Bolting cloth is a fine woven fabric, of silk, tightly woven linen or metal screen, stretched on a frame or a rotating drum so that fine flour falls through and bran stays on top. Rate any cloth by sifting 1 kg of meal through it and weighing what passes: 0.74 kg through means 74% of the meal became flour. A hand frame suits a household; a reel driven from the mill shaft suits a village.
Part of Community Grain Mill
Cloth, frames and machines that separate flour from bran after milling: what the cloth is made of, how to test it, and how a hand sieve differs from a powered bolting reel.
Finer Cloth Makes Whiter Flour And Less Of It
The carrier lists three cloths: silk (traditional, the finest sifting), fine linen (adequate for most grades) and metal screen (the most durable, and salvageable from kitchen sieves). The corpus mill design also mentions horsehair and fine wool (Grain Mill Design). A coarser cloth passes more of the meal and takes out only the largest bran flakes (the carrier’s “first clear” flour); a finer cloth passes only the finest particles and gives whiter flour.
The cloth itself is a textile problem. A tightly woven linen needs fine, even yarn from flax processing and a loom that can beat the weft up close (Weaving); a cheap loose cloth will not hold back bran.
Hand Sieve Or Powered Reel
A hand sieve: a round or square frame with the cloth stretched drum-tight. Shake or tap it over a tub. It suits a family and costs almost nothing, but sifting a village’s flour by hand is days of work.
A bolting reel: a hexagonal drum on a slightly tilted axis, covered with cloth, that turns slowly. The meal is fed in at the raised end and travels down the drum as it turns, and flour falls through the cloth into a chest while the bran travels out of the far end (Grain Mill Design describes it as driven from the mill’s own shaft). Using different cloths in sections of the drum sorts several grades in one pass.
The reel takes power from the mill, so add its load to the power sum for the site before building it (Bushels per Hour by Power Source). I have not measured the reel’s load; a light cloth reel takes much less than the stones, but time the mill with and without the reel engaged before relying on a figure.
What You Need
- Cloth of at least two fineness grades, silk if you have it, fine linen if not, or metal screen
- Frames and a drum stiff enough not to sag, and a chest or bin to catch the flour
- A scale accurate to about 10 g, a stopwatch, and a clean tub
Method: Rating A Cloth
Step 1: Weigh 1.00 kg of freshly ground whole-grain meal.
Step 2: Sift it by hand through the cloth for 2 minutes with a steady shake, then stop.
Step 3: Weigh what passed. Say 0.74 kg passed: that cloth gives 74% flour and 26% bran-rich overs.
Step 4: Judge the flour by feel. It should feel smooth between thumb and finger, with no gritty specks of bran.
Step 5: Choose the cloth for the grade. Use the coarser cloth for brown flour and the finer for white, and keep a record of which cloth gave which result.
Step 6: Brush the cloth after each session. Meal packed into the weave blocks it and shortens the cloth’s life.
Worked Example
A reel is fed a day’s grain of 68 kg (the carrier’s 100-person village). If the chosen cloth passes 74% of the meal, 68 x 0.74 = 50 kg of flour comes out and 68 - 50 = 18 kg of bran-rich overs goes to the animals or back to the stones for a second grind. Re-grinding the overs is a way to raise the flour yield without finer stones, at the cost of a second pass through the mill.
Common Mistakes
- Using damp meal. Meal from damp grain, or in humid weather, clogs the cloth; dry the grain (Grain Moisture and Milling Readiness).
- Loose cloth. Slack cloth passes bran and tears at the corners; stretch it drum-tight.
- Mixing grades. Keep a record, or white and brown flour end up in the same bin.
The bolter is the dustiest room in the mill
The enclosed reel and chest hold a standing cloud of fine flour, which explodes if a spark, a hot bearing or a flame reaches it. Keep open flame out of the room and out of the mill, ventilate, sweep the dust daily and never test for dust with a flame. The stop for a powered reel is the drive belt, which must be thrown off its pulley from the floor, and the drum coasts a short time; do not reach into the drum or the drive while it turns.
What’s Next
- Learn next: Specialty Grinds: Cornmeal, Oat Flour - grains that do not sift like wheat
- Combine with: Whole Grain vs Sifted Flour - when to sift at all