The mining development in the Roman era consisted mainly of excavations and shafts—some more than one hundred meter deep—using hand tools and sometimes fire-setting to shatter rocks. Workers were free miners first, called "metallari" and from about 190 onwards were slaves and prisoners called "damnati ad effodienda metalla." In 369 the emperor Valentinian II decreed that each ship landing at Sardinia should pay a tax of 5 solidi for each metallarus on board. Afterwards the emperors Gratian, Valens and Valentinian II prevented any metallari from moving to the island. There was a fear that the extraordinary richness of Sardinian ore bodies might threaten the silver mines of Spain that were owned by the Emperor.