What is depletion gilding?

This may be a dumb or obvious question, but what is “depletion gilding”?

–Kathy Johnson
Feathered Gems Jewelry
http://www.featheredgems.com

** Hanuman's note **

Check out http://www.ganoksin.com/borisat/nenam/depletio.htm
Some Notes on Depletion Gilding
 This may be a dumb or obvious question, but what is "depletion
gilding"? 

Allowing an acidic solution (such as pickle) to deplete the alloying
metal from the surface of a piece, leaving essentially fine gold or
silver in a thin layer.

Ron Charlotte – Gainesville, FL
@Ron_Charlotte1 OR afn03234@afn.org

Hi Kathy, it’s done when you want to enamel on sterling. You anneal
at 1450 for a couple minutes, then into sparex, repeat until no more
fire scale or coloration comes up when heated. I’m sure you could
find out additional info and the reason in any beginner enameling
book. I just read about this myself and I’ve never done it.
Good Luck, Jennifer

Kathy, Generally, depletion gilding is carried out like this: The
precious metal alloy is heated to cherry red in an oxidizing
environment. The result is an oxide rich material on the= surface,
which is removed with the appropriate acid, leaving a thin layer of
pure or nearly pure precious metal on the s= urface. Repetition of
this process increases the thickness of the surface layer. The
result is a layer of nearly pure pr= ecious metal which is very
resistant to abrasion. Great way to give a high karat gold look to
lower karat alloys. It is also a necessary part of the process used
in preparation for torch texturing 80/20 silver.

Wayne