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Decontaminating metals

Jewelry Discussion
#1

One of the techniques that I was taught for “cleaning” contaminated
gold or silver was to add a pinch or two of Potassium Nitrate to the
crucible while the metal was molten. This results in the formation of
a black “clinker-like” crust, which is then removed with a graphite
rod. The remaining metal is then poured into an ingot mold. The Black
cindery crust or scab is then left in Hydrochloric acid until it
dissolves and any remaining original metal (silver or gold) can be
recovered from the acid as small pieces. It may sound like a “bush
remedy” but it works. My question to the knowledgeable orchidian
community concerns the chemical process that takes place. Has anybody
done the same? What is happening when the KNO3 flashes and the black
crust forms? I have searched the web to no avail. I would appreciate
any light shed on this as I do like to know what I am doing from time
to time.

Also would like to take the opportunity to wish all Orchid members
the very best for 2009.

John Bowling

#2 
concerns the chemical process that takes place. Has anybody done
the same? What is happening when the KNO3 flashes and the black
crust forms?

It works quite well, but what must be kept in mind is that you will
get higher carat of gold, because what stuff does, it reacts with
base metals and converts them to flux soluble compounds. If you use
a lot of it, you can achieve almost pure gold. You must constantly
stir, because gas can form on the bottom of crucible any that can be
dangerous. It is not recommended practice because of
unpredictability and potential hazards.

Leonid Surpin
www.studioarete.com

#3

Basically, it has the same reaction as when you use borax flux. It is
an oxidation/reduction reaction where the excess oxygen is given off
as a gas [NOx] and the impurities form a stable oxide or glass phase.
These will readily dissolve in acid wheras your precious metal wont.
You can recover the small amounts of precious metal oxide by various
processes including dissolving in nitric acid and then reducing the
salt with carbon but it is very energy intensive and not worth the
bother at home. It is one of the oldest known smelting processes for
precious metals. Read ‘ancient mining techniques’ by Ronnie Tylecote.

nick

#4

G’day; Potassium nitrate is a powerful oxidising agent, and would
probably oxidise any copper in gold or silver alloys to black copper
oxide. Oxides of silver and or gold cannot be formed in this manner
and are not black, anyway. Placing the black crusty material in any
acid solution including usual pickle would also dissolve the oxide
and produce a pale blue solution.

Cheers for now,
JohnB of Paraparaumu NZ