"By 3000 BCE the Sumerians, perhaps while heating copper to make it more malleable, had discovered that more copper could be retrieved from the fire if the metal were heated with certain types of dirt and stones—that is, certain earths. These earths were the metal s, and the process they discovered, ', reduced metal salts to pure metal by the action of in the fire. The process of changing metal salts into pure metal is known as reduction because the metal without the accompanying oxygen, , or of the salt weighs less than the ore. Eventually metal workers learned to distinguish various metal-bearing ores by color, texture, weight, flame color, or smell when heated (such as garlic odor of ores) and they could produce a desired material on demand."