When the insulation is removed from the copper magnet wire the copper becomes oxidized under ordinary exposure to air. Copper becomes coated with green basic carbonate when exposed to moist air containing carbon dioxide. That’s why protecting the coils from moisture is important and potting solutions are used. Copper wire is said to be hard-drawn, medium-hard or annealed, the latter being the softest condition. Annealed copper wire used in coils has a tensile strength of about 30,000 lb. per square in.