http://agathe.gr/democracy/the_verdict.html
The Verdict After the speeches and other evidence had been presented, the members of the jury voted by casting ballots. A series of vase paintings of the early 5th century B.C. show a mythological story, ... We are told that in voting on whether Orestes should live or die, the contesting parties placed their ballots (pebbles or mussel shells) into one of two urns, one for the prosecutor, the other for the defendant. Although it is hard to see how this procedure could be secret, since the choice of urn might be observed by anyone present, it seems that the Athenians had devised a way to maintain secrecy, for in the vote over Orestes' fate, the outcome is in doubt until the votes are counted. Athena, casting her vote in Orestes' favor, says: If the other votes are even, then Orestes wins.