Refiner.
God is the refiner of His people, as the precious metals have to be separated from the dross that clings to them. Proverbs 25:4 (where the A.V. has 'finer'); Isaiah 48:10; Zechariah 13:9; Malachi 3:2, Malachi 1:3. So God tests the believer's heart (1 Thessalonians 2:4), and his faith, which leads to endurance. James 1:12; 1 Peter 1:1.