During copper smelting, oxygen is added to the metal to help remove impurities. Oxygen-free copper has been refined to reduce the amount of oxygen in the metal in order to increase conductivity and resistance to oxidation. There are three levels:
- C10100 (oxygen-free electronic or OFE) — 99.99% pure copper. With OFE, even silver in the copper is considered an impurity. This is the most expensive type, as the copper must be refined under oxygen-free conditions.
- C10200 (oxygen-free or OF)) — 99.95% pure copper. OF may contain silver.
- C1100 (electrolytic tough pitch or ETP) — 99.9% pure copper ETP may contain silver.
Oxygen-free copper is used for industrial applications, such as for the windings in large electrical generators where the wire is constantly exposed to high temperatures and vibration, as well as for audio and video cabling, where the increased conductivity is believed to improve low-frequency transmission.
