What is metal moulding?

There are frames made from metals such as brass, copper, and silver, but when picture framers speak of metal frames they are typically referring to extruded aluminum frames.
The first metal mouldings to catch the public’s eye were made for the Museum of Modern Art in New York City, in the late 1950s. Metal moulding became available in kit form in 1968. More recently, ready-made back loaded metal frames became available.
Anodizing and painting are the processes used to color aluminum moulding. Anodizing produces a translucent dye-like coloring, while painting on the other hand produces a solid opaque finish.