Depletion gilding with ferric nitrate

I’m experimenting with the use of ferric nitrate to remove the
silver component of the alloy in 18 karat yellow gold, in order to
enrich the surface of the 18k.

The ferric nitrate is much easier to use than the various acid
combinations but it seems to take a long time. Can anyone recommend
a way of speeding up and enhancing the effect of the ferric nitrate
on removing the silver?



If the nitrate ion really is an effective scavenger of Ag from the
solid matrix, forming a silver nitrate solution, then adding a
chlorine containing salt to the liquid solution should cause AgCl
(silver chloride) to precipitate out, both removing Ag+ ion and
regenerating nitrate, all of which should drive your reaction to
completion. I suggest using plain old table salt (NaCl) as your
chloride ion source. Don’t use hydrochloric acid! (see below)

One bonus is that you can collect the silver chloride sludge. Most
refiners will take it, as they are used to reclaiming from
photographic solutions. Or if you are feeling adventurous, you can
reclaim metallic silver from it using zinc powder and acid. (Check
the web with keywords like silver, chloride, recovery, and zinc.)

What do you say, Bill? I’m willing to split the patent rights if you
want-- I’m thinking 60/40 but am open to negotiation! :wink:

Things to watch out for:

Don’t let the solution become acidic or you risk consuming gold.
Chloride ion, nitrate ion, and H+ ion is the basis of aqua regia. A
small amount of sodium bicarbonate (baking soda) should ensure you
stay basic enough.

Try not to let silver nitrate precipitate out directly on the
workpiece. Keep the piece suspended and get a slow stir going. AgCl
is rather dense and will settle even in a slight current.