# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DchtuRzAz5a
- **Gondola URL:** https://gondola.cc/posts/69727802-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/80ee76b2c3.jpg
- **Posted:** 2026-08-27T02:30:08.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

An obsidian blade can terminate in an edge roughly three nanometres wide. A surgical steel scalpel, honed to its practical limit, stops near 500. The difference is not craftsmanship. It is the absence of structure.

Obsidian is volcanic glass. Rhyolitic lava at 70 to 75 percent silica, so viscous and so rapidly quenched that the silicon-oxygen tetrahedra never organise into a lattice. No grains. No crystal boundaries. No preferred cleavage planes. It is frozen liquid, isotropic all the way down.

That matters because a metal edge is limited by its own microstructure. Steel is a mosaic of grains and carbide particles, and the finished edge can never be finer than the features it is built from. Grind past that limit and the carbides tear out, leaving a ragged, serrated line under an electron microscope.

Glass has nothing to tear out. Fracture it correctly and the crack front propagates through a continuum, terminating in an edge only a few molecular layers thick.

The mechanism is conchoidal fracture:
- A Hertzian cone of stress radiates from the point of impact
- The crack follows the stress field rather than any internal plane
- The result is a curved, shell-like surface with a feather edge at its margin

Aztec knappers exploited this industrially. Pressure flaking from a prepared prismatic core yielded long parallel blades, dozens from a single nucleus, each one sharp the instant it detached. Bedded into a hardwood paddle with pine resin, they became the macuahuitl. Spanish accounts describe horses opened to the bone in a single stroke.

The same edge reappears in the operating theatre. Obsidian scalpels cut with lower resistance, sever fewer cells laterally and produce narrower scars in comparative trials. Their limitation is brittleness: fracture toughness below 1 MPa m^0.5, and an edge that chips the moment it meets bone.

The sharpest thing a human has ever held was not manufactured. It was cooled.

#stemantics #obsidian #materialscience #geology #archaeology

## Stats

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## Tags

archaeology, geology, materialscience, obsidian, stemantics

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