What is the cause of greening observed when porcelains are fired on alloys that do not contain silver?

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Evaporation of silver from the walls of furnaces contaminated with silverSilver migrating across the surface from the edge of the metalAccelerated silver movement within glass containing sodiumIonization of silver due to porcelains with elevated oxygen potential

Explanation

Greening occurs primarily because silver vaporizes from the walls of furnaces that have been contaminated with silver. This vaporized silver then interacts with the porcelain during firing, causing the greening effect. Other factors such as silver diffusion from the metal surface or ionization by porcelains are less significant in this context.