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Karat gold tarnish

Hello everybody. After placing finished 21 Kt yellow gold pieces in
the showroom for a few months they are turning into a darker shade of
yellow. Our alloy contains 65% copper in addition to silver and
nickel. It is possible that the copper content is causing this
tarnishing effect(oxidation). Has anyone experienced this, and if so
what was the solution implemented? Do you know of any additives to the
alloy that might inhibit this process? or possibly a post finishing
procedure!! I would greatly appreciate your help. Thanks…

Fouad, perhaps you could give us some more details of the alloy you
are using? 21kt is rather an unusual grade. OTOH, if, as you say,
it contains 65% copper then that works out to just 8.4kt… The
lowest legal grade here in the UK is 9kt, which does tarnish,
certainly, but not really to any noticeable degree.
Kevin (NW England, UK)