John carefully identifies six stone water jars 'after the manner of the purifying of the Jews.' Their material, purpose, number, and capacity are unusually concrete details in the Cana story.
They belong to the world of Jewish ritual washing, not merely to ordinary kitchen storage.
Archaeology from the late Second Temple period has uncovered extensive Jewish use of stone vessels, especially in Judea and Galilee. Stone was valued in Jewish purity practice because it was treated differently from many pottery vessels with respect to ritual impurity. John's jars are very large, each holding two or three metretai; estimates vary, but the combined capacity would amount to well over one hundred gallons.
The servants fill them to the brim before drawing from them.
The material background shows that John wants readers to picture a very large quantity, not a small household pitcher. The abundance is part of the sign.
The vessels' explicit connection with purification also gives the transformation symbolic resonance within John's Gospel, while the narrative itself remains clear about the physical sequence: purification jars are filled with water, and what is drawn becomes excellent wine.