Bark Mill

Part of Mill Construction

Building and operating mills for grinding bark to produce tannin extract for leather tanning.

Why This Matters

Leather production is fundamental to a rebuilding society — it provides shoes, harnesses, water-resistant containers, bellows for forges, and countless other items. Tanning leather requires tannin, and the most abundant source of tannin in most temperate regions is tree bark, particularly oak, chestnut, spruce, and hemlock. A bark mill grinds this bark into a fine pulp that releases its tannin into water efficiently.

Without a bark mill, bark must be pounded by hand — an enormously labor-intensive process. A single tannery serving a community of a few hundred people requires enormous quantities of ground bark. A water-powered bark mill can process in an hour what would take a team of workers a full day by hand, making viable leather production possible.

The bark mill is also a relatively simple watermill application — the crushing and grinding requirements are less demanding than grain milling, and it makes an excellent first mill project before attempting the precision required for grain flour production.

Bark Mill Design

The classical bark mill uses a series of wooden stamps or rollers to crush and grind bark. Two main types:

Stamp mill design: A series of heavy wooden or iron-shod stamps (vertical hammers) are lifted by cams on a rotating shaft and allowed to fall on the bark in a trough below. The stamps pound the bark repeatedly, reducing it to fine fragments. Simple to build and very durable — the mechanism is entirely wood and iron with no precision fits required. Requires more water power than the roller design but is more robust.

Roller mill design: A grooved or corrugated roller, typically granite or hard stone, rolls in a circular trough, crushing bark under its weight as it turns. A central post drives the roller via a horizontal arm. The bark is fed into the trough manually and raked to keep it under the roller. Produces finer, more uniform output than the stamp mill.

Edge runner (pan mill): Two heavy stone rollers mounted vertically, rolling around a circular stone pan. The rollers rotate on horizontal axles attached to a central vertical shaft (the runner). This design is adaptable to many materials beyond bark: grain, ore, clay, and dyestuffs. More complex to build but far more versatile.

Building a Stamp Mill for Bark

The stamp mill is the easiest to build from rough materials:

The cam shaft: A horizontal hardwood shaft (100-150mm diameter) running across the top of the mill frame. Cams — wooden or iron projections — are set into the shaft at intervals. As the shaft rotates, each cam lifts the stamp by its lifting arm and then releases it. Space cams to lift stamps sequentially (not all at once) to distribute load on the waterwheel.

The stamps: Heavy vertical poles or rectangular timbers, 50-80mm square, 1.5-2m long. Iron shoes on the bottom face (50mm thick, hardened where possible) take the wear. A lifting arm (a horizontal peg through the stamp) is engaged by the cam. Guide the stamps in slots cut in the upper and lower crossbeams of the frame to keep them vertical.

The trough: A hardwood or stone trough beneath the stamps, slightly wider than the stamp width. The bark is loaded to 150-200mm depth and the stamps fall repeatedly on it. Drain holes in the bottom allow water to be added (wet grinding releases tannin better) and excess to drain away. The bottom of the trough takes heavy wear — line it with renewable hardwood inserts or flat stone slabs.

Drive train: The cam shaft connects to the waterwheel via a simple gear or direct coupling depending on the required speed. Stamps should fall 2-4 times per second for efficient processing — this requires cam shaft speed of about 120-240 RPM.

Water Power Requirements

Bark milling requires moderate power — more than grain milling per unit volume, because bark is fibrous and does not crush as easily as grain. A stamp mill with four stamps, each weighing 40kg, falling 300mm at 3 strokes per second, requires about:

Power per stamp = mass times gravity times height times frequency = 40 times 9.8 times 0.3 times 3 = approximately 350 watts per stamp. Four stamps = 1.4 kW minimum, plus losses. A waterwheel delivering 3-5 kW can comfortably run such a mill.

For a roller mill or edge runner, power requirements are lower (0.5-1.5 kW) but the mechanism is more complex and requires a more reliable steady drive — a buckshot flow variation in the water supply is more problematic for a roller mill than a stamp mill, which accommodates fluctuation gracefully.

Bark Preparation and Processing

Fresh bark is stripped from felled trees in spring when the sap is rising — this is when the bark separates most easily from the wood and has the highest tannin content. Strip bark in sheets, dry briefly (a few days) to reduce weight, then feed to the mill.

Oak bark yields 10-15% tannin by dry weight. Chestnut bark yields 8-12%. Hemlock bark (North America) yields 8-12%. The ground bark is called “spent bark” or “ground bark” and is mixed with water to make the tanning liquor.

For pit tanning (the traditional slow-tan method), ground bark is layered with hides in stone or timber pits. The tannin slowly penetrates the hide over weeks to months, producing firm, durable leather. A single bark mill can supply a tannery processing dozens of hides simultaneously in multiple pits.