One of the most iconic sights in Pompeii are the bakeries with their donkey mills. These hourglass-shaped mills were one of several types in use in the Roman world to mill grain ready to make bread. The donkey mills have a solid lower stone, called a meta, which was bell-shaped. Over the top was a hollow upper stone, which was shaped like an hour glass; this was called the catillus. The catillus functioned like a hopper into which the grain would be poured for grinding. The stone used is hard and abrasive to help with the grinding, often basalt. As their name suggests, these mills were probably rotated by donkeys. In addition to these extant examples, there are also numerous depictions on tombstones and sarcaophagi. These come not only from Italy, but also from North Africa, the Aegean and Gaul, which attests to how widespread they were in the Roman world.
One of the objects we have selected for our exhibition comes from a different kind of mill: a rotary hand quern. These are also made from hard, abrasive stone to help grind the grain. Rotary querns are flatter than donkey mills, but also have an upper and lower stone. These kinds of mill would have ground smaller amounts of grain and would have been sufficient for a domestic household.
At the other end of the scale are watermills, which could grind grain on an industrial scale. Studies of Roman watermills have suffered in the past from the traditional view of technological stagnation in the ancient world. It was thought that slave labour and lack of interest in investment in new technologies caused a lack of water milling technology. As a consequence, for a long time, all water mills were thought to date to the early medieval period at the earliest. This idea was challenged by Orjan Wikander in the 1980s, who started to find archaeological evidence for watermills belonging to the Roman period.
There are two main types of watermill that are distinguished by the orientation of the wheel: vertical or horizontal. The vertical wheel requires a right-angle gear and turns a horizontal wheel shaft on the other end of which is a vertical cogwheel. This cogwheel drives a horizontal cogwheel and the millstones above. The horizontal wheel does not require a gearing system and is driven by water conducted either by a steep chute onto oblique paddles on the wheel or from a nozzle at the base of an arubah penstock (which functions like a tube delivering water to the wheel), or exceptionally, as a turbine.
In most parts of the Roman world, mills are of the vertical-wheeled type. In some cases, as at Barbegal in France, there were 16 wheels in a single installation. This multi-wheeled mills give a strong impression of the scale of grain milling that was occurring in the Roman world. They suggest that grain was being milled at a near-industrial scale and so that there was a large market for grain and bread. This market may have been related to the annona, a system of free grain (and later bread) given out to the populace – this is where the phrase ‘bread and circuses’ comes from i.e. that these were given to the people to keep them happy and peaceful.
In the Near East, the majority of the 25 known watermills seem to be of the horizontal type with an arubah penstock. The arubah penstock is usually a 6-10 m high stone tower that contains a column of water. This column of water is let out through a narrow orifice at the bottom of the tower, so the wheel is driven by a jet of water under pressure. It seems that this type was used in the Near East because it is a particularly efficient design to use when there is a limited flow of water.
Dating watermills is fraught with difficulty, so most of the Near Eastern mills I looked at in my PhD thesis were only tentatively dated to the Roman period. A case in point is the mill complex at Lejjun, which was thought for several years to date to the Roman period, but excavation subsequently revealed that it dates back only as far as the 19th-century AD Ottoman use of the area (McQuitty, A. “Watermills in Jordan: technology, typology, dating and development,” in Amr, K., Zayadine, F. and Zaghloul, M. (eds) Studies in the History and Archaeology of Jordan 5 (Amman: Dept of Antiquities, 1995), 745-751.). In some rare cases, the mills can be dated from literary references, for example the mills at Amida were mentioned by Ammianus Marcellinus (18.8.11) and so can be firmly dated to AD359.
For more info and a list of all possible Roman watermills in the Near East, see my book (Zena Kamash 2010 Archaeologies of Water in the Roman Near East (Gorgias Press) and http://archaeologydataservice.ac.uk/archives/view/kamash_2006/
For a full account of grain mills and milling, in the ancient world, see LA Moritz 1958 Grain Mills and Flour in Classical Antiquity – old, but still good on donkey mills and rotary hand querns.