John Calvin Commentary Jeremiah 19:10

"Then shalt thou break the bottle in the sight of the men that go with thee," — Jeremiah 19:10 (ASV)

Jeremiah summoned witnesses, so that the confirmation of the prophecy might be more fully attested to the people. Regarding the history of this transaction, we may add that he was first sent to the potter's house, from where he procured the bottle; he then went to Tophet and there spoke against their impious and corrupt superstitions; and finally, to seal the prophecy, he broke the bottle in the presence of the witnesses whom he had brought with him.

And we have said that it was necessary to deal in this way with a people not only ignorant and stupid but, what is worse, perverse and obstinate. There was not only importance in the sign, from which they might learn the doom of the city and of the whole land, but it was also a solemn sealing of the prophecy. And for this reason he was commanded to break the vessel, so that he might show by a visible act the near approach of God’s vengeance, of which the Jews had no apprehension.