William Brooks is a geologist and mineral commodity specialist with the U.S. Geological Survey in Reston, VA. He has extensive mapping and exploration experience in Latin America and has applied his knowledge of regional geology to mineral resource use in ancient societies on archaeological field projects in Belize, Peru and Bolivia. He presented this interdisciplinary study at Harvard University during the 22nd Annual Northeast Conference on Andean Archaeology and Ethnohistory, Nov. 1-2, 2003. He was a co-convener of Utilization of Geologic Resources by Ancient Civilizations, a topical session at the Northeast/Southeast combined section meeting of the Geological Society of America in spring 2004.

COAL AND CREMATION IN ANCIENT PERU

Sidebar:
Pre-Columbian silver mining

After my visit to the adobe-walled archaeological site of Chan Chan, near Trujillo in northern Peru in the summer of 2000 (Geotimes, August 2003), my guide asked if I would like to see the metallurgical furnaces used by the Chimú, ancient residents and master metalsmiths of the region. Chan Chan was the capital of the Chimú Empire (A.D. 1100-1400) and the largest pre-Columbian city in the Americas. These furnaces, my guide explained, were where Andean gold, silver and copper ores were smelted and fabricated into jewelry, masks and plates sought by the Spaniards. We left the main part of the complex, followed a dusty trail, and arrived at a site marked by fresh-looking, redbrown, clinker-like debris.

There are four large burned areas at Chan Chan — one of these, the Tschudi burn, is approximately 25 by 70 meters and is surrounded by partially eroded adobe walls that are now approximately 3 meters high. Intrigued by these structures, I took a soil sample and later analyzed it; oddly, the sample showed no trace of metals. Thus I began a geochemical quest to understand if these burned areas were in fact used for ancient metallurgy. My findings thus far have revealed the possibility of a far more morbid role for these furnaces in ancient Chimú society.

Interior of the Tschudi burn in northern Peru shows a research trench cut in the 1970s. Field of view is approximately 20 meters, and the weathered adobe walls, to the right and in the distance, are approximately 3 meters high. Soil samples at the site show coal use, with little evidence of metalwork; rather, the soil indicates a potential crematory nature of the site. Photo courtesy of William Brooks.

Past accounts

American journalist and diplomat George Squier visited Chan Chan in 1877. In his book Peru — Incidents of Travel and Exploration of the Land of the Incas, Squier, who was not trained in mining or metallurgy, described these burned areas as "ancient furnaces with thick walls that were burned deep with slag of copper and silver ores." In a very different account, Colonel La Rosa, a Peruvian advisor to Squier, said that "this area was filled with calcined human bones … and this should be considered as a place of the burning of the dead." Still, Squier noted that "few traces were found at the time of my visit" and that human burning was "a practice of which I found no traces elsewhere."

Given the quantity of metal objects produced by Chimú craftsmen, Squier's conclusion seemed sound. However, in the 1970s, Massachusetts Institute of Technology (MIT) archaeologists analyzed the "slag" and showed that it was simply adobe that had been melted and vessiculated by the intense heat. That study did not support Squier's observations; it showed no geochemical evidence for metal processing or smelting at the site.

I explained this enigma to a geochemist familiar with ancient mining and smelting techniques described in Agricola's 1556 classic text De Re Metallica. The geochemist suggested that a soil traverse across the area would be the best test of whether or not metals had ever been processed at the site. And so I decided to return to Chan Chan a year after my first visit, determined to find any possible geochemical evidence of metallurgy.

The role of coal

My field research for the U.S. Geological Survey's (USGS) World Coal Quality Inventory of South America showed that coal is abundant in the Alto Chicama region of northern Peru, and sea coal is found on the beach near Trujillo. Although "seacoale" collected from beaches in 12th-century England would eventually lead to widespread use of coal in the Industrial Revolution, the early users and sellers of this resource that produced "filthy vapor" were often tortured or executed.

Modern coal mining at the La Victoria mine, Alto Chicama region in northern Peru. Photo courtesy of William Brooks.

The archaeological literature indicated that ancient people in South America used coal for mirrors and jewelry and, in North America, ancient people mined coal and used it to fire pottery. However, despite its abundance, I found only two obscure references to the use of coal as a fuel in Peru; one indicated that coal was found near an Inca wind furnace, which implied, but did not necessarily prove, use of coal as a fuel.

Therefore, geochemistry of the ash horizon would help determine the original fuel. Perhaps it was charcoal or other plant material referred to in the archaeological literature; or, given the regional availability, perhaps it was coal.

Back to Chan Chan

I obtained permission to sample at the Tschudi burn from the Instituto Nacional de Cultura, the organization that oversees field archaeological research in Peru, and returned to the site in summer 2001.

The 1970s MIT archaeology team had exposed the stratigraphy of the Tschudi burn in a research trench. In-place river gravels formed the lowest unit or floor of the burned structure; next, a 15- to 20-centimeter, fully combusted, pink to gray ash horizon; and, finally, 30 to 40 centimeters of blocky, lava-like, scoriaceous adobe that graded horizontally into the partially melted and scorched adobe walls.

Using a plastic trowel, I took eight soil samples at 10-meter intervals across the Tschudi burn — four samples from within the burned area and four samples from outside the burned area to provide background data — from depths of 20 to 30 centimeters. I sieved them in the field using plastic to eliminate the risk of sample contamination; if copper or other metals were present in the soil samples, it would be apparent in the analytical data.

I sent ash samples to USGS laboratories and the soil samples to a Nevada exploration geochemistry lab for analysis. Geochemical data on the ash from the Tschudi burn indicated high (approximately 50 percent) silicon dioxide (SiO2), low calcium oxide (CaO) and high zirconium (Zr) content. I compared these data to recent data from ashed Peruvian coal samples in the USGS World Coal Quality Inventory of South America database. That comparison indicated that the fuel used at the Tschudi burn was coal — the first geochemical evidence for the use of coal as a fuel in the pre-Columbian archaeological record of Peru. In contrast, charcoal or plant ash typically has low (approximately 5 to 6 percent) SiO2, high CaO and low Zr content.

The soil geochemistry confirmed what the MIT archaeologists had already published: There was no anomalous copper or other metal content in the soils either from the exterior or the interior of the structure. Physical evidence had again shown that the Tschudi burn had no metal signature and therefore had not been used for metallurgy at any time in the past.

But the soil geochemistry held more information. Calcium, phosphorus and sulfur content of the samples was low outside of the burned area and high within the burned area. Colonel La Rosa had described "calcined human bones" — could the high calcium and phosphorus content indicated in the soil data be related to "burning of the dead"? I contacted archaeological colleagues and the Cremation Association of North America for answers.

Making the link

Researching the history of coal and cremation, I learned that coal was used for fire 70,000 years ago in France and that the oldest-known cremation took place 50,000 years ago in Australia. Bronze Age tribes in Wales used coal for funeral pyres 3,000 years ago; coal was used for cremation at the first U.S. crematorium in Washington, Pa., in 1876; and the poet Robert Service wrote about coal as the fuel in his 1908 Yukon tale "The Cremation of Sam McGee."

But while cremation was an acknowledged worldwide custom, it was not a pre-Columbian mortuary practice, according to my archaeology colleagues. There were rare examples of secondary cremations where bones of the deceased had been burned, but there was no evidence for cremation as a primary mortuary procedure in pre-Columbian Peru.

Pre-Columbian mortuary customs in the region are quite interesting. For example, the elite were wrapped in elegant textiles with gold, feathers and ornaments for the afterlife, while the poor were given simple burials with no objects for use in the afterlife. Criminals, however, were buried alive, and the bodies of some prisoners left to decompose on the surface show signs of intentional mutilation.

At Chan Chan, archaeologists described Chimú multiple burials and vaults with 13 female skeletons "stacked like cordwood;" they estimated that as many as 300 may be buried at other sites. Perhaps the Tschudi burn was a mass cremation intended to destroy the bodies and thereby deny an afterlife to the victims.

A coal expert at USGS indicated that the presence of sulfur in the soil data was especially interesting. If coal were burned, then the sulfur, available from pyrite in the coal, should have volatized. However, in modern power plants, limestone or other high-calcium materials are added to the burning coal. The calcium combines chemically with the liberated sulfur. In a modern power plant, this reaction is called flue gas desulphurization and produces synthetic gypsum. Also, this is environmentally important because it limits sulfur releases to the atmosphere that would otherwise contribute to acid rain. Both the calcium and sulfur content of the soil samples was high, perhaps indicating the presence of limestone, shells or even bone as sources of calcium.

With La Rosa's words about human bones in mind, I continued down the crematory path, looking at how temperature and the duration of the Tschudi burn compared to the physical conditions at a modern crematorium. According to lab research carried out by the MIT archaeologists, Tschudi burned for 16 to 40 hours at temperatures greater than 1,300 degrees Celsius. A modern crematorium operates for 2 to 3 hours at temperatures of approximately 1,200 degress Celsius. The conditions of the Tschudi burn, therefore, exceeded the requirements of a modern crematorium.

The geochemical evidence — such as high calcium and phosphorus content of the soil from human remains and the interrelationship of calcium burned in the presence of sulfur from pyrite in the coal — has led me to believe that the Tschudi burn was used for cremation. However, collaborative work between geochemists and archaeologists is necessary to reach a conclusion. The excavation skills and recognition of fragmented, charred human remains, provided by an archaeologist or physical anthropologist can determine if La Rosa's words are eerily true.

Sidebar
Pre-Columbian silver mining by Megan Sever

In the Bolivian Andes, people have been mining and smelting silver for much longer than previously thought, according to recent research from lake sediments. For some time, archaeologists have known that the Incans treasured the precious metal, as did the Spanish colonists after them, but there had been little evidence that any cultures prior to the Incans had harvested silver.

Geologists Mark Abbott of the University of Pittsburgh and Alexander Wolfe from the University of Alberta cored Laguna Lobato, a small but deep lake 6 kilometers from the largest silver deposit in the Bolivian tin belt, looking for evidence of past climate change. What they found surprised them, Abbott says — evidence of pervasive pre-Incan silver mining.
Abbott and Wolfe's findings are a completely new and unexpected contribution to the archaeological records of the pre-Incan period, says Marc Bermann, an archaeologist at the University of Pittsburgh. "Nobody would have predicted to see silverworking on the scale the lake cores indicated, because silver is essentially absent from pre-Inca sites in highland Bolivia," he says.

The stratigraphic evidence in the cores included layers of metals associated with smelting, such as lead, antimony, bismuth, silver and tin, at much higher levels than natural. Pronounced metal-enrichment events coincide with the late stages of the Tiwanaku Empire (A.D. 1000 to 1200), and with the Incan period (A.D. 1400 to 1650) and the Spanish Colonial period.

Abbott says the researchers dated samples spanning from 2000 B.C. to A.D. 1000. They found stable, low concentrations of the metals. Then concentrations suddenly rose well above background levels, reached a peak around A.D. 1130 to 1150, then decreased during the following period and increased again with the Incan Empire, Abbott and Wolfe reported in the Sept. 26 Science.

"Metalworking on this scale is a hallmark of highly complex societies," Bermann says. These findings are the first evidence that such societies existed in this area as early as A.D. 1000. "It's very exciting," he concludes.

Bermann and Abbott both want to solve the mystery of what happened to the tremendous amount of silver that was mined once it was produced. "Was the silver recycled in later cultures, looted or does it remain buried?" Abbott says.

For now, Bermann will join Abbott and Wolfe in the next trip to the Andes to search for more lakes that might have preserved similar records to Laguna Lobato. By studying a series of lakes from the region, they hope to determine the history of metallurgy across the region.

MINERAL RESOURCES OF PERU'S ANCIENT SOCIETIES

Northern Peru has an exceptionally rich archaeological heritage that includes metalwork, ceramics and textiles. The success of at least a half-dozen pre-Columbian societies dating back 3,000 years and subsequent Spanish colonization in the 1400s has rested on the effective use of northern Peru’s abundant resources. In the summer of 2000, my son Matt and I learned about that connection firsthand by volunteering at the Santa Rita B archaeological site in the Chao Valley near Trujillo in northern Peru. Riding donkey-back through the Andes and talking with local people, we got our hands dirty in the rich archaeology and geology of the area. We were able to correlate mineral occurrences to their various roles in society — opening a window into the region’s fascinating past. From construction to metallurgy, pre-Columbian societies flourished and advanced because of their understanding and use of the available mineral resources.

Metals

Although recent mining activity over the past few years has overprinted evidence of pre-Columbian mining, several features we observed at a small copper mine near Santa Rita B indicate the many-thousand year history of mining in the area. Adits (tunnels) in the mine have undulating walls, originally opened by firesetting — an ancient mining technique in which miners set a fire adjacent to the face and then threw cold water on the hot surface to make the rock shatter and facilitate excavation of the ore with bone or stone tools. Also, the floor of the adit slopes downward approximately 30 degrees, representing a style of excavation, not used today, known as “medio barreto.” The adits are only 3 to 4 meters in length because pre-Columbian miners rarely penetrated beyond the distance where the sun would easily illuminate the adit. On a flat area nearby, we saw 20- to 30-centimeter-high stone foundations that outline three 4-by-6-meter structures that were workshops or living quarters for the miners. We identified several ground sluices (water passages) carved into the slope of the hills near the mine — artifacts of an antique method of mineral separation. Abundant thick-walled ceramic fragments also signaled that this was a pre-Columbian work site.

“Tumbaga” is the archaeological term that describes an alloy of gold, copper and silver crafted in pre-Columbian workshops that ultimately fed the Spanish passion for metals in the 1600s. Some of the metals used in Peru came from local sources, while others, based on our geological field work, may have come from a distance.

A Spanish colonial adobe smelter in the Chao Valley in northern Peru marks a time of great Spanish interest in metals in the area. Photo by William Brooks.

Copper was perhaps the most widely used metal in pre-Columbian cultures, used for everything from jewelry and tweezers to foils and small “tumis,” or knives. Ores from Andean porphyry copper systems may also contain gold and silver. Copper is abundant in Peru; a few major porphyry copper deposits include Antamina, Cerro Verde and Tintaya. On a smaller scale, copper occurs near Santa Rita B, with a small copper mine less than 2 kilometers south of the site. We found shallow adits and veins containing hematite, goethite, manganese oxides and quartz in the oxidized cap. We also found loose chunks of chrysocolla, locally “cardinillo,” an ore of copper valued by pre-Columbian civilizations for use in jewelry because of its blue-green color.

The major source of pre-Columbian gold appears to have been alluvial, which may also have contained silver. Alluvial, or placer gold, is abundant and easy to mine, and when alloyed with copper has a lower melting temperature. Its malleability permitted minimal preparation before crafting. Today, northern Peru hosts two of the country’s three major areas of alluvial gold production; abundant goldwork items correlate with these areas of abundant placer gold. Of Peru’s 14 gold-producing districts, the Pataz district in northern Peru is known to have produced the precious metal from gold-quartz-pyrite veins since pre-Columbian times. This district also has the largest historical production of gold in Peru.

Some goldwork objects and mask and craft decorations contain platinum nodules. Both metals have similarly high specific gravity. Gold-platinum alluvial occurrences exist in Columbia, Bolivia, Ecuador and Peru. Because of platinum’s brittleness and high melting point, the Spaniards who colonized the area discarded platinum artifacts in favor of the tumbaga artifacts that could easily be melted and returned to Spain.

While silver occurrences are numerous throughout the Andes, no evidence exists for silver mining in the Chao Valley. Polymetallic veins with copper, lead and silver are abundant near Santa Rita B; but the absence of lead artifacts, slag and wind furnaces for smelting argentiferous galena indicate that their silver must not have come from a local source. We found a large Spanish colonial adobe-walled smelter, indicating Spanish interest in metals in the area; however we found no ore minerals or tailings at the site.

Pre-Columbian miners used mercury to mix with gold flakes, creating a paste that they applied to copper objects and then heated to volatize the mercury — leaving a thin layer of gold on the object. Even then, the Inca recognized the toxic effects of mercury fumes and mining, as recorded by a half-Spanish, half-Inca scholar named Garcilaso de la Vega in the 1500s. The pre-Columbians also used cinnabar as a pigment, as a cosmetic to indicate social status and as decoration on funeral masks.

Fuels

Archaeologists think that pre-Columbian societies used charcoal, dung and plant material as their main fuel sources; however, the geology of the region indicates the availability of coal in northern Peru. The Northern Anthracite Field, also known as the Alto Chicama (which means coal in Quechua), is the most extensive and well known coal province in Peru. Coal is mined today on a small scale at La Victoria and Banos de Chimu, with the coal burned for domestic use or processed into briquettes for use in small, unvented coal burners in rural areas. The largest and most well known Peruvian coal mines, Goyllarisquisga and Callacuyan, however, are closed.

But pre-Columbians knew about coal and used it for mirrors, metallurgy, beads and cremation. Sea coal, available today on some of the beaches of northern Peru, may have provided another source of coal to ancient people.

Fertilizer

Guano, deposited by sea birds on numerous islands along the coast of Peru, provided fertilizer for the agricultural needs and regional expansion of pre-Columbian society. The guano beds were as thick as 40 meters, with pre-Columbian metal and wood as well as Spanish colonial artifacts found in the some of the deposits. Of the organic fertilizers, Peruvian guano is exceptionally rich in nitrogen, phosphate and potash. These nutrients stimulated the growth of cotton that was dyed and woven to become part of the archaeological richness of northern Peru.

I was surprised to see that reed boats or “caballitos de totora,” are still used today on the beaches of northern Peru. This style of boat, depicted in pre-Columbian drawings, transported the guano to the coast for agricultural use. Peruvian guano was an important commodity and was exported to Europe during the 19th century.

Construction, decoration and ritual

Altered volcanic rock associated with porphyry intrusions provided fine-grained clay as an industrial material for making stirrup pots and figure molds for metal casting; it was even fashioned into flutes and horns. The pre-Columbians may have used coal to fire some of the ceramics and used white clay to make a final creamy coating, or slip, for ceramic decoration. River clay and silt, mixed with straw and shell fragments, provided another industrial material to make adobe bricks, used to build temples and walled cities such as Chan Chan. Hematite, limonite and manganese oxides, also sourced from these areas, were used as pigments to decorate ceramics and murals.

Manganese oxide was a preferred resource for mummy preparation in parts of the pre-Columbian world. Obsidian, attached to sticks with bitumen, was fashioned into surgical instruments or was used for ritual defleshing. A mummy skull in the Gold Museum in Lima has a gold plate that was surgically implanted in the skull premortem. The shell of Spondylus, a spiny, coral-colored oyster, was valued for jewelry and its possession indicated social status because it could only be obtained from deeper waters off the coast of Ecuador.

Bitumen, found in northern Peru, near Talara, had uses as cement for dimension stone, as a setting for gold, and as a glue to attach arrowheads and secure Spondylus shell fragments as decoration on wooden figures. The Spaniards used bitumen to caulk ships, waterproof rigging, or seal wine bottles.

These reed boats on a beach near Trujillo off the coast of Peru once transported guano and are now used for fishing. Photo by William Brooks.

Pre-Columbian construction workers selectively used rounded river cobbles, 20 to 30 centimeters in diameter, in the construction of dams and irrigation canals, as hammer stones to finish dimension stone or process ore, and to grind grain. Rounded pebbles, 1 to 2 centimeters in diameter, were stockpiled and used as sling-stones. The workers preferred angular cobbles, however, for domestic walls at Santa Rita B.

While exploring the region’s geology, I gained a vast and rich understanding of the importance of mineral resources in pre-Columbian society. Research into the role of mineral occurrences in the growth of ancient societies is an exciting field that combines geology and archaeology and promises further discoveries.

 

 

 

 

 

 

Also of interest:

WORLD COAL QUALITY INVENTORY -- PERU
By William E. Brooks and Jason C. Willett