We are facing problems of hardspot in White gold jewelery specially
14, 10 and 9 kt.
It is not observed frequently but on reqular intervals say every
4-5months and then gets solved after sending the alloyed gold to
refining and changing the batch of alloy. (Alloy used is UPMR 982
for 14kt and 980 for 10 and 9 kt) This spots are visible after
polishing only and then we need to reject the pcs and production
suffers a lot
Please suggest how we could avoid the occurance of hard spots
Sounds like a nucleation problem, cause could be one of a dozen
things from too much superheat of the metal in melting, wrong
cooling cycle due to mass of casting tree, overlarge sprues, unusual
shape of castings etc.
Your best advice will probably come from Jim Binnion. There are
computer modelling programs that might help you sort this out if you
cant get any joy from working it out empirically. Send me a reminder
and I will ask my old work colleagues if they can offer assistance on
that front.
We are facing problem of hardspot in white gold jewelery specially
in 14,10 & 9 kt.
Our Technical Dept. has not been notified of your problem. We will
contact our people in Mumbai and ask them to contact you regarding
this problem. Neither one of the alloys you are using # 982 with
nickel and the #980 silver base white alloy have been prone to hard
spots. In India many of the factories have their own gold refineries,
occassionally left in the gold after refining due to some of the
processes used. This has proved to be troublesome and variable in the
past. One of our tech people will be contacting you. One other
problem in India is counterfit alloys packed in knock off containers,
it is something to be aware of.
Best regards,
Jim Sivertsen
United Technical Dept.
Please suggest how we could avoid the occurance of hard spots
Refine more often. I have no experience with that particular alloy
but endless recycling of trees and scrap leads to problems. Metal
mold reaction products, oxidation, dirt all eventually mean your
alloy will not preform like you want. So keep good records and
determine how many cycles before you see the hard spots and refine
before that many cycles.
You can eliminate the hard spot occurrence by raising the
temperature by a few degrees. You can go up maximum by 50 degrees
centigrade. How many times do you recycle the gold? What is the fresh
gold component you are adding is crucial for avoiding hard spots.
Hard spots means your alloy is not a homogeneous melt.
UPMR has the technical details on their website. Check it out.
These hard spots in your white gold are probably caused by
investment contamination from recast buttons. You can avoid this by
tumble polishing your buttons before remelting them and steaming them
well before tumbling. Frank
I used to have a problem that sounds like the same thing. In my case
it was that the caster was miking the master alloy and fine gold
grain in the casting crucible and small bits of the harder,
nickel-rich master alloy wasn’t completely dissolved. This would
leave a very hard spot that a file would slide off, usually right
where the sprue connects. Pre mixing the alloy with a good stir
solved the problem. If you are lucky this is what is happening to
you, because it is easy to fix. The scenarios that Jim B. and Jim S.
describe sound like more of a hassle. –