Cut the furrows of a full-size millstone about 10-13 mm (3/8 to 1/2 inch) deep at the eye, tapering to about 3 mm (1/8 inch) at the rim, in 6-10 sectors called harps, and crack the flat lands between them with fine, shallow cuts. The 30-40 mm eye depth given on one wiki page is roughly three times too deep for a full-size stone, and the 4-5 mm given on another suits only a small hand quern.
Part of Community Grain Mill
The layout and cutting of the grinding faces of a millstone: harps, furrow depth and taper, the sharp and sloped sides of a furrow, and the fine cracking of the lands.
Furrows Are Deepest At The Eye And Run Out Shallow At The Rim
The furrow does three jobs: it carries grain from the eye outward, carries flour to the rim, and presents a sharp edge that shears against the matching furrow on the other stone. Grain is large at the eye and must have room, so the furrow is deepest there; by the rim it is flour and the gap is at its tightest, so the furrow runs out shallow. The stone face is also dressed slightly recessed around the eye (the swallow) so whole kernels are drawn in and started (Stone Dressing: Cutting Grinding Surfaces).
For a full-size stone the traditional figure I am working from is about 10-13 mm (3/8 to 1/2 in) deep at the eye, tapering to about 3 mm (1/8 in) at the skirt. This is a recalled figure from millwrighting references, not a measurement made for this wiki, and it is flagged for checking; the reasoning below shows why the two corpus extremes cannot both be right.
Three Reasons The Corpus Extremes Fail
- A wheat kernel is only a few millimetres thick (about 2-3.5 mm, and 5-8 mm long, recalled). A furrow 10-13 mm deep is three to six times a kernel’s thickness, ample room to take it in; a furrow 35 mm deep, the middle of one page’s eye range, is ten to seventeen times the thickness and lets flour pack into it instead of travelling outward.
- Cost of cutting. The worked example below removes about 7.7 kg of granite when all furrows are cut at 10-13 mm tapering to 3 mm. At a 30-40 mm to 6-12 mm taper the same pattern removes about three times as much, and every re-dress that restores that depth costs the same again.
- The small-quern figure is a different scale. Furrows 4-5 mm deep at the center and 2-3 mm at the edge are a sensible size for a hand quern 30-40 cm across, but the rim value of 2-3 mm agrees with the full-size 3 mm while the eye value of 4-5 mm is less than half of 10-13 mm.
| Page | Eye depth | Rim depth | Verdict for a full-size stone |
|---|---|---|---|
| Stone Dressing (construction) | 30-40 mm | 6-12 mm | Too deep, about 2-4 times |
| Stone Dressing: Cutting Grinding Surfaces | 4-5 mm | 2-3 mm | Right at the rim, too shallow at the eye; suits a small quern |
| Mill Construction | 10 mm | not given | Consistent |
| Community Grain Mill | 1/4-3/8 in (6-9.5 mm) | not given | Consistent, slightly shallow |
| Furrow Patterns | 10-20 mm (master furrows) | not given | Overlaps; the upper end is deep |
The eye depth on the first row is about 8 times the eye depth on the second, which is the disagreement this article resolves in favor of the traditional range above.
Layout: Harps, Master Furrows And Lands
Divide each face into 6, 8 or 10 equal sectors called harps (the corpus gives the carrier at 8-10 and the furrow page at 6, 8 or 10). Each harp carries a master furrow and several smaller furrows beside it, 4-6 in all according to the carrier. The furrows are not radial: they are cut at a slant to the radius (the mill-construction page gives about 15 degrees from the radius line) so that the runner’s furrows cross the bedstone’s like scissor blades as it turns. The bedstone is dressed as a mirror image of the runner, so that when the runner is turned face down its furrows cross the bed’s.
Worked Example: One Full-Size Pair
A 1.2 m runner has a rim circumference of pi x 1.2 = 3.77 m. Cut in 8 harps, each harp spans 45 degrees and a rim arc of 3.77 / 8 = 0.471 m. With 5 furrows in a harp and each furrow 20 mm wide (the mill-construction page’s 2 cm), the furrow pitch at the rim is 471 / 5 = 94 mm and the land between furrows is 94 - 20 = 74 mm wide.
Stone removed: 8 x 5 = 40 furrows, each about 0.5 m long and 20 mm wide, with an average depth of about (3 + 11.5) / 2 = 7.25 mm. One furrow is 7.25 x 20 x 500 = 72,500 mm3, or 72.5 cm3; forty are 2.9 litres, and at 2,650 kg/m3 about 7.7 kg of granite. With the 30-40 mm to 6-12 mm taper (average about 22 mm) the same layout would remove 220 cm3 per furrow, about 3 times as much.
Cutting A Furrow: Sharp Edge One Side, Sloped The Other
Each furrow has one steep, sharp-edged side and one sloped side, as in the corpus’s quern page. The sharp edge does the cutting; the slope lets grain and flour move on. Work from the eye toward the rim with overlapping bill strokes of constant angle, and clear the floor of dust with a brush. A steady dresser works at 60-80 strokes a minute (Furrow Patterns).
Crack The Lands Lightly
The lands are roughened with fine, closely spaced cuts so that they grip and shear fine particles. The corpus gives two sets of figures: pecking at 3-5 mm intervals no more than 1-2 mm deep, and diagonal stitching 3-4 mm wide and 2-3 mm deep at 10 mm spacing. I have no sourced figure to choose between them; start with the lighter one, test the flour, and cut deeper only if the stones glaze fast. The aim is a slightly rough, open surface, not deep grooves.
What You Need
- A mill bill and thrift, hardened tool steel, sharpened every 20-30 minutes (Tool Sharpening)
- A staff, a dead-straight board 1.5-1.8 m long, coated with red ochre
- A pattern board or a cord and chalk for the harp lines
- Crane or tackle to turn the runner face up
Method: Dressing A Pair
Step 1: Lift and turn the runner face up on supports, with the bedstone cleaned in place.
Step 2: Check flatness with the staff. Rub the ochre-coated staff across the face; high spots take the color. Peck them down until the marks are even.
Step 3: Lay out the harps with chalk from the eye to the rim.
Step 4: Cut the master furrows first, then the smaller furrows, to the depth above.
Step 5: Crack the lands.
Step 6: Dress the bedstone as a mirror image.
Step 7: Test. Sprinkle dry sand between the stones and turn the runner by hand; the sand should move steadily outward and emerge evenly all round (Mill Construction).
Common Mistakes
- Cutting furrows too deep. It wastes stone and clogs with flour; keep to the traditional taper.
- Furrows the same depth to the rim. Grain leaves coarse; taper them.
- Polished lands. Glazed lands slip over fine particles; crack them.
- Dressing by the wrong page. The quern figures on a 1.2 m stone, or the 30-40 mm figures on a quern, both fail.
Chips, silica dust and a heavy runner
Stone chips fly at speed and can blind: eye protection is mandatory, and the dresser sits so chips travel away from the face. Dressing makes silica dust that causes silicosis; work outdoors or with all doors open, wet the stone first, and take air breaks (Dust and Respiratory Protection). The runner weighs several hundred kilograms; never stand under it while it is lifted.
What’s Next
- Learn next: Stone Dressing Schedule - how often to redo it
- Combine with: Runner and Bedstone Mechanics - gap, balance and drive