Flex nibs require considerable skill to make and were typically manufactured with a 14k gold alloy. This alloy gave the nib enough flexibility to support the classic Spencerian or Copperplate writing styles. At the same time, a higher proportion of gold would be liable to bend permanently. Steel was rarely used because the steels of the time had not developed resistance to corrosion. Inks were acidic and would cause the steel to corrode, limiting the longevity of the nib. The Esterbrook 9128 nib is one example of a steel flex nib.