Mischna zu Wajikra 19:15

Water may be poured over the hands out of any kind of vessel, even out of vessels made of animal dung, out of vessels made of stone or out of vessels made of clay. Water may not be poured from the sides of [broken] vessels or from the bottom of a ladle or from the stopper of a jar. Nor may one pour [water] over the hands of his fellow out of his cupped hands. Because one may not draw, nor sanctify, nor sprinkle the water of purification, nor pour water over the hands except in a vessel. And only vessels closely covered with a lid protect [their contents from uncleanness]. And only vessels protect [their contents from uncleanness] inside earthenware vessels.