We see St.Paul writing to the Corinthians in 2 Cor 4:6-9 (NRSVCE):
For it is the God who said, “Let light shine out of darkness,” who has shone in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ.
But we have this treasure in clay jars, so that it may be made clear that this extraordinary power belongs to God and does not come from us. We are afflicted in every way, but not crushed; perplexed, but not driven to despair; persecuted, but not forsaken; struck down, but not destroyed
The jar of clay is evidently a metaphor which stands for the missionary spirit of Paul and his team. Now, solid treasures, say gold, kept in a clay jar remain the same even if the jar breaks, but a liquid treasure, say vintage wine, stored in a clay jar gets lost if the jar is broken. St Paul goes on to say that his spirit is not fragile as a clay jar is (verse 9). So, a doubt arises as to why he used the metaphor of clay jar.
My question therefore is: Is the use of metaphor of a clay jar used by St Paul in 2 Cor 4:6-9, appropriate to context?