Workings of a Grist Mill
The following is expanded from material provided by John Goff.
The US government’s HABS efforts produced a few drawings of mills including the Leffert's Tide Mill. These machines were also in the Souther Tide Mill Grist Mill (which we know from the fact that it was a grist mill, also from Josiah Adams Fenno's descriptions printed in old Quincy newspapers). Note some of the basic core items which characterized these mills: a) Crane: used to pick up the top millstone, and to remove it for sharpening. It often looked like a pair of iron tongs, attached to a large iron screw and a pivoting gallows-like device. The tong parts would grab the runner stone at two opposite points, then pivot and swing it away where it could be flipped over for sharpening by the miller, using a mill pick or mill bill.
A. The Crane
Crane is at upper left in illustration.
HABS drawing of Van Wyck Lefferts Tidemill
Illustration from Mill: The History and Future of Naturally Powered Buildings by David Larkin. New York, 2000.
A millstone could be either a runner (top) or bed (bottom) stone; used in pairs to rotate on a common axis, and to cut the grain into meal--which was then sifted to produce (in progressive finenesses) flour, middlings and bran. Each millstone would be typically about 4 feet in diameter, cylindrical, about a foot thick, and very heavy. The Souther used millstones made locally of Quincy granite (we think) for grinding yellow corn into cornmeal, and also used imported French millstones--called French buhrstones, which were used to cut and create the whiter flour.
The bottom stone was stationary. The top stone or runner was connected to the turning shaft that came through the bottom stone with the bracket shown. The stone on the grounds of the Perkins mill site is turned upside down to show the notch where the bracket would fit.
Exploded isometric view of a pair of millstones in Windmills & Watermills by John Reynolds. New York, 1970.
Millstone on the grounds of the Perkins Mill in Kennebunkport, ME.
Mill stone being quarried in Medfield, MA.
French buhr stone made up of blocks. Pieces are cemented together with plaster of Paris and the whole bound around with iron hoops. Illustration from Mill: The History and Future of Naturally Powered Buildings by David Larkin. New York, 2000.
The stones were housed within a hoop or tun. Hopper on top of tun enclosing mill stones with horse and hopper.
Inverted pyramidal wood funnel, used to focus the pour of the grain into the eye of the millstones below. d) Horse: Four legged wooden contraption used to hold up the hopper above the hoop or tun. Often had nicely turned legs--like these on the Lefferts Mill horse below the hopper. e) Hoop or Tun: Octagonal or cylindrical wood cover used to fit over the stones, and to keep the flour and meal from spraying all over the milling floor during milling. These covers could be removed when access to the millstones was needed, such as for sharpening them.
This illustration shows a stone in quarter dress pattern.
Four legged wooden contraption used to hold up the hopper above the hoop or tun. Often had nicely turned legs--like these on the Lefferts Mill horse below the hopper.
E. Hoop or Tun
Octagonal or cylindrical wood cover used to fit over the stones, and to keep the flour and meal from spraying all over the milling floor during milling. These covers could be removed when access to the millstones was needed, such as for sharpening them.
Illustration from Early American Mills by Martha and Murray Zimiles. New York, 1973.
Mill Stone Patterns from Wikipedia: 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 furrows and lands are arranged in repeating patterns called harps. A typical millstone will have six, eight or ten harps. The grooves provide a cutting edge and help to channel the ground flour out from the stones. When in regular use stones need to be dressed periodically, that is, re-cut to keep the cutting surfaces sharp. Millstones come in pairs. The base or bedstone is stationary. Above the bedstone is the turning runner stone which actually does the grinding. The runner stone is supported by a cross-shaped metal piece (rind or rynd) fixed to a "mace head" topping the main shaft or spindle leading to the driving mechanism of the mill (either water or wind powered). 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. 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.
The following is from http://www.engr.psu.edu/mtah/projects/millstones.htm
In order for a millstone to cut the grain into flour - for that is exactly how grain is ground - the millstone faces need to have grooves cut into them. The pattern of these grooves varied from stone to stone and from region to region, and stone dressers developed both elaborate and subtle variations for different grains, different grinds (much like the range of grinds of coffee available today), and different grain conditions. The patterns on the stones working together act like a pair of scissors to cut the grain as the stones rotate. When we speak of a grindstone, we can either mean each of the pair or the two together, for you must have a pair to do any useful grinding. The bottom stone is called the bedstone and it remains stationary - in fact, it is set into a bed of concrete or mortar to make it literally rock stable. The top, rotating stone is called the runner stone, and it has a hole in the middle through which the grain is fed. Both stones have a pattern of grooves cut into their working faces (top face for the bedstone and bottom face for the runner stone) that act as feed channels for the flow of the grain to the outside edge. As the two stones rotate against one another - never touching! - the grain is repeatedly scissored between the grooves, whose gap varies from well less than 1/8" near the center down to thousandths of an inch at the perimeter. Individual grains are cut again and again on their journey from the center out to the edge. A stone dresser would come to the mill once a year and re-face the stones to keep the grooves crisp and the stones milling smoothly through the year.
Illustration from Mill: The History and Future of Naturally Powered Buildings by David Larkin. New York, 2000. A mill with a vertical wheel used gearing to transfer the power of the water to the vertical shaft to turn the mill stones. A mill with a horizontal wheel often used the shaft of the wheel to turn the mill stones directly.
Horizontal mill from Ramelli, Le Diverse ed Artificose Machine, 1588 as published in British Water-Mills by Leslie Syson. London, 1965.
F. Lighter or Lightening Staff and Bridge-tree
I don't know as all mills had these; I think this was one way, similar to tentering devices, used to lift the pivot point upon which the vertical axle that held the runner stones--rotated--so that in fact the runner stone could be raised or lowered, to adjust the thickness of the flour or meal ground in the mill. Thomas McErlean, Harnessing the Tides: In horizontal-wheeled mills, the mechanism for adjusting the space between the mill-stones—the tentering system—is relatively simple but effective. A vertical timber called the ‘lightening-tree’ extended from the floor of the millhouse to connect with a horizontal timber called the ‘bridge-tree’ at the bottom of the wheelhouse on which the base of the waterwheel rotated. Raising or lowering the lightening-tree resulted in a similar movement of the bridge-tree and the waterwheel and, via the shaft and spindle, ultimately the upper millstone bia the rynd on top of the spindle.
from website for Molinos Nuevos in Murcia: http://www.waterhistory.org/histories/murcia/
Elevation showing bridge-tree.
Illustration from Harnessing the Tides by Thomas McErlean and Norman Crothers. 2007.
A plan showing the bridge-tree. Illustration from Harnessing the Tides by Thomas McErlean & Norman Crothers. 2007.
the following is from http://www.angelfire.com/journal/millrestoration/bottleweights.html
The Young Mill-Wright and Miller's Guide, by Oliver Evans, was first published in 1795, and ran to 15 editions by 1860. One section deals with tentering, and the use of the lighter staff and bottle weight in that process. "The lighter (staff) is fixed in front of the mill-beam, at such a height as to be handy to raise and lower at pleasure; a weight of 4 lbs. is hung to the end of it by a strap, which laps two or three times round, and the other end is fastened to the post below, that keeps it in its place." This weight is called a bottle weight. The lighter staff is used for raising and lowering the runner stone. It is called the lighter staff, because its movement lightens the grinding effect of the millstones upon the grain. Raising and lowering the lighter staff, a term called tentering, adjusts the distance between the millstones for the desired type of grind. The bottle weight is then what holds the lighter staff in position to maintain uniform result. Other than in drawings found in his book (often reproduced elsewhere) there is no further mention of how this weight is used. Yet, the weight is a mill feature that is not widely understood or even recognized when taken away from the mill. Note from John Goff: The Souther Tide Mill Grist Mill (Quincy, MA) definitely had crane(s), millstones, hopper(s) horse(s) and hoop(s) or tun(s) when it was functioning--also other devices like conveyors and water (speed) controls too.