Choose millstone rock that a steel point cannot scratch, that rings clearly when struck, and that is evenly fine-grained and slightly porous; quartz-rich burr, vesicular basalt or fine sandstone and granite all work, while slate, marble, limestone and flint do not. Split the block with the plug-and-feather method, expect 3-6 weeks of work from quarry face to usable stone, and plan the haul around a blank that weighs about 750 kg.
Part of Community Grain Mill
Finding, testing and splitting rock for millstones: the properties that matter, the rocks that work and those that fail, and how to get a block out and home.
A Good Millstone Is Hard, Even, Slightly Porous And Free Of Flaws
The longer comparison is in Millstone Selection; this article keeps the field tests and the quarrying. Four properties decide:
- Hardness. The stone wears against grain and the pick marks must hold. A steel point should skid, not cut a groove, on a fresh face.
- Porosity. A fine, even porosity grips the grain and carries heat away; coarse voids make a surface that crumbles.
- Uniformity. Hard and soft bands wear unevenly and leave the face wavy; strike the face at intervals and listen for a consistent ring, not dull thuds.
- Clean fracture. When dressed, the stone must chip cleanly along the pick marks without shattering.
The carrier lists freshwater quartz (burrstone), granite and sandstone: quartz as the historic best, granite as hard and widely available, sandstone as easier to work but quicker to wear. Granite’s feldspar can weather out and leave soft spots, and coarse-grained granite is worse than fine (Millstone Selection).
Rocks To Avoid
Slate splits along its planes, marble is too soft, limestone is too soft and attacked by acids in the grain, flint is too hard to dress, and any rock with visible voids or hard and soft bands fails early. A stone that sheds grit contaminates the flour and wears the eater’s teeth; this is a health problem as well as a waste.
What You Need
- A sample of each candidate rock, a fist-sized piece, from the outcrop face rather than loose surface rock
- A steel nail or chisel, a mason’s hammer, and a bucket of water
- Drills or chisels, iron wedges (feathers) and plugs for splitting
- Levers, rollers or a sled to move the block, and a team
Method: Testing And Quarrying
Step 1: Scratch test. Draw a hardened steel point across a fresh break. A groove means the stone is too soft for a runner.
Step 2: Ring test. Strike the sample and large blocks in several places. A clear, even ring means sound rock; a dull thud means a crack or soft pocket.
Step 3: Water test. Drop a little water on a fresh face. It should soak in slowly and evenly; pooling suggests glassy rock, instant soaking suggests rock that is too open.
Step 4: Split a sample. Break a piece with the hammer. The fracture should be controlled, with sharp edges, not crumbly.
Step 5: Mark the block. Draw the disc diameter plus allowance for trimming on a sound face, avoiding veins and cracks.
Step 6: Plug and feather. Drill holes along the split line at intervals, set a pair of iron feathers in each with a plug between them, and drive the plugs in turn, a little at a time, until the stone splits along the line.
Step 7: Rough-shape at the quarry. Knock off the excess with a mason’s hammer; it is much easier to move a rough disc than a square block.
Step 8: Cut the eye with a star drill (a chisel rotated a little between blows) in a circle.
Worked Example: How Heavy Is The Blank?
A 1.2 m runner blank, 0.25 m thick, is a cylinder of radius 0.6 m: area pi x 0.6^2 = 1.131 m2, volume 1.131 x 0.25 = 0.283 m3. At 2,650 kg/m3 (a typical granite density, recalled) the solid blank weighs 0.283 x 2,650 = 749 kg, about 7.3 kN. The squared block that encloses the disc is 1.2 x 1.2 x 0.25 = 0.36 m3, which is 0.36 x 2,650 = 954 kg before it is rounded. A team using levers and rollers can move that on a prepared track; a sled with greased runners works on level ground; lifting needs a tripod or crane with rated rope, because a rope that parts on a 750 kg stone kills.
A Composite Stone Is The Fallback When No Large Block Exists
The corpus describes building a stone from pieces of good rock set in a hydraulic lime and sand matrix, held with iron staples and a shrunk iron band (Millstone Selection). French burr stones were historically built this way. The result is a little weaker than a monolith and must be handled more carefully, but it lets you use the best local rock whatever the size of the outcrop.
Common Mistakes
- Testing loose surface rock. Weathered stone is softer and more cracked than the same rock a metre in; test fresh faces.
- Quarrying before testing. Weeks of labor go into a block that fails the ring test.
- Driving the plugs too hard. The split runs off the line; tap each plug in turn.
- Moving the block without a plan. The haul is where people are hurt.
Rock splitting, chips and silica dust
Splitting throws shards and a falling block crushes. Wear eye protection, keep everyone clear of the block’s path, and chock it before and after every move. Cutting and dressing stone releases fine silica dust that causes silicosis over years: work outdoors, wet the work surface, wear a damp cloth over mouth and nose, and rest from dust often (Dust and Respiratory Protection).
What’s Next
- Learn next: Dressing and Furrowing Patterns - cutting the faces
- Combine with: Stone Types - the corpus page on millstone rock