you have to be able to pay attention to fine detail in order to understand the prophets. in chapter 18, jeremiah uses the potter as a metaphor for the house of israel.
clay is an earthly thing. clay is what the lord made our flesh from. and guess what?
1 Corinthians 15:50
Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.
so get your eyes off the world and on spiritual things; things of the father.