A French burr millstone is, in effect, a three-dimensional jigsaw puzzle for the millwright. As you'll see from this bed-stone, the millwright has to take a dozen, or more, individual pieces of burr stone and fix them perfectly together to produce the required size. Further Examples.
The runner-stone is made in just the same way (though obviously the other way up) but because it needs to be heavy, and the burr stone used in its construction is not heavy enough in itself, a layer of rubble is added at the top. This can be seen in our overgrown layer-cake.
French burr stones could be purchased ready assembled from the quarries in France or from manufacturers in England (such as this set here from Hughes of London). They may have been assembled locally, as was the case for our layer cake which bear the initials (IA) of Mr Apsey the local millwright and the date (1832).
Once the basic stone has been prepared it must be "dressed". Dressing the stones basically involves cutting a pattern of grooves in the surface of the stone. It is the grooves that actually provide the milling action. As they cross, the grain is not so much ground as cut to pieces. The grooves are known as furrows and the raised areas in between are known as the lands. Once the grain has been cut by the furrows the resulting meal will work its way up on to the lands where final grinding takes place before finally issuing around the edges of the stones. "Dressing" is discussed further here.
Once a set of stones has been installed, or after routine maintenance, the runner-stone must be balanced to ensure that it spins true. In use the gap between the stones is on the order of a fraction of the millimetre, any wobble in the runner-stone would therefore bring it into contact with the bed-stone which must be avoided at all costs. It is said that a perfectly set pair of stones shall be separated at the rim by the thickness of a piece of tissue paper!
This balancing may be achieved in one of two ways: either molten lead is poured on to the rim of the stone or else boxes for balance weights are set into the top surface of the stone (see the Hughes example above). The millwright had to achieve this balancing act without the aid of any of the technology that we take for granted today whenever you take your car to the garage to have the wheels trimmed.
As the meal emerges from between the stones it is trapped inside the circular wooden container known as the "vat" or "tun". The meal inside the tun is drawn around by small paddles, attached to the runner-stone, until it reaches the hole in the floor that comes out as the meal chute downstairs.
The flat upper surface of the tun provides a convenient platform on which to place the hopper and its supporting frame, known as the "horse", which feeds the grain to the millstones. Grain must be fed into the eye of the millstones at exactly the right rate for their speed and separation. This is achieved by passing grain from the hopper into a short trough, known as the "shoe", on a very gentle gradient. Grain will not naturally flow along the shoe so it is caused to oscillate by means of a spring pulling it against a square peg attached to the top of the drive shaft. The rate of flow of grain can now be accurately controlled by adjusting the gradient. The square peg is known as the "damsel", allegedly because it spends all day chattering to itself. No hoppers, horses or shoes have survived at Whitemill, the pictures here were taken at Eling Tide Mill.