In his book Dictionary of Glass, Charles Bray describes three techniques for producing iridescence on the surface of glass: reducing flame that alters the metal oxides already in the glass, fuming, and application of lustres.
All three methods have in common a final result where a very thin layer of metal is deposited on the surface of glass.
Fuming refers to a process of placing the hot glass in vapors created when metal salts are subjected to high temperatures. For creating iridescent glass, the most common of these is stannous chloride (tin salt). This method is the one used by most manufacturers to produce the metallic surfaces most often associated with iridescent glass.
A broad palette of iridescent colors is possible by changing both the metal used and the thickness of the thin metallic layer created during fuming.