I do a lot of repairs and sometimes run up on a problem of yellow
gold turning reddish at the place where soldered. I use a fire coat,
and Batterns flux at the place I’m working on.
fire scale prevention is much easier than fire scale removal. your boric
acid solution maybe old or weak. if you are using acetylene, that seems to
generate more fire scale (i know i’ll hear it from all you acetylene
users) than a cleaner fuel. propane will generate firestain as well, but
one can use an oxygen poor flame to consume oxygen around the joint while
being soldered. actually, it is oxygen binding with the copper when your
alloy reaches a sufficiently high temp. if there is little or no oxygen
coming in contact with the metal, there will be little or no oxidation. if
one solders too long, the firecoat can break down allowing oxy to contact
your metal. not good. also, if one has old pickle with lots of copper salt
in it and if one is dousing hot into the pickle, it can cause coper to
plate out onto the hot part of your piece (where it was just soldered).
depending on how deep the firestain goes, one can remove it by electro
stripping, abrasives, or by ‘bombing’. soaking the piece in cyanide won’t
do much with the firestain, but bombing will if you do it many times. if
one must use this stuff, know what you are doing+ACE- most jewelers have
quit using it, rarely use it, or are not supposed to be using it. a few
years back, some downtown l.a. jewelers we’re feeding our fishes with it
by pouring it along with the heavy metals it contains down the drain.