Stained glass colored throughout its body by adding metal oxides to the molten batch is called pot-metal glass.
In pot-metal glass, coloring agents are mixed into the molten glass before it is rolled or blown into sheets. Because the color runs through the material, cutting or grinding the sheet does not reveal a clear, uncolored layer beneath the surface. Medieval glassmakers used this method extensively, with compounds of metals such as copper, cobalt, manganese, and iron producing different hues.
Pot-metal glass differs from flashed glass, which is made by applying a thin layer of colored glass over a clear or differently colored body. It also differs from surface painting, where pigment is fired onto the glass after the sheet has been formed.
The phrase “pot metal” refers to the glassmaking pot or crucible in which the colored mixture was melted. It does not mean that the finished window contains metallic pieces.