Obsidian can be melted to a liquid state. It does, after all, start life as molten lava which then rapidly cools, producing the glass-like substance in question here. Casting molten obsidian can also be done, but the real question is why would anyone do such a thing?

Melting and then casting obsidian would be a tremendous waste of time and materials. Obsidian produces excellent blades by simply fracturing the stone. Also, while obsidian is fairly hard and can produce extraordinary sharp edges, it is a brittle material. Smaller knives of obsidian were made and used, but a long “sword” of obsidian thin and light enough wield would very quickly snap and shatter.

Humans in the past very astutely recognized the properties of obsidian and other stones used to make tools. Rather than devising high temperature kilns to melt these rocks, they instead innovated numerous techniques to maximize the cutting edge and durability of their materials. In the late Pleistocene, the major innovation was the use of microblades. By applying pressure to the edge of a larger “core” stone, a blade the length of the core can be flaked off and then used by itself or attached to a haft. Microblades are not exclusive to obsidian, but can be created from any number of crystalline or vitreous rock.

The properties of obsidian, however, make it an excellent material for creating blades from cores. Central Mexico, as a volcanically active region, has abundant sources of obsidian. The highest mountains in Mexico today are a mix of dormant and active volcanoes. One of these peaks, in the chain of mountains separating the valleys of Mexico and Puebla, is still active and goes by the Nahuatl name Popocatepetl, which literally translates to “Smoking Mountain.”

Archaic Mesoamericans were quick to adopt the plethora of obsidian into their toolkit. One of the distinctive technologies of Mesoamerica was the use of prismatic blades. Like the microblades described above, prismatic blades were flaked off a core, but these were typically larger in size and were ideally shaped like a long, flat trapezoid with razor sharp parallel edges. Obsidian is a superb material for making these items.

While present and common enough throughout the history of Mesoamerica, it was not until the Postclassic period starting around 900 CE that this form of stone tool became the dominant form of blade used throughout the region (Healan 2009). Prior to this, weaponry had employed various kinds of stone points as well as other sharp objects (e.g., shark teeth), but the production of prismatic blades really took off in the Postclassic. There is no clear explanation for this shift, and it could include factors as diverse as lack of consistent obsidian supply, competition from other forms of blade and tool making, and sumptuary control of prismatic blades as elite items.

One other factor in the spread of prismatic blades was the growing centrality of the macuahuitl in Mesoamerican warfare. While often called a “sword,” the macuahuitl was more like a hybrid between that and the spiked/bladed clubs that preceded it. There are different styles and variants depicted, but in general there were two styles: a larger “two-handed” version about 2-3 feet long, and a smaller “one-handed” version about 1.5-2 feet long. The core of the weapon was a wooden haft resembling something like a cricket bat made from hard wood (possibly encino oak), with grooves on the sides where prismatic blades would be attached using a hard-drying resin (Obregon 2006).

While there are antecedents to the macuahuitl in Mesoamerican history, objects that can definitely be given that name are absent until the Postclassic (Taube 1991). There a few edges cases (e.g., Uaxactun Stela 5), but the standard form would not coalesce until that era. The explosion of growth in microblade production could be posited as linked to the standardization of their use in military applications.

Now that’s a bit speculative, and prismatic blades were, by the Late Postclassic, ubiquitous items with a number of domestic, agricultural, artisanal, and even religious uses. The explosion in the use of prismatic blades in central Mesoamerica is, like many cultural shifts, multifactorial.

States in the Postclassic did, however, have a vested interest in ensuring access to obsidian sources to supply both military and domestic needs. This is reflected in archaeological remains. In Tlaxcala, for instance, an Aztec embargo left them cut off from most of the main sources of obsidian, but they still had control over the El Paredon source. Tlaxcalan cities were thus able to maintain an obsidian processing industry. Analysis of obsidian artifacts at several sites in the Puebla valley found between two-thirds and three-fourths were from the El Paredon source. Prismatic blades also dominated the assemblage; they made up >80% of all obsidian items at one site in the elite urban core of Tepecticpac (Corral et al 2021).

The Aztecs likewise invested time, people, and resources into securing and processing obsidian, though they had access to broader array of sources. The most important of these was the Sierra de las Navajas site near Pachuca. Obsidian in this region is not only plentiful, but has an ideal glassy appearance along with a green tint which was highly prized. As a result, the Navajas site has been worked basically since the dawn of Mesoamerican history and obsidian sourced from the site turns up across the region. In the Aztec era, hundreds of workers were present year round in camps, excavating large chunks of obsidian from deep pit and shaft mines to be processed on site for shipment to urban centers for refinement into finished products (Pastrano & Carballo 2016). Large scale obsidian industries in the Postclassic were buoyed not just by state interest, particularly as relates to the spread of prismatic blades. Remember that these blades were formed by striking them off from a core stone. In the Late Classic/Early Postclassic, the city-state of Xochicalco innovated a change in processing that would become the standard way to prepare cores for mass blade production. This was to make small chips in the base of the core to get it mostly flat, and then grind it leaving a flat, roughened surface. This technique not only made the base more stable, but allowed for easier and more consistent flaking of blades (Healan 2009).

The “deskilling” of prismatic blade production acted in a feedback loop with military innovation, domestic demand, and state interest to produce the Postclassic obsidian manufacturing industry. More readily available and more standardized prismatic blades meant greater availability in their use for weapons and tools. Weapons and tools that could effectively utilize prismatic blades produced a demand for more prismatic blades. Thus the state had an interest in securing obsidian supplies and supporting an industry to craft blades for use both on the homefront and the battlefield.

Melting obsidian down to cast into a single blade is the stuff of wacky youtube videos and (probably) anime. As the pontificating above shows though, obsidian use in Mesoamerica was not some static “Stone Age Caveman” technology, but was a toolkit with constant transformation and improvement. The technological shift towards mass produced prismatic blades enabled a military innovation in the form of the macuahuitl which was an incredibly effective weapon replacing clumsier bladed clubs and thrusting spears as the primary weapon of Mesoamerica (Hassig 1992).

Macuahuitl blades, like all obsidian, were brittle, but extremely sharp. Hence, the Spanish accounts of a horse decapitated in a single blow or of men cleaved almost in two after a strike. Though the blades could shatter, fragments of obsidian could be left embedded in bone and the fractured blade could still cut (Obregon 2006). Breakage could also be mitigated by utilizing the weapon to slash, rather than bludgeon or stab. Put together, the macuahuitl allowed for what is now the archetypal Mesoamerican battle kit. The macuahuitl could be wielded with a shield, but did not encumber as much as a spear, enabling the use of an atlatl. Put all this together and we get the classic image of the Aztec warrior, charging towards the enemy hurling atlatl darts to break their line, more darts clutched in his shield hand,with a macuahuitl tied to his wrist at the ready.

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Corral et al 2021 Core and Periphery: Obsidian Craft Production in Late Postclassic (a.d. 1250/1300–1519) Tlaxcallan, Mexico. Journal of Field Archaeology 46(7), 480-495

Hassig 1992 War and Society in Ancient Mesoamerica. U California Press.

Healan 2009 Ground platform preparation and the “banalization” of the prismatic blade in Western Mesoamerica. Ancient Mesoamerica 20(1), 103-111

Obregon 2006 The macuahuitl: an innovative weapon of the Late Post-Classic in Mesoamerica. Arms & Armour 3(2)

Pastrano & Carballo 2016 “Aztec Obsidian Industries” in The Oxford Handbook of the Aztecs eds. Nichols & Rodriguez-Alegria. Oxford University Press

Taube 1991 Obsidian polyhedral cores and prismatic blades in the writing and art of ancient Mexico. Ancient Mesoamerica 2(1), 61-70