Other similar metaphors are employed today. We should not think that Jesus' baptism of His disciples with fire is strange - men baptize with fire for the same reasons. Our faith is tested by fire. This gold is a genuine faith which has been tested and refined with fire. With this gold, we are spiritually rich.
Salt is a preservative.
No work will stand in the end except the work which is according to the word of God. He provides a way of escape for every temptation. Jesus baptizes His disciples with fire. Those who are refined by the fire have an eternal home in heaven. Those who are not willing to endure the testing of their faith will be eternally lost.