Querns for Grinding Ores
Ore breakage in Greek and Roman times was mainly by human muscle power, although cattle mills were introduced about 200 bc. The usual procedure was for rocks to be broken with hammers, large pebbles to be broken in mortars and pestles up to 1.5 m deep and 4 m in diameter, and small pebbles to be broken in rotary querns. Screens made of hazel twigs or horsehair were used to separate the fine particles.
The Japanese woodcut in Figure 3.13a shows an example of how the rotary quern was used to break ores, with more detail illustrated in Figure 3.13b. The ore grinders took ore that had been broken to 5-mm pieces in the mortars and broke it further to
0.25-mm pieces in the rotary quems. A mill produced about 4 tons per 24 hours, was driven by up to six slaves, and used about 0.5 kW of energy. The coarse product from the screen was returned to the mill for further grinding.
Men and women grinding ores must have worked in conditions that were even worse than for those grinding grains because the pebbles were much harder. The conditions were described in a footnote in De Re Metallica.
Those over 30 years of age take a piece of rock of a certain quantity, and pound it in a stone mortar with iron pestles till it be as small as a vetch; then those little stones so pounded are taken from them by women and older men, who cast them into mills, that stand together there near at hand in a long row, and two or three of them being employed at one mill they grind a certain measure given to them at a time until it is as small as fine meal. No care is taken of the bodies of these poor creatures, so that they have not a rag to cover their nakedness, and no man that sees them can choose but commiserate their sad and deplorable condition. For though they are sick, maimed or lame no rest or inter-mission in the least is allowed them; neither the weakness of old age, nor women’s infirmities were any plea to excuse them; all are driven to their work with blows and cudgelling, till at length, overborne with the intolerable weight of their misery, they drop down dead in the midst of their insufferable labours. (Agricola 1950)
The multiholed stone was used 3,000 years ago to grind ore that had already been broken to a powder of less than 5 mm for mineral separation. High-grade particles were placed in a depression in the large fixed stone and broken with a smaller rounded stone held in the hand. When the depression deepened to the point at which breakage was difficult, the operator moved to a new position on the stone.