Hi, I would like to depletion gild my 22k pieces without having to
use nitric acid. I used nitric acid (% solution?) 20 years ago and
remember it gave agorgeous rich gold matt finish but I am a little
nervous about the safety issues and somewhat reluctant to use it
again.
I heard from a tech guy at Rio Grande that sodium bisulfate (Sparex,
Rio pickle) will remove both the copper and silver from the surface
after heating to oxidize, but that the process of heating and
quenching has to be repeated several times to be effective. But in
both Oppi Untracht and Brepohl’s booksit says sodium bisulfate does
not remove silver from the surface, only copper. I’m confused. I
have seen that I get different colors on my gold if I put various
components of a piece through different heating/pickling cycles, not
paying attention to making sure they get the same treatment. And
even just using Sparex or Rio pickle, some pieces have more of that
look of 24k on the surface and others are much less rich in color,
obviously more silver left.
Any suggestions?
And if I decide to go with nitric acid, what % solution? If I have a
hood with a strong fan is that sufficient to avoid the fumes? And
can those fumesbe just vented outside safely? Or should I work with
it outside?
Thanks for your help,
Margaret
I think you are wise to be uncomfortable handling nitric acid
Margaret. Over38 years I’ve had 3 different bad acid related
accidents in the shop, all caused by the momentary carelessness of
individuals. I think the safest, smartest thing to do is not to
allow dangerous acids in your shop.
Mark
Margaret- The easy fix would be to 24 kt plate the piece.
Have fun and make lots of jewelry.
Jo Haemer
timothywgreen.com
Hi Margaret,
I don’t have an answer for you. However, they are all using the same
basic method to remove the copper and silver - an acid.
bisulphate is a very weak acid
nitric is a very strong acid.
The strength of the acid determines the rate. so dilute nitric at
the same pH will dissolve e. g. copper at (approximately) the same
rate as the equivalent pH bisulphate.
However, some acids will or will not dissolve different metal, and
this is determined by the salt that forms, its solubility and the
nature of the species that coat the surface of the metal.
simple version, if the metal and acid form a soluble salt (with
exceptions) it will dissolve the metal.
I know nitric will work on silver and copper but not gold. it is
safer when dilute, so the trick isn’t to avoid conc nitric it’s to
use dilute nitric in everyday work, and only handle nitric when
making up a fresh solution.
can you buy already diluted nitric?
always add acid to water to dilute - not water to acid. Think of it
this way. All the energy is in the acid. you only want to add a
little energy at a time. when you add water to acid, all the energy
available local to the water causes it to boil and spit…
Nitric has a love affair with carbon and starch. ruined a few pairs
of jeans leaning against a spill on a bench un-noticed. it will turn
wood brown. which is still better than turning melamine brown…
several explosives are made by using nitric acid to nitrate
starches… 
Thank you Paul for your response. I experimented with some 22k gold
and I found that if I left the piece on my bee hive kiln for several
minutes and let it turn very dark before quenching it in Rio pickle
(sodium bisulfate) thecolor was distinctly more rich gold than when
I just annealed and quenched. So it would appear the silver is
getting removed from the surface as well as the copper when I let it
really oxidize. In the end, all I want to do is create a color I
love that will last, and if I can do that without nitric acid, that
would be ideal. Of course what I really should do is compare the
results to those with nitric and see what I like best. And yes, you
can buy nitric acid at a 10% dilution if you wanted to.
Thanks again,
Margaret
Hi Margaret - that totally makes sense - oxidise the metal and then
remove it with pickle - which it is intended to do.
wrt nitric you could go to 1% or less - and more time… &/or
heat…
experiment.
I’d expect at least twice as long for each 50% dilution so 1% should
take 9 times longer than 10%…
of course at some point it will stop working altogether - even if
because there isn’t enough material left to attack the available
metal…