In Biblical times clay jars were a very common household article. They were used to store grain, oil, water, wine, bread, coins and even bible scrolls - like the Dead Sea scrolls. Most were plain, inexpensive and quickly discarded when cracked or broken. In today's culture they have been replaced by the ubiquitous plastic bag.
In 2 Corinthians 4:7 Paul writes, 'But we have this treasure in clay jars to show that its extraordinary power comes from God and not from us'. The treasure that Paul was referring to was the Word of God (the Old Testament writings), all the teachings of Jesus, and the miracle power and presence of the Holy Spirit. The clay jars (vessel) referred to his own humanity, with all its frailties and weaknesses.
As I meditated on this verse I asked the Lord to give me more revelation about the meaning of the passage. The image that immediately came to me was a clear incandescent light bulb. I laughed at myself and said "that's not even a vessel!" "Look again," I heard. As I looked again I realized that the light bulb was indeed a sealed glass vessel containing a wire filament. As I watched further, the bulb was suddenly 'switched on' and became a dazzling white light and I understood that this represented the presence of the Holy Spirit shining forth from the vessel.
It's a big stretch from a clay jar to an incandescent light bulb, but no distance is too far a stretch for the imagination of our God.
Postscript: I recently upgraded my laptop to 'Word 10 ®'; as a result, I receive a beautiful scenic image when logging into my computer each day. Last week I received an image of an incandescent light bulb, not attached to any electric socket, but shining brightly. I burst out laughing at God's creativity, and how God could use the good folks at Microsoft to bless my day!