Phosphor Bronze Kerly Music strings are seasoned through a patented metal tempering process that exposes them to minus 320F and plus 350F for 24 hours. The treatment strengthens the string at the molecular level, ads tonal crispness, increases durability and dramatically resists corrosive elements that reduce the life and tone of your strings. Tuning is less frequent, and a smoother surface feel is created with stellar bendability.