It has been experimentally shown for the first time that metallic copper in glasses can play the role of a reducing agent for other metallic ions, in particular, resulting in the formation of metal nanoparticles. Silver ions have been introduced into an alkali silicate glass containing copper nanoparticles by ion exchange. This has resulted in the formation of silver nanoparticles accompanied by the disappearance of copper nanoparticles initially contained in the glass. Such a process has never been observed and described anywhere. The process has been explained by the redox reaction Cu0 + Ag+ → Cu+ + Ag0 taking place in silver ion exchange. It has been shown that this reaction can go only in one direction (from left to right) as a result of the relative positions of copper and silver in the activity series.