The exact proverb is: En pitho ten kerameian manthanein (εν πίθω την κεραμείαν μανθάνειν) [Liddell & Scott, Dimitrakos].
And the exact meaning is: In breaking many pots, the potter learns his craft. (According to Liddell & Scott: Of those who undertake the most difficult taks without learning the elements of the art.)
Google for: kerameian.
But please note that the translation given in some sites about learning and practising together is wrong.
And, incidentally, this proverb is very relevant to an ongoing discussion about translation and translators.