Early metal windows were made out of wrought iron by blacksmiths, and were expensive to produce. Steel is now the dominant material, with modern windows designed to be energy efficient and corrosion resistant.
Wrought iron: Relatively durable, early wrought-iron frames and casements were usually made of flat sections, but this meant that they often provided little resistance to wind-driven rain.
Cast iron: Used from the mid 18th century, cast iron was reasonably resistant to corrosion but it was brittle.
steel: In the 19th century, the introduction of mass-produced rolled steel enabled precise and more complex sections to be formed. This meant manufacturers, like dominant Crittall, could produce better-sealing casements.