Millstones and pumice were also exported, but for the former the more gritty lava of Rocca Monfina was later on preferred.
15 ), and millstones, salt (from a marsh at the east end of the harbour), and gypsum are still exported.
Minerals produced in small quantities include gypsum, millstones, salt and sandstone, and among those found but not produced (in 1902) in commercial quantities may be mentioned allanite, alum, arsenic, bismuth, carbonite, felspar, kaolin, marble, plumbago, quartz, serpentine and tin.
The slurry, which is wet enough to flow, is ground between millstones so as to complete the process of comminution begun in the wash mill.
The grinding of the hard rock-like masses of clinker is effected between millstones, or in modern plants in ball-mills, tube-mills and edge-runners.