http://agathe.gr/democracy/ostracism.html
Ostracism In addition to the legal assassination condoned in the Law against Tyranny, a less extreme method was also available for removing powerful but dangerous men from public life. This was a formal, ... Officials at the entrance collected the sherds and kept the citizens inside the enclosure until all had voted. The sherds were then tabulated; if more than 6,000 votes were cast, the man whose name appeared on the greatest number was sent into exile for 10 years. ... Stray sherds from the whole area represent later votes of ostracism and provide the names of most of Athens’ prominent statesmen (21, 22). 22.