January 1, 2011
M is for Millstone
Among the many European millstones, the geology of the French burrstone was prized for the effect of its quartz grain.
‘mil-st¯on A portmanteau created by blending the words ‘mill’ and ‘stone.’ “I can never cease to hanker for the rumble and grumble of the busy mill, and the solemn murmur of the millstone . . .” Remarks, 1891, by Bill Nye, p. 102.
By Bill Langer
Millstones have been used to mechanically grind indigestible seeds into nourishing flour since the time of the Romans. The flour they make has become part of the daily diet of a third of the world’s population, and without millstones, civilization would probably be quite different from what we know.
Millstones are used in pairs. Those that run horizontally are called face grinders. The lower millstone, called the ‘bed stone,’ is stationary; the upper stone, called the ‘runner stone,’ turns about its axis. Millstones that are oriented vertically and ground on their edge are used to grind harder materials such as barite, cement, feldspar, gypsum, phosphate rock, mica, and quartz. These millstones are called edge-runners or crushers.
In the Colonial United States, the early mills generally used millstones from Europe. Many of these millstones, called composite millstones, were shipped in pieces that could be carefully fitted together, cemented, and wrapped around the perimeter with iron banding.
Colonists generally preferred millstones from their homelands. Mills constructed by millwrights from Holland and Germany commonly used millstones made from bluish-gray lava flow rock first quarried near Köln (Cologne), Germany. These stones, called ‘Cullin’ (Köln) stones, were first quarried by the Romans and were popular throughout most parts of the European Continent.
The English colonists favored quartz-conglomerate millstones cut from English quarries in the Peak District of southwest Yorkshire and the northeastern perimeter of Derbyshire. The number of millstones imported to America from England sharply decreased following the Revolutionary War and the War of 1812 due to political, practical, and economic reasons, and the use of native stone increased greatly. During the heyday of milling, it is likely that millstone quarries were present in most areas of the United States where there was a suitable hard stone and where grain was milled.
However, no millstone could match the world-famous French burrstone, a freshwater quartzite quarried in Northern France. A special property of those stones was that, as they wore away, new sharp edges of quartz grain were exposed. Burrstones produced a whiter flour from wheat because it broke the bran (the dark, outer part of the grain) into large flakes. Other stones tended to finely shred the bran, which would pass through the flour dressing machinery, resulting in a darker colored flour. French burrstones were found in most American mills in addition to other imported or native stones.
No machinery was available to quarry and shape a millstone — only hand-held tools and human muscles. Drill holes were beat into rock using hand-held bits hit with sledge hammers. Next, feathers (pairs of inverted L-shaped objects) were placed into the holes, wedges were put between the feathers and tapped, one after another, until the stone split. Calipers made from forked tree branches were used to scribe a circular outline of the stone, and specialized hammers, chisels, and drill bits were used to shape to the line and create the hole for the axle. Other specialized hammers and picks were used to cut furrows in the stone, which was called ‘dressing’ the stone.
I never cease to hanker for the rumble and grumble of the busy quarry, the sharp sound of steel on stone, and a slice of warm bread made from millstone-ground flour.
Bill Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey and can be reached at 303-236-1249 or via e-mail at email@example.com.