A cutting edge

Flint, chert, obsidian and fine quartzite break with a conchoidal fracture — a smooth curved scar like the inside of a shell, leaving a razor edge. Granite, sandstone and most river cobbles never will; the problem is the rock, not you.

Test: strike a small piece off an edge. A curved, glassy flake with a sharp feathered edge means yes. A square, granular or crumbly break means no. Bottle glass and thick porcelain flake identically and are usually easier to find; a bottle base is a good core.

Flakes are surgically sharp and they fly further than you expect. Eye protection, a thick leather or folded canvas pad on the thigh, a glove or pad on the hand holding the core, closed shoes, and nobody within 2 m in front of you.

Work outdoors. Flint is silica, and knapping throws fine dust that causes silicosis with long, repeated exposure in a closed room. Work over a cloth so the debitage can be collected — both because it is useful and because it will otherwise lie in the grass cutting feet for years.

A flake comes off where the edge angle is less than 90° — ideally around 70–80° between the platform you strike and the face the flake will run down. Square or obtuse edges crush instead of flaking.

Abrade the platform with a coarse stone before striking: rubbing off the thin overhang strengthens the edge so the blow is transmitted into the core rather than crumbling the lip.

Hard hammer: a rounded, fist-sized cobble of quartzite or tough sandstone, a few hundred grams. Soft hammer: an antler or dense hardwood billet, which removes thinner, longer, flatter flakes and is how the final shape is made.

Strike 3–5 mm in from the edge, not in the middle of the core — a blow in the middle shatters it. Aim into and down through the core, as if the flake's path were a line passing inside it, and follow through. You are detaching one long flake, not breaking rock.

Hold the piece in a padded palm. Place an antler tine or a nail set in a wooden handle on the edge, press firmly inward and then down so a small flake pops off the underside. Work along the edge in sequence, turning the piece over to alternate faces for a straight, even edge.

This is slow and precise — minutes per centimetre — and it is what makes an arrowhead or a knife edge usable rather than ragged.

Pitch glue: melt pine or spruce resin, then stir in roughly 1 part powdered charcoal to 4 parts resin and a little fat or beeswax so it does not shatter when cold. Keep it warm, not boiling.

Split or notch a handle, warm the tool and the glue, seat the flake in pitch, then bind with wet rawhide, sinew or cordage, which shrinks tight as it dries. Coat the binding with more pitch.

Any scrap steel is worth more than good stone — it bends rather than shatters, and it can be resharpened thousands of times.

Grind on a coarse stone, then finish on a fine one, holding a consistent angle: about 15–20° per side for a knife used to slice, 25–30° per side for an axe or a chopping tool. Work until a burr — a fine wire edge — can be felt along the whole of the opposite side, then remove it with lighter strokes on alternate sides. A consistent angle matters far more than any particular one.

Keep the debris pile — small waste flakes are perfectly good knives for a single job, like butchery or cutting fibre, and throwaway edges mean nobody saws at meat with a dull one.

Record who in the group can knap, and teach at least two others. A knife cut is an infection risk, so the quality of first aid is part of this step — see wound care.