Monthly Archives: October 2016

Metallurgy in pre-columbian Central Mexico

28 October 2016 Comments (2)

If dazzling jewelry was your weakness, then you might have found it hard to pass through a marketplace or workshop areas of Tenochtitlan or any other major Mesoamerican altepetl/city-state without spending much of your hard earned goods or local currency – cocoa beans and cotton cloths – on too many beautiful trinkets. Glittering bracelets, earring and anklets of copper and gold or brilliantly polished precious stones were always in high demand, and the canny Mesoamerican traders knew how to tempt a customer with most charming, intricate, lavish designs, causing jewelers and other artisans work long and hard to supply the demand.

Pre-Columbian metalworkers toiled in their workshops, located usually in the less prestigious parts of the city, along with other craftsmen and their shops – feather-makers, stone-workers, weapon-makers and such, organized into guilds, represented well in their districts, taxed but respected, the heart of the middle class and the spine of it. Unlike their fellow other craftsmen and artisans, the metal-smiths’ working areas required special facilities – powerful braziers, specialized tools, considerable supply of fuel. Braziers were typically made out of stone, with special openings for pipes crowned with clay tips to be inserted into the raging fire in order to make it rage fiercer, reach desirable temperatures by blowing into it constantly.

Copper, for one, needed to be heated to over six hundred degrees (Celsius) in order to separate it from the most obvious excess of other minerals it was extracted with from the earth, then reach 1250C in order to make it into a workable material for smelting. Blowing reed pipes with clay tips achieved that, but to maintain such long standing fires plenty of firewood was required. A problem for the big cities where most of the metal smiths’ workshop were located; less so for the miners out there in the country, those who didn’t produce the finished products but still needed to do the first round of heating in order to separate copper and silver from other minerals those raw materials were mixed with.

In Nahuatl, the lingua franca of Central Mexico, the term for mining was ‘in tepetl auh in ozototl’ which means ‘the mountain and the cave’, indicating typical location of precious stones and minerals. The term for digging up a mine was ‘tlallan oztotataca’‘to dig caves in the earth’. The word for copper was ‘tepoztli’ and ‘tepoztli iohui’ meant the ‘copper vein’. In Western Mexico, where metallurgy was even more wide-spread the dominant Pu’repecha language is full of appropriate terms.

The easiest and most wide-spread technique of mining was surface collection, where the ore was simply available on the surface, either in streambeds or on the ground. The erosive power of streams would break the ore and the heavier metals would settle on the bottom in areas of slower flow. Those were also the easiest to recognize because the deposits of cooper that are naturally dull gray in coloring, when exposed to the weather conditions of the surface brighten into vivid green or blue. A wonderful lead for the miners to follow, to collect what’s on the surface and dig short tunnels in order to reach the hidden treasures in the correct places. This technique is called ‘open-pit mining’.

The Underground Mining was also used when the deposit occurred deep below the surface in the form of a vein in a hard rock – the term tepoztli iohui means copper vein. In this case, tunnels were excavated in the rock to remove the ore, narrow vertical shafts driven through the rock, widening out to horizontal galleries where the ore was found. Pre-Columbian miners preferred to drive adits – nearly horizontal entrances to a mine – or tunnels into rocky slopes over digging shafts, which made drainage and haulage much easier.

In Mesoamerica, evidence of underground mining, including sizeable adits, shafts and galleries dug with hafted hammer-stones dates back to the beginning of AD, not only in order to extract metals but of course in order to haul out precious stones as well – cinnabar, turquoise and obsidian mines, even though the obsidian mines did not required digging adits.

Evidence indicates that the tools used to excavate mines and extract the ores were varying, consisted of stone hammers, large stone mortars, either portable or fixed on the walls of the mine, pestles upon which minerals were probably ground, bone scrapers, and digging sticks, ceramic ladles, obsidian blades, and wooden wedges. Remains of ocote-torches, and vegetal fibers impregnated with resin, baskets, ropes and ceramic pots, have also been recorded often.

As mentioned before, to separate metals was crucial, so the miners would work the found treasures on the spot, using what we call today pyrometallurgy when the ore was ground, mixed with charcoal and heated in a crucible or brazier. At the right temperature, up to 1300C, copper would separate from other components and merge into droplets. Adding ash or sand helped to melt the slag, so the copper would sink while the rest of the liquid would float, ready to be picked off while still hot, or broken off while cooled.

However to created refined, beautiful or useful items, the purified metal would have to be sent to the cities and into the hands of the urban craftsmen, the metal-workers. Styles of fashioning final products were many and diverse: hard-hammering or cold-hammering (working the metal when its cold), annealing (heating the metal after cold-work reduced its plasticity), casting (shaping metal when in its liquid state). Decorating techniques included gilding, embossing, soldering;lost-wax casting, gilding, low-relief decorations (created by hammering from the reverse side of the object), sheathing and so on.

Cold-working involved changing the form of a metal object by bending, shaping, rolling and hammering. As the metal being shaped internal stress serves to harden the part. Heat also serves to harden the material. Bells, needles, tweezers, rings, awls, axes, ornaments were usually made by cold-working from an ingot cast with occasional round of annealing. However if concentration of tin or arsenic was high enough to cause brittleness, hot working or forging was employed.

Small open rings for earrings or hair ornaments were very popular. Cold-hammered, then annealed through several sequences, those would fetch good prices on the marketplaces of big cities. After 1200 AD such rings were forged in high-tin bronze. Silvery rings were made by silver-copper alloy.

Tweezers were as popular, made by two symmetrical blades joined by hinge that was fashioned from a continued piece of metal. In earlier times they were hammered out from a solid piece, then bent over a wood piece or other solid material in a shape of a hinge, then cold-worked into a final shape and the excess metal cut. Later tweezers were made out of alloys and have been of a high quality, hot-worked into shape. Tweezers made of gold were saved for leaders and foreign offerings alone.

Sheet-metal ornaments were made of extremely thin cold-worked sheet of copper-silver, copper-gold, copper-silver-gold or silver-gold mixes. Those were used to ornate breastplates, shields, headbands, pendants, earrings, disks and bracelets. Copper-silver alloy was the most popular for such ornamenting purposes.

Axes were made from copper or bronze, mostly for symbolic use as it seems. Those were cold-worked, annealed, then cold-worked again. Copper (like silver and gold) is not an optional metal for cutting wood, but naturally occurring copper, due to its metallic impurity, can be relatively hard, useful for splitting wood. However, even such axes lost their edge quickly and needed to be reshaped. Bronze alloy axes became wide spread in Post Classic period (after 1200 AD). Tin-bronze, copper-arsenic and copper-arsenic-tin were added to enhance the tool. Those were three times thinner at the edge and harder, made by pouring molten alloy and into the mold (Florentine Codex), then shaped by hammering and annealed and cold-worked again to harden them.

Needles and awls, hoes, fishhooks, digging stick-points, thin leaf-like objects were made out of arsenical copper, usually cold-worked and annealed, sometimes used as tribute payment. Copper bells and later on bronze bells were created for decorative purposes, their shapes vary from round, to oval, to cylindrical, with suspended ring at the top and a narrow slit opening at the base, with loose clapper made of metal, ceramic or pebble. Such bells sported beautiful, elaborated designs. Some were made from coiled threads of wire, forming complex vertical and horizontal patterns. The original models for these belts were made entirely out of wax, winding piece of wax thread around a clay core.

Mesoamerican smiths experimented lavishly, producing alloys of copper-silver, copper-gold, copper-arsenic, copper-tin, as well as more complicated mixes of copper-silver-gold, copper-silver-arsenic, copper-arsenic-antimony, copper-arsenic-tin. Copper-silver alloys were reached by smelting copper and silver ores separately, then melting the two together (as there are no ores to contain both metals in satisfactory amounts together, such alloys could be nothing but intentional product). Copper-arsenic alloys could be achieved from the same ore and the same smelting (in West Mexico it was probably achieved by smelting chalcopyrite and arsenopyrite together). Copper-tin alloys were produced either by cassiterite in order to win metallic tin then adding tin to the molten copper, or by smelting cassiterate together with copper ore minerals. Such bronze was manufactured only in two areas in Americas – Andean highlands and West Mexico. In West Mexico it dates back to 1200 AD. Copper-gold alloys usually contained plenty of copper and much less gold, mainly to give the product a shiny appearance.

So while strolling around the better parts of Tenochtitlan or other important city-states of the 14th-15th century Central Mexico, one was likely to have one’s vision assaulted by fierce glint of the noble people’s jewelry or the glittering of the ornamented walls and temples. Then it would be a high time to visit the most sought out jeweler oneself, or to look for his mat on the marketplace.

An excerpt from the upcoming novel “Obsidian Puma”

There was no room for mistakes in this trade, his benefactor would repeat over and over. With the sort of the fire they maintained and the sort of the metallic liquid they dealt with, one single mistake could cost a person his life or, at least, his ability to live properly. Still, there were times when he didn’t care one way or another, not heedful of the warning of his employer, or rather a slaver. There was a limit to a person’s ability to crouch next to the blazing braziers, blowing to make them rage fiercer. One couldn’t do it all day long for many days in a row.

The other workers, both sons of the owner and one disinterested nephew named Patli, did other things, hammered and scraped to refine the half ready products, worked with blades and ceramic ladles on the less delicate ornaments, rushed around with bee-wax and pottery. Learned the trade! While all he, Miztli, did was to slave in the melting room, tending the fire and not letting it go down the insanely high heat, allowed to pour melted goods into various clay and stone utensils sometimes, starting his day earlier than anyone and finishing way after the others were well away at the main house or wherever, loitering and having a good time.

He wasn’t a son or a nephew, or any other sort of a family member, but his father wanted him to learn how to work the precious metals and not only how to extract those from the earth, and so here he was, living in misery for more than three moons, blowing into the fire to make it rage fiercer. Some learning!

Grimly, he blinked the sweat away from his eyelids, watching the greenish powder that he was made to scrape from a solid piece of copper earlier in the day, in the blissful coolness of the outer room. There was another pile of powdered stone poured to mix in the pot this time, not gold but a duller looking mineral. It created better results, a stronger metal that was easier to work with, sturdier but more flexible at the same time. Magic. It was a beautiful sight, those simmering liquids of various colors, a pretty show to watch. In the beginning, it thrilled him to no end, the ability to turn something solid into a workable flow to be shaped to one’s desire, any form, any size, a jewel or a brick, or just an impossibly thin sheet of metallic wonder to create detailed reliefs for noble establishments upon their request.

These days, it bored him to death.

The outer screen screeched, announcing newcomers, quite a few of them, judging by the voices and the draft that managed to sneak in through the cracks in the wooden screen. Miztli ground his teeth and let his fingers crush the straw he worked with. To throw the remnants of his tool into the raging fire made him feel better. In less than a heartbeat, it was consumed, ceasing to exist – one moment there, the other gone.

Twisting his lips contemptuously, he reached for another pipe, a whole pile of those, reed straws being as plentiful as the mud upon the shores of the Great Lake, but old Tlaquitoc would grimace all the same, scolding his apprentice for carelessness and lack of concentration. If only there was a way to feed this entire establishment to the fire.

The draft made his work momentarily easier, igniting the flames in both braziers, as the screen shielding the entrance to his backroom moved, letting a thin surge of the fresher air in.

“Niltze!” Instead of the squat, wide-shouldered figure of his stocky employer, the lithe form of Chantli slipped in, thousand-folds more welcome. “Still working on that copper from the morning pile?”

Pleased to notice her moving into the corner of his eye, Miztli smiled with the free side of his mouth, nodding ever so slightly. When busy with such fiercely raging flames, one could take his attention off of it up to a very small limit.