After the basic stone has been prepared, the surfaces are smoothed and a pattern of grooves is cut into them. It is these furrows that do the real work.
As the stones are used, the furrows gradually become worn and must be refreshed or "dressed". This work was often done by the miller himself, but in many areas you would find itinerant stone dressers, who would travel from mill to mill doing a few days work here and a few days work there.
The initial preparation of the stone depends very much on its source. Peakstone "blanks" would be cut from the ground as a single piece, but a set of French burr stones has to be hand assembled from a number of distinct pieces.
To turn his lumps of burr stone into a usable millstone, the millwright will first prepare the pieces of his jigsaw puzzle such that they fit together to give him the overall shape and size he requires. A typical millstone will be about 4ft in diameter. Once the pieces have been put together, a set of iron tyres are then fitted around them. This would most likely have been done in exactly the same way as a wheelwright would prepare a wooden wagon wheel. When cold, the tyre would be just a little bit too small to fit around the stones, but when heated it would expand until it was just large enough to drop into place. As the metal cools, it will attempt to shrink back to its original size, thus gripping the pieces of the stone very tightly indeed.
The stones will be sealed with plaster of paris. This is poured into every crack and crevice and is ultimately used to "ice" the stones such that the entire outer surface is covered. As well as helping to cement the stones together, this plastering also seals the stones to keep dirt out and freshly milled meal in.
Millstones always come in pairs: Bedstone and runner. The bedstone is fixed immovably in the floor and the runner spins above it.
The runner stone needs a good weight to it (about three quarters of a ton) if it is to work well. The typical thickness of burrstone alone will not provide this, so a layer of ballast is cemented onto the back of the burrstone. A peakstone runner starts out quite heavy enough, but as time wears it down it will become too light, at which point it is normally recycled as a bedstone.
Now that the basic stone is ready, the working surfaces must be prepared. Contrary to what most people expect, the millstones do not rub against one another, but they do spin very close together, so the two opposing surfaces must start out absolutely flat if they are not to collide at some point in their rotation. If the stones are allowed to come into contact then there is a very real risk of sparks which, in the flour laden atmosphere of a working mill, could readily cause an explosion. Bear in mind also the fact that the runner stone will typically be revolving at somewhere between 100 and 150 RPM. The millwright achieves the required degree of flatness by "staffing" the stone. He takes a piece of timber which he knows to be truly straight and spreads red orchre paint along its length. This staff is then dragged across the surface of the stone. Any areas standing higher than those around them will get red paint on them, the millwright then rubs away at the painted areas and repeats the process over and over until he has an enitely even surface.
The surface of the runner stone will then be very slightly dished, of which, more later.
Now the furrows are cut into the surface. The furrows are invariably cut to the "harp pattern", which can trace its origins back to Roman millstones. The reason for the name is obvious: The stone is dressed in 10 segments, each of which resembles a harp. It has always been known that this was a very good pattern for millstones, but it was only in the late 19th century that anyone actually sat down and proved it mathematically. See Flour Manufacture.
Looking at how the furrows work, we see that grains of wheat accumulate in the furrow in the bedstone, but as this fills up the upper grains get cut by the passing of the furrow in the runner stone. The smaller pieces resulting from this will be drawn up onto the lands and further reduced, by the stitching, the larger parts remain behind and get cut again by the next furrow. The furrows also act to draw air between the stones thereby cooling the stones and helping to draw the ground meal out to the edge of the stones.
To produce the finest possible meal, the lands of the burrstone (but not of a gritstone) will have secondary furrows, known as stiching cut into them. These are very much finer than the major furrows and, in the very finest work, may be cut as many as sixteen to the inch. It should be noted at this point that burrstone is an INCREDIBLY tough material.
The tools of the millwright are the "millbill" and "pick", which are made from a very specially tempered steel (much of it from Germany) and only the very best blacksmiths could keep them in good repair. The bill or pick was set in a wooden handle known as a "thrift", as we see here. Holding the thrift in both hands, the stone dresser will peck away at the stone for hours at a time. Even today there are no better tools for dressing a burrstone... a pnuematic chisel tends to do too much collateral damage, and an angle grinder (unless fitted with diamond edged discs) just isn't tough enough - they also lack the finess to do the stitching.
In order to promote a smooth flow of material from grain at the eye of the stones to meal at the skirt, the runner stone is usually cut so as to be very slightly concave. In the idealised set of stones, the gap at the eye should be the thickness of a piece of brown paper but the gap between them at the skirt should only be the thickness of a piece of tissue paper! Few millers manage that degree of excellence today, typically settling for a differerence of between an eighth and a quarter of an inch.
Millstones require redressing at regular intervals to maintain efficiency. For a gritstone this might be every few weeks, for a burrstone every few months (figures of 200 to 500 milling hours are quoted). Fully dressing a pair of stones might well take 3 days and may consume as much as three eighths of an inch from the depth of the stone.
When dressing the millstones, the hardness of the French burr would often cause small flakes of metal to break away from the point of the mill bill, some of these would become embedded in the backs of millwright's hands. It was possible to tell, from an examination of his hands, just how much experience the millwright had of dressing millstones. Hence the phrase "show us your metal, mate".
If the animation is behaving itself, you will see here how the rotation of the stones causes the furrows to cross one another giving the effect of hundreds (if not thousands) of pairs of scissors. You can also see how the angle at which the furrows cross will cause the ground meal to work its way out towards the edge of the stones. In real life, the runner stone (coloured red) would be rotating at approximately 100 to 150 revolutions per minute. The bedstone (coloured black) remains static.