Gold Plate Turns Dark
Why does gold plated over silver turn dark?
Q. I am an artist and routinely plate small items with gold and silver. Recently I tried plating gold over some silver pieces I made. After a couple of weeks of exposure to the environment in my studio, the pieces lost their characteristic gold color and turned dark (tarnished). What is going on?—K.Y.
A. You are observing the migration of silver into the top gold layer. As the silver migrates into the gold layer, it will tarnish, just like unprotected silver plate or solid silver. To prevent this migration, you have to deposit a barrier layer on the silver before plating with gold. In the past, nickel was routinely used as a barrier layer, but this is much less acceptable if the pieces you are plating are jewelry. (Many people are allergic to nickel.) If that is the case, you will have to use a different plate as a barrier layer, such as albaloy, miralloy, optalloy or copper/tin/zinc alloys.
I believe you will be able to salvage the pieces you have already plated by removing the tarnish with a diluted nitric acid dip, and then replating with nickel or nickel substitute followed by gold.