There are good impurities carbon, maganese, silicon, nickel, magnesium, etc. (aloy elements) with benefitial properties and bad impurities like sulfur, hydrogen, oxygen, nitrogen (gases traped in the iron) that are essentialy holes in the crystal structure. The best way to get gases out of iron is to heat it up till its liquid and spin it in a centrifuge. The elements will layer themselves depending on their density. Gases will move to the center of the centrifuge and escape out of the metal.