The Grist Mill at Colvin Run Mill Park

Feature photo: Fairfax County Park Authority

Article and other photos by FMN Stephen Tzikas

Just off Route 7 in Great Falls, Virginia, is a rare working grist mill from the early 19^th century. The Colvin Run Mill campus is part of the Fairfax County Park Authority, and is an opportunity to see nature and engineering working together.  Grist mills offer the opportunity to observe engineering principles related to powder technology in a park setting.  In 2002, my first visit to a mill, known as Evins Mill, coincided with a chemical engineering course on powder processing I attended in Tennessee. The instructor wrote the Size Reduction and Size

Enlargement chapter in Perry’s Chemical Engineering Handbook. The simple grist mills of the past represented the beginnings of powder technology engineering. Powders are used in a variety of industries including the pharmaceutical, consumer products, food and industrial chemical sectors.  The handling of powders requires a knowledge of properties related to particle size, moisture sorption, surface area, surface chemistry, hardness, density, and flowability. Operations include segregation and powder sampling, as well as key unit operations of mixing, grinding, agglomeration, classification, fluidization, drying and compaction, as well as solids transport between unit operations.

The Colvin Run Mill is powered by a 20-foot waterwheel. The use of elevators, moving belts, and Archimedean screw conveyors made the milling operation more efficient and profitable, as an early example of mass production. The wheel’s axle transmits power from the turning waterwheel into the mill. The greater face gear attached to the axle in the mill basement turns the wood gears that operate the grinding stones, grain elevators, and sifting machinery. The waterwheel produces up to 26 horsepower, turning (outside) at a rate of 10 rpm, resulting in a top grindstone rate of 100 rpm.  Premium burr stones were imported from France to grind the grain.

The tour of the mill’s four floors includes the main grinding floor, the

basement, the gear pit, and other processing units. The mill offers the ability to load, convey, sift, grind, separate and dry grain into various desired grades. Each of the four floors of the building served a specific function in turning the grain into flour or cornmeal.

The engineering processes seen at the Colvin Run Mill tour will include:

• Gears: Falling water moves the water wheel and all gears throughout the mill’s operation. The speed of the water wheel can be adjusted, thereby controlling the speed of the
• Lever: A beam scale weighs grain from farms. A receiving hopper gathers the grain.
• Pulley: Pulleys move grain upstairs for cleaning. The grain flows down a chute where it is captured by cups, and transported to the top floor and emptied into another chute.
• Screens: Screens remove dirt, seeds, sticks, straw, chaff, fungus, smut, broken grains, and bugs. Shaking shifts the grain.  There are different size mesh screens for final processing of the milled grain.  They range in size from super fine to coarse.
• Inclined plane: A inclined plane sends the cleaned grain to the grinding operation.
• Grinding: Special French millstones mill the wheat. A screw lifts the mill stone into place for grinding. Wedges line the millstone where they grind the grain. These special French millstones are fossils called burr stones. They rip and shred the grain.  Shafts in the stones move the grain to the next operation.
• Raking:  Since crushing the wheat generates heat and warms the grain powder, rakes (known as hopper boys) help cool the wheat and prevent caking and clumping