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)