Calvinist James White writes on Jeremiah 18:
Just as we had to express our amazement at the insertion of acts of “free will” into Romans 9:16, so too here we cannot help but point out that the main point of the entire passage is overthrown and literally contradicted all to maintain the supremacy of the free choices of men! Read Jeremiah 18 and see if the point of the parable of the potter and the clay is that there is something in the clay that determines what the potter will do?
White, James. The Potter’s Freedom: A Defense of the Reformation and a Rebuttal To Norman Geisler’s Chosen But Free (p. 225). BookMasters. Kindle Edition.
Yes, is the answer.
Jer 18:4 And the vessel he was making of clay was spoiled in the potter’s hand, and he reworked it into another vessel, as it seemed good to the potter to do.
The clay is spoiled and the potter then determines to do something else with the clay other than what he originally planned.
God is Open 
Great point Christopher!
This reminds me that Calvinism evolved via a Gnostic/NeoPlatonic dualism – synchronized into Catholic doctrine by Augustine. And Calvin swallowed Augustine’s dualistic camel.
This is why we observe in Calvinism, almost everything appears in good-evil conceptual pairs.
This good-evil dualism is what forces the Calvinist to assert [A] now and deny [A] later.
In the case if the potter and the clay:
For any “good” outcome from the clay, the Calvinist will deny the attribution of free-will.
For any “evil” outcome from the clay, the Calvinist will assert the attribution of free-will.
The Calvinist has a love-hate relationship with the dualism inherent in his own theology.
And double-think always leads to double-speak. :-]
So what James White denies now, he will eventually assert later.