Copper is a reddish metal with a ductile and malleable consistency. It is an excellent conductor of heat and electricity and has good corrosion resistance and low chemical reactivity. In humid air it slowly develops a greenish surface film called patina that protects it from external attacks. It is a very common mineral that occurs naturally in the environment and is used in industry and agriculture especially for the manufacture of electrical equipment and construction. Pure copper is very soft and must be hardened to be processed, while copper alloys (such as brass, a zinc alloy, and bronze, an alloy of tin) are hard and resistant.