Hard-drawn copper is bare copper wire that hasn’t had heat applied to it after it has completed the drawing process through the dies. The more times the wire is pulled through a die, the more “work hardened” it becomes. After a certain point, the wire becomes brittle and could break due to stress.

By forgoing heat treatment, hard-drawn copper has a much higher tensile strength than annealed copper. It also has higher resistivity due to its “hardness.” This is because as the wire is pulled through the dies, the crystalline structure within the copper itself breaks down. As a result, it’s harder for electrons to flow through this copper because they’re too busy being jostled around by the irregular crystals.