Millstone Selection

Part of Mill Construction

Choosing the right millstone material and size determines the quality of flour produced, how often stones need re-dressing, and how long they last.

Why This Matters

The millstone is the heart of the grain mill. Everything else — the water wheel, the gearing, the building — exists to turn two stones against each other. The quality of those stones determines the quality of every meal the mill produces. A poor stone grinds unevenly, contaminates flour with grit, overheats the grain and destroys nutrients, or wears so quickly that you spend more time re-dressing stones than milling.

In a civilization rebuilding context, knowing how to identify good stone, quarry it, and work it into serviceable millstones means the difference between a mill that produces fine flour and one that produces coarse, gritty meal. You may not have access to the traditional premium materials — French burr or German blue basalt lava — but every region has stone that can be evaluated and used if you know what to look for.

The selection process is not complicated, but it requires understanding what properties matter and why. Stone that fails in use wastes enormous labor — cutting, transporting, and dressing a millstone takes weeks of skilled work.

What Makes a Good Millstone

Four properties determine whether a stone will make a good millstone:

Hardness — the stone must be hard enough to resist wear from the grain itself. Grains like wheat and corn are surprisingly abrasive at milling pressures. Stone that wears too fast contaminates the flour with grit and requires constant re-dressing. The stone should scratch iron easily (Mohs hardness 5+). Test by scratching the stone face with a nail; the nail should leave no mark.

Porosity — some porosity is actually desirable. A slightly porous surface grips and cuts the grain more effectively than a completely smooth, glassy stone. The pores also carry away heat generated by friction. But excessive porosity means the stone breaks down quickly at the surface. Ideal porosity is fine and evenly distributed — not coarse voids or pitting.

Consistency — the stone must be uniform throughout. Hard spots alternating with soft spots cause uneven wear, which leads to a wavy stone surface that grinds unevenly. Test by striking the stone surface with a hammer at intervals — the sound should be uniform (a consistent ringing) across the whole face. Dull thuds in some areas indicate softer zones or hidden voids.

Fracture pattern — when the stone is dressed (the furrows cut), it must fracture cleanly along the pick marks and not chip excessively. A stone that shatters unpredictably creates sharp fragments that contaminate flour and make dressing dangerous. Test on a small piece by striking with a mason’s hammer — the fracture should be controlled and the edges sharp, not crumbled.

Traditional Stone Materials

The best traditional millstone materials, in rough order of preference:

French burr (freshwater silica) — the gold standard, quarried near La Ferté-sous-Jouarre in France. Composed of freshwater siliceous rock with a cellular structure that provides ideal porosity. Stays sharp longer than any other stone. If you find naturally occurring chert or flint-like freshwater silica in your region, it may behave similarly.

Cullin (German blue basalt lava) — volcanic lava from the Eifel region. Dense, tough, and naturally porous from the gas bubbles trapped during volcanic cooling. Any vesicular basalt (porous volcanic rock with a fine texture) from your region may serve comparably.

Millstone grit (coarse sandstone) — a coarse-grained quartz sandstone used widely in England and the American colonies. Easier to find and quarry than burr or basalt, but wears faster and needs more frequent re-dressing. Still produces acceptable flour. Look for fine to medium-grained sandstone with quartz grains that are well cemented.

Granite — hard and durable, but the feldspar in granite weathers out over time, creating soft spots. Coarse-grained granites are particularly prone to this. Fine-grained granite (sometimes called “bluestone”) is considerably better. Granite stones run hotter than burr stones and can scorch flour if the stone gap is not carefully managed.

Limestone — generally too soft and too susceptible to the acids in grain. Limestone millstones wear rapidly and produce flour with high calcium carbonate contamination. Use only if nothing else is available, and expect to re-dress frequently.

Avoid these materials

Slate splits along cleavage planes and disintegrates in use. Marble is too soft. Any stone with visible large voids or alternating hard/soft bands will fail quickly. Flint is too hard to dress — pick marks glance off rather than cutting furrows.

Sizing Millstones

Stone diameter and thickness are both important:

Diameter — larger stones grind more grain per revolution but require more power to turn. A single-family mill (grinding 20–30 kg/day) can use stones 60–75cm in diameter. A community mill grinding for 50+ families needs stones 1.2–1.5 meters in diameter. The water wheel must be sized to match.

Thickness — the bedstone (the stationary lower stone) should be thicker than the runner stone (the upper rotating stone) because it takes more stress from the runner pressing down. A 1.2-meter diameter stone should be about 25–30cm thick at the center.

The eye — the central hole in the runner stone through which grain feeds. For a 1.2m stone, the eye should be 15–20cm in diameter. Too small, and grain backs up and doesn’t feed; too large, and grain passes through unground.

Quarrying and Shaping

If the stone must be quarried locally:

1. Identify a suitable outcrop and test several samples for hardness, consistency, and fracture pattern
2. Use plug-and-feather technique to split large blocks: drill holes at intervals along the desired split line, insert iron wedges (feathers) with a central plug, and drive the plugs in sequence until the stone splits along the line
3. Rough-shape the stone by working around the perimeter with a mason’s hammer and chisel, knocking off excess material
4. Cut the central eye with a star drill (a chisel rotated slightly between hammer blows, used in a circular pattern)
5. Dress the working face flat using a long straightedge to check for high spots; work high spots down with a pick

The entire process from quarry face to usable stone takes 3–6 weeks of steady work for an experienced mason. Building a stock of spare stones before the mill opens is strongly recommended.

Composite Stones

When no suitable large stone can be found, composite millstones can be built from smaller pieces of good stone set into a mortar matrix of lime and iron filings, held together with an iron band around the circumference. French burr stones were actually made this way — individual pieces of burr stone were cemented together and banded, allowing use of a superior material that never occurred in large blocks.

To build a composite stone: select pieces of the best available stone, roughly rectangular, 15–25cm in each dimension. Lay them in the desired circular form on a flat surface, fitting them as tightly as possible like a mosaic. Mix a mortar of hydraulic lime and clean sharp sand. Pack it into all joints. Drive iron staples across joints at intervals to add mechanical strength. Once set, dress the face flat and cut the furrow pattern. Finally, heat and shrink an iron band around the circumference to hold everything under compression.

Composite stones are slightly weaker than monolithic stones and must be handled more carefully, but they can be made from the best available local material regardless of the size of natural outcrops.