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Differentiating gold using aqua regia


#1

Some six or seven years ago, I was learning how to differentiate
gold karats from one another using aqua regia. Unfortunately, my
mentor left this earth at an early age, before he completed teaching
me this, and several other things… I still remember the concept
quite well, but I have forgotten some of the details.

A significant part of my stash is the remnants of commercially made
jewelry, some marked, and some with any and all markings worn away.
The rest is bench scrap: buttons from castings, small pieces of
broken jewelry, snippets from piercing or cutting out jewelry parts,
etc. I have found most of the local buyers somewhat deceptive, never
clearly giving a rate of return, understating the karat, shorting on
actual weight, etc. I NEED to know exactly what I have before I walk
in to the one or two that seem to be relatively honest. I hope ya’ll
can help me.

I decided to stay with aqua regia for several reasons. Given that I
have access to both hydrochloric and nitric acids, I don’t have to
order it, buy it or have it shipped to me. Also, it is a traditional
method, and the method my mentor was teaching me to use. I am
therefore interested only in using pure aqua regia, and not the more
modern graded acids. Yes, I am stubborn! Yes, aqua regia is
considered old-fashioned. Nonetheless, this is the way in which I
wish to go.

I mix aqua regia in relatively small amounts. I graduated an acid
test bottle using a small pipette to determine the total number of
drops of water the bottle will hold. I emptied the bottle, dried it,
and put three-fourths the capacity of drops into the bottle. I marked
that level by scoring the bottle with a file. I then added the
remaining one-fourth the number of total drops, and marked that
level. When I need to mix aqua regia, I use a larger pipette to put
reagent hydrochloric acid in the bottle to the level of the first
mark. I then add reagent nitric acid to bring the total liquid to
the top mark. Do any of you see any problem with this method?

I have since seen a recipe for aqua regia containing three parts
hydrochloric acid, one part nitric acid, and one part water. Which
will work better? Will added water slow down the process, or will
the process be affected in some other way?

When I rub the gold sample on the stone, how much of a “rub” is
best? My mentor used a single firm stroke. The GIA recommends four
strokes on top of each other. Most jewelers or gold buyers I have
seen lately attack the stone with the sample quite vigorously, like a
three-year-old coloring on paper, leaving a massive scribble. Which
is best?

My “readings” are inconsistent. If I lay down comparison strokes of
10K from my test needles, a known 14k sample to be tested, and a 14k
stroke from the needles, I don’t always get the same results. With
either the one- or four-stroke methods, (I haven’t been able to
bring myself to use the ‘mad scribbler’ method) sometimes the 10K
fades, leaving the two 14K strokes quite visible, as it should.
Sometimes the 14K from the needles remains strongly visible, and the
10K and the 14K sample fade. On one occasion the 10K remained strong,
and the two 14K strokes faded. Why is it doing this? What can I do to
make my tests more consistent? Am I doing anything wrong here?

Then there is the question of white gold. One battered ring in white
gold is marked 18k. The other scrap pieces are unmarked. I am
reasonably sure that it is nickle white gold. How do I test it? Can I
just test it using the yellow gold needles for comparison? That
doesn’t seem quite right because of the nickle in the sample and not
in the needle affecting the test, but after all, both are
seventy-five-per-cent gold. Without a test, it will be hard to value
the unmarked white gold.

Any advice or any of you ladies or gentlemen can offer
would be greatly appreciated.

Thank you in advance.

Marrin T. Fleet
Sometimes, part time independent maker of jewelry and other small items!


#2

I rarely use gold, so its a bit outside my daily skill set, however
to save a lot of fiddling about, whats the total weight of all the
gold scrap excluding the white gold?

Can you melt it into a single ingot? then youl have only one test to
make. If my memory serves me right, you mark the stone with the
unknown carat gold, then put your needles in ascending order
underneath from 9 to 24 carat, apply your aqua rega to them all and
compare the result to establish the carat.

If you can establish the accurate carat of your ingot, you could
offer it on Ebay at a price cheaper than new but more than from your
scrap dealer.

What with the way the price of gold is going you might just be
better keeping it for a year or more as the price will always keep
creeping up.