Nano-Carbon Shield AOT Coated Round Core Phosphor Bronze Acoustic Strings. BlackSmith acoustic guitar strings are made with the finest materials and manufactured using state-of-the-art automatic servo controlled winding equipment. USA made phosphor bronze and steel ensures a long lasting string with increased sustain and purer & warmer tone. The Nano Carbon Shield AOT (anti oxidation technology) coating prevents oxidation and dirt build-up on the wound strings without altering the tone – a BlackSmith exclusive!! Sub-Zero treated plain steel strings offer brilliant tone with a silky feel. With BlackSmith coated Round Core phosphor bronze acoustic guitar strings, you will experience the perfect balance of consistency, tone, projection, sustain, and playability. Vacuum Sealed in Patented Anti Kink Packaging Prevents Oxidation and String Damage.