Like steel and aluminum, bronze is strong and stiff, excellent attributes for a window frame supporting the weight of up to four layers of glass. Bronze frames are a sandwich of bronze on the inside and outside with an insulating layer between them to limit heat transfer by conduction just as with modern “thermally-broken” steel and aluminum window frames.

Bronze needs no protection from the elements as it will not rust but simply darkens as it ages.

Some manufacturers offer wood frame windows covered on the outside (“clad”) with bronze instead of aluminum.