Gristmills turned by water have been around for many centuries, some as early as 19 BC. Although the terms "gristmill" or "corn mill" can refer to any mill that grinds grain, the terms were used historically for a local mill where farmers brought their own grain and received back ground meal or flour, minus a percentage called the "miller's toll. Millers have made a pretty fair living by charging a portion of the grain as payment for their labors. Cuthbert Grant took 10% of the grain as payment for his services.
There are three major parts to a gristmill: the raceway, water wheel and grinding stone. The raceway channels the flowing water to the wheel. The water forces the wheel to turn. The turning wheel powers the grinding stones by a series of shafts and pulleys or gears and shafts. The grinding action of the stones breaks the grain into small, usable pieces like flour, cornmeal, and grits.
There are many types of water wheels that were used. The first and least efficient looked like a great paddle wheel and was called "undershot." It was simply propelled by the flowing water pushing on its blades. Grants Old Mill is considered an undershot.
Below is an example of an undershot wheel
The "overshot" wheel was much more effective and took more careful planning and placement. Water was directed to the top of it and it had blades that were more like buckets or troughs to catch and hold water. At the very least its blades were slanted rather than set perpendicular to the wheel. The weight of the water was the driving force and it built up momentum as it turned. It needed to be placed so that it was never in the stream as the motion of flowing water would slow it down.
Below is an example of an overshot wheel
Breast wheels were another type that had the water directed at about 1/2 the way towards the top of the wheel. It did better than an overshot wheel if water levels rose as it would not be drastically slowed by the water flow and was often used where it would be too difficult to use the more efficient wheel. It was half way between the two previous types.
Below is an example of a breast wheel arrangement
Sluice gates controlled the flow of water going to the wheels. The gear mechanism that caused the grinding stones to turn varied greatly. Mostly there was a gear attached to a shaft turned by the water wheel. The diagram below shows an undershot arrangement similar to the arrangement in Grants Old Mill.
The grinding stones were mostly granite and purchased from afar. The best ones came from Europe at great expense, but could last for a hundred years. Each stone weighed between half a ton and 2 tons and had to be lifted each year to be refaced. The wear on the grooves must be carefully repaired so that grains would be ground to the correct size. Uneven runner stones would also damage the underlying or "bed" stone. Stones were arranged so that the miller could raise them up or let them down to control the fineness of the product. Roughly cracked corn might be required for animal feed while fine meal was used for baking and intermediate corn particles were used as grits.
The millstones themselves turn at around 120 rpm. They are laid one on top of the other. The bottom stone, called the bed, is fixed to the floor, while the top stone, the runner, is mounted on a separate spindle, driven by the main shaft. The distance between the stones can be varied to produce the grade of flour required; moving the stones closer together produces finer flour.
Dressing a millstone
The surface of a millstone is divided by deep grooves called furrows into separate flat areas called lands. Spreading away from the furrows are smaller grooves called feathering or cracking. The grooves provide a cutting edge and help to channel the ground flour out from the stones.
The furrows and lands are arranged in repeating patterns called harps. A typical millstone will have six, eight or ten harps. The pattern of harps is repeated on the face of each stone, when they are laid face to face the patterns mesh in a kind of "scissoring" motion creating the cutting or grinding function of the stones. When in regular use stones need to be dressed periodically, that is, re-cut to keep the cutting surfaces sharp.
Millstones need to be evenly balanced, and achieving the correct separation of the stones is crucial to producing good quality flour. The experienced miller will be able to adjust their separation very accurately. Below are mill stone examples