God is often said to be our potter, we are often said to be His clay. He molds us and shapes us into the creation that He would have us be. The term pottery is used to describe a technique where clay is mixed with other minerals and is formed into various objects, including vessels. The shaped clay body is fired in a kiln to induce permanent changes that result in increased strength
So if we are God's clay and He is our potter, He must be mixing us, His clay, with other things and forming us into vessels to be placed in fire to induce permanent changes that will increase our strength.
What are these things, these minerals, He mixes us with? To be classified as a "true" mineral, a substance must be solid. It must also be naturally-occurring. God mixes us with naturally solid things.
II Peter names some of those naturally solid things.
Chapter 1 :5 And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge; 6 And to knowledge temperance; and to temperance patience; and to patience godliness; 7 And to godliness brotherly kindness; and to brotherly kindness charity. 8 For if these things be in you, and abound, they make you that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ.
What fire does He places us in?
Matthew 3:11 speaks of the fire. I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:
Can permanent changes occur that increase the strength of these human vessels?
In Acts Chapter 9 we read of the most amazing story of permanent change ever told. Saul who was a persecutor of followers of Chirst, after an encounter with Holy Ghost fire, became Paul became a mighty follower who preached the gospel of Christ.
I Peter 4 describes that fire.
Chapter 4: 12 Beloved, think it not strange concerning the fiery trial which is to try you, as though some strange thing happened unto you: 13 But rejoice, inasmuch as ye are partakers of Christ's sufferings; that, when his glory shall be revealed, ye may be glad also with exceeding joy. 14 If ye be reproached for the name of Christ, happy are ye; for the spirit of glory and of God resteth upon you: on their part he is evil spoken of, but on your part he is glorified.
We each are continually in need of being on the Potter's wheel to have solid things mixed in, shaping us into His chosen vessels. As God places these vessels in fire, permanent changes occur and He increases our strength.