but since few survived more than a year in the mines, a life sentence was no great matter.” 21 Cave-ins occurred constantly and many slaves died “the first time their pick strikes” as mine owners used minimal timber-shoring because of the expense: “Vast quantities of timber were required in the smelting process, and the wood had to be hauled over great distances. It was cheaper to replace workers than pay for timber.”

Progress in the advancement of mining methods was slow. The horse- or mule-powered whim (malacate), employed for lifting ore, water and rubbish in bags suspended by ropes, did not become widely used until the 18th century.20 Blasting powder, employed in Hungary in 1627, 22 was not used in New Spain until 1703.19

The Spanish period of mining came to an end with the War of Independence (1810-1821),19 during which time many of the mines were abandoned, went into decay, caved in, and/or filled with water. A number of mining archives, going back some three centuries, were destroyed.23 In 1823, the new Republic decreed a mining law which allowed foreigners to enter into mine development and abolished many of the prohibitory taxes and Spanish monopolies.8 English capitalists began to invest heavily in the mines of Mexico, followed by the Germans and Americans. Cornish miners arrived with their mining expertise and culture, and it was believed that the application of steam power to drain mines, raise ore to the surface in kibbles (iron buckets), and crush the ore, would revolutionise mining. ‡

Cornishman Captain Garby, in a letter dated 1824,24 stated that there were many of the mines, from 90 - 270 m in depth “that have no shaft or machinery for discharging either water or stuff, but all accomplished by manual labour on men’s shoulders.” Only in the deeper mines was the animal-powered whim used. The largest and deepest mine was La Valenciana which had very large entrance galleries but, according to Garby, “in this immense mine there is not a single level that you can put a barrow through” to move ore from the face.24 In the network of very small passages the underground workers would have needed to crawl in near pitch-blackness.25 Thomas Brocklehurst, on his visits to the silver mines of Pachuca in 1881, reported, “From the galleries of the lowest levels we ascended by vertical ladders to the upper ones and we had to creep through holes barely large enough to admit us.” 26

For more than three centuries Indian and black slaves, and forced and free workers, would have faced dangers such as deep narrow shafts, long narrow tunnels, wooden ladders and supports, and limited-use tools which amounted to very hard labour, flooding and poor mine ventilation.27 The usual 12-hour shifts (sol a sol) in the colonial mines “led to fatigue and decrease in resistance to contagion.” 2 Once they had completed the excavations, they had to bring the ores to the surface by hand, up narrow ladders and over flimsy rope bridges suspended above underground chasms.28


Thermal environment

If they arrived safely at the surface, those emerging from warm humid shafts into the cold air, even at moderate altitudes, could catch colds and pneumonia. Many poorly clothed tenateros (those who carried the ore in bags on their backs to the surface) died in this way.2 The temperature at the bottom of Valenciana mine (with a perpendicular depth of 513 m) was measured by Humboldt at 34° C. As the temperature at the mouth of the mine in the open air in winter (at an altitude of just over 2 000 m) could drop to 4 or 5° C, tenateros could be exposed to a change of temperature of 30° C or more.29


Heavy ore loads

Humboldt criticised many aspects of mining in New Spain (and Peru): the waste of blasting powder, the erroneous choices of mine ventilation, and neglect of the subterraneous geometry, until the establishment of the School of Mines in Mexico City in 1792. On the enormous expenses of ore transportation at the Valenciana mine, with the large numbers of men employed to carry the ore to the surface on their shoulders, he commented they “remain continually loaded for six hours with a weight of from 225 to 350 pounds [102-159 kg], and constantly exposed to a very high temperature, ascending eight or ten times successively, without intermission, stairs of 1800 steps.” 29 He considered that “Well contrived operations would facilitate the extraction of minerals and the circulation of air, and would render this great number of tenateros unnecessary, whose strength might be employed in a manner more advantageous to society, and less hurtful to the health of the individual.” 30

Figure 2 illustrates tenateros employed at a Central American silver mine carrying ore loads of around 57 kg on their shoulders in the 1850s.


Figure 2. Use of tenateros (ore carriers) at a silver mine in Honduras Source: Harper’s New Monthly Magazine, May 1856


Poor mine ventilation

West2 reported that only crude methods were utilised. At Parral mines, small shafts were sunk from the surface to ventilate the upper levels, and air shafts were constructed between levels. As the forcing of air by means of bellows, water devices, or heating machines was seldom practised, working conditions were poor in the lower levels where the air became stagnant and fouled by toxic gases from the burning of candles and fire-setting.2 While the candles lit the way, they also created carbon monoxide and smoke. In poorly ventilated areas, miners could have died directly from carbon monoxide poisoning or suffered injury from falling after becoming unconscious. In around 1730, the silver mines of Zacatecas consumed some 80 tonnes of candle wax and more than 1.5 tonnes of wick, annually.20


High dust exposures

Work in the silver mines was gruelling; miners dug, loaded and hauled rocks in near darkness by candle light for days at a time.31 Through the 16th and 17th centuries, those breaking ore with picks and bars would have been exposed to very high dust levels which would have increased with the introduction of blasting in the 18th century, producing suffocating clouds of dust and cave-ins. Fine airborne dust, containing respirable quartz (crystalline silica),* would have travelled throughout the mines, affecting all those underground.

Humboldt stated that: “This occupation of Tenateros is accounted unhealthy, if they enter more than three times a week into the mines.”29 But the labour which ruined the most robust constitutions was that of the barenadores, who blew up the rock with powder:“These men rarely pass the age of 35, if from a thirst of gain they continue their severe labour for the whole week. They generally pass no more than five or six years at this occupation, and then betake themselves to other employments less injurious to health.”

Doris Ladd, in her review of the miners’ strike at Real del Monte in 1766 and associated working conditions, described the ‘bitter wages’ of death and disability, either by silicosis or mining accidents.33 Respirable dust levels were further exacerbated by the introduction of compressed air rock drills for drilling holes for explosive charges. This led to the first cases of silicosis being recognised at Real del Monte in 1898.34



Around 1536, Germans skilled in mining techniques brought the knowledge of smelting silver ores. Following their arrival, the shipping of mined silver to Spain began to assume importance.35 Detailed information on silver refining and associated environmental contamination with lead and mercury is provided by Saúl Guerrero.36,37 During the colonial period, almost 40% of silver was refined by smelting in the presence of lead, and just over 60% was obtained by amalgamation with mercury.


Smelting with lead

Sophisticated processing of silver ore to extract silver is attributed to the Chaldeans in about 2500 BC, using the ‘cupellation’ process.38 From 650 to 350 BC, the Athenians extracted 7 000 tonnes of silver, and more than two million tonnes of lead from the large deposits at Laurium, using slave labour.39 Early methods of separation were inefficient; more than one-third of the silver mined by the Athenians was lost in the slag.40

In New Spain, smelting of lead sulphide ores with lead was conducted under high temperatures and reducing conditions to produce a silver-rich lead with worthless gangue components (unwanted mineral) separated in the slag. In the second step (cupellation or refining stage), oxygen was blown onto the molten lead in a cupel (shallow, porous container) to form litharge (lead oxide) which entrained the majority of other metals, leaving the silver. Litharge (‘greta’ in Spanish text) was skimmed off to be recycled. That absorbed by the bottom of the cupel (made from bone ash or the ashes of plants) to form ‘cendrada’, was also recycled to recover any silver lost in the ash. The health hazards of this process were mentioned by Bartolomé de Medina in the mid-16th century: “And so I have seen how such ores are processed in many places using greta and cendrada and with great cost to the owners of the mines and with great risk to the life and health of those involved in their processing, both of Indians and Africans.” 36,41


High lead exposures

Workers at the smelting and refining furnaces would have been exposed to airborne lead fumes, and the fumes and smoke would have drifted throughout the refining hacienda, affecting the entire workforce. Exposure to airborne lead dust would have occurred when handling the bars of molten lead and associated slag, working with greta or cendrada, and from wind-borne dust from the stockpiles of lead-containing material within the compound. Hands, arms, hair, clothing and food would have been contaminated. Wives working as crushers and/or washing contaminated clothes would have been exposed to lead dust, as would the children playing in the dirt of these compounds.36 Gamboa wrote that, between smelting runs, the inside of the furnace was scraped with a crow or iron bar and “here the unfortunate smelters suffer much, during an hour of great labour; for the furnace is hot in the extreme, the crow is heavy, and the incrusted matter adheres very closely.” The “smoke and vapour from the slag”, quenched by water, was poisonous and the smelter workmen were subject to “violent pains in the stomach.” 1,36

In addition to the lead health hazard caused through inhalation and ingestion at the workplace and living quarters, extensive lead pollution from the chimney stacks affected local woodlands, livestock and surrounding communities.36 Woodlands were also depleted as charcoal was required as fuel for smelting.


Amalgamation with mercury

The use of mercury for amalgamation with gold goes back to antiquity.7,42 It was used in the sub-Saharan gold fields in Africa from the 12th century, and on an industrial scale well before its use in New Spain.43 From 1460 to 1485, eight tonnes of African gold was produced via amalgamation in Egypt and north Africa, with an additional three tonnes produced in Europe, consuming a total of 45 tonnes of mercury from Almadén.36,43

Amalgamation of silver ores is attributed to the Venetians as early as 1480.36,44 and was adopted for use with silver ores in New Spain by Medina.7,44-47 The so-called patio process, where the amalgamation slurry was spread out in tortas (cakes) placed in a courtyard (patio), was used from the 1550s, and quickly spread through the Spanish New World. Silver was extracted from finely ground silver ores by mixing saltwater brine and a magistral, or reagent (usually copper or iron pyrite), with mercury to form the torta. The salt started a chemical reaction in which the reagent released the silver from the ore to form an amalgam with the mercury,44 which could be heated to recover the silver, and the mercury for reuse. Amalgamation remained substantially unchanged until the end of the 19th century when it was replaced by the cyanide process.


High dust exposures

In the production of finely ground ore, the mills were very dusty places and mill workers would have suffered from acute and chronic silicosis and silico-tuberculosis. Water or animal-driven stamping mills lifted iron ‘heads’ weighing up to 68 kg to drop and crush the concentrate to sand-like consistency on stone platforms,18 or in a large wooden mortar, well lined with iron.1 The ore, when reduced to powder, was passed through sieves of iron wire.1 A 1681 report on Pachuca indicated that some of the Indians “have been made so sick from the dust of the mills that they have died from it.” A 1737 report indicated that, of the repartimiento Indians sent to the mines of Taxco, some worked in the mills from sunup to evening prayers, “with the result that many of them die vomiting blood.” 2,48 To address the dust problem, now commonly known as silicosis, José de Gálvez, the Visitor-General of New Spain (1765-1771), suggested that research should show ways of avoiding this by using water and carrying on the processes in enclosed troughs instead of in the open.49 Humboldt pointed out that, in some great amalgamation works of New Spain, the arrastras for wet grinding (circular mills of ancient origin introduced into New Spain about the same time as the patio process in the 1550s) were unknown and the dry stamping and sieving process was the norm. He reported that unequal and coarse-grain crushed ore did not amalgamate well and “...the health of the workmen suffers greatly, in a place where a cloud of metallick dust is perpetually flying about.” 30 In his letter of 1824, Captain Garby described the dry stamping of the ore with mills carrying eight heads of 34 - 45 kg worked by four mules at a time. In 24 hours, about five tons of ore could be crushed. This was followed by a wet grinding process using 20 arrastras, each with four large stones dragged around in a circular pit by two mules at a time, changing every six hours, to produce an ore slime in 24 hours. The slime was then drained to a consistency of a thick paste for amalgamation. To pulverise the five tons of ore required around 100 mules and 30 men.24

Figure 3 shows some examples of arrastras from small operations. At the top, a mule-powered arrastra with a rotating wheel moving in ever-expanding circles at Villa Alta in the late 1700s is depicted. The sketch in the centre shows an arrastra using a large flat stone that was dragged around in a circular pit to crush and grind ore. The lower images are of a Honduran hammer boy breaking up ore into a size suitable for use in an arrastra, and of a ‘Chilean Mill’, a development of the arrastra, with two round stone wheels driven by an overhead geared mechanism powered by water wheels.


Figure 3. Examples of mule- and water-powered arrastras Sources: Figura No. 2, 2014; Harper’s New Monthly Magazine, May 1856 and April 1860


Mercury exposure

Mercury exposure would have occurred during various stages of the amalgamation process conducted outdoors, such as the manual addition of mercury to each amalgamation torta, and mixing by men treading the torta. Higher mercury vapour exposures would have occurred when the amalgam was squeezed through cloth to force out the excess mercury, particularly during the heating stage to separate the silver and mercury from the amalgam, and in the final casting of the silver bars when any remaining mercury was volatilised. The workers did not use gloves or other personal protective equipment to avoid skin contact with mercury.

Brocklehurst visited Mexico in 1881 and published a book which included a chapter on the rich silver mines of Pachuca, where the patio’ system, which had its origin there in 1557, was still followed.26 The beneficiating hacienda he visited at Loretto consisted of a five- to ten-acre flagged yard, around which were placed the ore-crushing mills, stables, refinery, retorts, and various workshops. High walls provided security for the silver bullion and mercury.

Figure 4 illustrates the open air nature of the patio-mixing process with horses or mules driven around in circles to mix the torta which was contained by a circular curb. After mixing of ore paste, sulphate of copper and saltwater brine for four days, mercury was sprinkled over the mass by men who carried it in canvas bags, which they would shake or knock against their bare thighs as they walked about the torta. After 14 - 21 days of treading, the mixture was wheeled in barrows to a large tank, through which passed a rapid stream of water. In this tank, around a dozen nearly nude men stirred up the muddy mixture with their feet and legs, causing the clay particles to pass off with the stream, and leaving the amalgam and excess quicksilver at the bottom of the tanks. This residuum was scooped up in small iron jars and taken to a shed for removal of excess mercury and separation of the amalgam into silver and mercury by heating in retorts. The pure silver was finally melted into bars ready for the mint or for manufacturing purposes.


Figure 4. Sketch of the beneficiating hacienda at Loretto,Pachuca, 1881 Image: Wikipedia Commons


Brocklehurst commented: “None of the quicksilver is lost or wasted; even the vapour is brought by cold water into its original state, and is again and again made to fulfil its part in the beneficiating of silver... The men and children employed in the processes where so much quicksilver is used did not appear to suffer from its effects, but the old, worn-out horses and mules, used in the treading-out process, soon lose their hoofs and become in other ways unfit for service, except perhaps to yield their skins for leather.” 26

Humboldt also wrote that it was a remarkable phenomenon to see the men who trod the torta in the most perfect health: “The physicians who practise in places where there are mines unanimously assert, that the nervous affections, which might be attributed to the effect of an absorption of oxid of mercury, very rarely occur.” 29

In 1843, Saint-Clair Duport, who resided in Mexico for 16 years to improve the treatment of silver ores, reported that those who filter the amalgam and prepare it for distillation often feel an irritation of the nervous system which has no serious consequences.50He considered that those charged with distillation were the only workers who were exposed to any real danger of serious mercury poisoning, such as when the supply of cooling water was disrupted or a distillation vessel broke. He considered that such accidents resulting in high mercury vapour exposures occurred only rarely, especially in large workshops, where the distillation apparatus was handled with extreme care.

The chemical transformation of mercury to calomel (mercurous chloride) in the amalgamation process also reduced the emissions of volatile mercury. According to Guerrero:“The workers of the amalgamation haciendas and the civilian population of the New World were spared the ravages of mercurialism on a major scale both by the safeguards adopted during the heating cycle of the amalgam and by the chemistry of the amalgamation process that consumed mercury by converting it to solid, insoluble calomel.” 37

By the time of the visits by foreign Europeans to the amalgamation works of New Spain in the late 18th century and early 19th century, when the Indian labour on the mines was free, mercury exposures would have been lower than in previous times for Indian and black slaves. Helmut Waszkis commented that “We do not know how soon the Spaniards learned to recover the vaporised mercury” when heating the amalgam,19 but water traps came into common use for the production of mercury in the 17th century. Gamboa published a description of the patio process in 1761 and the use of a simple vessel of cold water to collect mercury vapour.1 He reported that:“The smelting and amalgamation works are noxious, and the maladies which arise from the moisture, fire and vapour, are incurable and frequent.” 1

West noted that a common form of mercury poisoning, Stomatitis (acute inflammation of the mouth and gums), was not mentioned by colonial sources, but was probably present in mining centres with amalgamation works. Duport, according to West,2 also reported the prevalence of shyness and nervousness (Erithismus mercuralis) amongst the mercury distillers of Fresnillo. Ladd, for the period of the miners strike of 1766, considered that distillation was the most dangerous of all phases of silver production, with distillers’ exposure to mercury vapour being prolonged and close.33

The custom of treading the torta by mules in New Spain was only introduced in the 1780s which reduced the expense of amalgamation by more than a fourth since it was no longer necessary to employ the large numbers of men who trod, barefooted, the torta.30The loss of mercury in storage and transit would have been reduced in New Spain by the early 19th century with the phasing out at Almadén of wrapping mercury in leather sacks (baldeses). Mercury started to be packaged at Almadén in iron flasks from 1793.36,46

In the late 1700s, European technical teams were sent by Spain to New Spain and Peru to improve the amalgamation process. The head of the mission sent to New Spain (1788-1798) considered that the European opinion was false with regard to the Spanish-American amalgamation process being harmful to health.51 This was in sharp contrast to that observed by the mission sent to Peru, where forced labour on the mines (under the so-called mita system) remained in place until 1812.

Before discussing mining in colonial Peru, Paper 2B will next address working conditions at the cinnabar mines of Almadén, Spain.

‡ The problems of transferring technology to Mexico were underestimated, with high transportation costs for equipment and spares, the lack of wood to fuel boilers, and the lack of workers familiar with the equipment. Mercury prices also increased greatly between 1825 and 1850, with the Rothschilds monopolising the world mercury market. By 1850, all but one of the British silver-mining companies in Mexico had failed.20


* Ore samples analysed from the principal mining areas were generally composed of “compact quartz, spotted or veined with metallic matters.” 32



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