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Tarnished gold bangles


#1

Dear All;

My name is Kumarpal. I manufacture customised jewellery in gold set
with Diamond/Cubic ZIrconia/ruby/Emerald Bangles. Of late we seem to
be facing peculiar problems with bangles being tarnished to the
extent that gold goes black in colour.Many customers have come up
with this problem.Even though the purity of the bangle is accounted
for, by way of hallmarking the customer is not satisfied with any
sort of explanation.

I The process of making the jewellery is as follows,

1). Gold in its purified form is very soft, Hence the mint
manufactures all over the world mix a certain percentage of copper
to give it strength, which is classified as 995.0 purity and 999.9
purity.The mixture of copper varies from 5.0 grams to 1.0 gram for
every 1000 grams.It is generally classified as 24Karat gold.

24 Karat gold is mixed with copper /silver to give it strength. The
mixture is about 8 grams of Copper to 100 grams of gold in case of
995.0 purity and 7 grams of copper /1gram of silver i case of 999.9
purity for 91.66 Purity (Hallmark standard for 22Karat gold). Gold at
this point melts at around 1000 degrees

  1. The Gold is casted to different designs of 2 Inch by 4mm stripes
    (Usually 6 Pieces to a bangle).The pieces are then soldered with a
    mixture of Zinc and pure gold (11 Grams of gold to 1 Gram of Zinc is
    the solder).

  2. The bottom portion of the bangle is then soldered with a 36Gauge
    strip of gold.

  3. The inner rim is then filled with wax.This is done in order to
    give support while setting the stones in the designs.

  4. A small piece of silver is then put in just before the stone is
    set in the design.The weight of this silver foil is approx 0.010 gram
    for every stone. This is done in order to give maximum reflection.

  5. The stone is now set in the design with the sides being trimmed
    for extra gold.

  6. The wax is now taken off after setting the stones.

  7. The sharp edges are now buffed to make the edges smooth. This is
    done with red oxidewhich is used by coating the buff of the
    machine.This gives the bangle a unique “red colour”

  8. This bangle is now washed with a weak solution of any regular
    shampoo.

  9. The Designs are now ready to be shipped to different retailers.

Problems.

The part where the pieces of gold strips are soldered shows signs of
Tarnish/Oxidation after a few months.

History

a) This happens only during the hot months of summer.

b) When the Tarnished / Oxidized Bangle is washed with a weak
solution of shampoo the oxidized/Tarnished portion retains the
original sheen of gold as in when the bangle is delivered to the
retailers customer… This bangle sometimes also returns to the the
state of Black after 15 to 20 days.The customer is perplexed to the
extent that the bangle is returned.This has been a source of deep
concern.

Observations

When the bangle is designed,the sides of the bangle are chipped to
give the design. At this point the solder (zinc and Gold in the ratio
of 1;11) also chips in with the design. When these chips are melted
the following are observed,

  1. The gold now melts at around 500 degrees instead of 1000 degrees.

  2. When the gold cools down to room temperature (Putting the gold
    rod in water) a greenish/blackish tinge is noticed on the surface of
    gold.

Corrective (May be) steps taken

The chipped gold is now refined to 24Karat purity instead of using
the gold containing the impurities (Zinc.) The problems still
persists even though this step is taken.

Possible Solution,

Could we use a solder which could be better or matching Zinc in its
fluid (Flow) properties.

Please advise.
Thank you very much in advance.

Regards
Kumarpal