I know that you can use ferric chloride to etch copper, brass and
I believe bronze, but that it will not etch sterling silver. Can
someone verify this?
That is correct. Just as hydrochloric acid doesn’t etch silver, it’s
salts, such as ferric chloride, also won’t etch silver. The reason is
that the product of silver and hydrochloric acid reacting, is silver
chloride, which happens to be insoluable in water. So when exposed to
the acid, or the chloride salt solution, the surface of the silver
reacts, forming silver chloride, but that product then does not
dissolve, but instead remains a surface layer, which then stops
further etching. So hydrochloric acid, or ferric chloride, will
damage the surface of the silver, giving it a rather dingy look (that
some people use as a form of aged patina) but they won’t etch beyond
that surface effect.
Instead, use ferric nitrate, a nitric acid salt. Or nitric acid
itself, diluted, of course. These work because silver nitrate, unlike
silver chloride, is water soluable.
A further, and sometimes useful, demonstration of the difference is
that you can use the insoluable nature of silver chloride to recover
the dissolved silver in your nitric acid or ferric nitrate solution.
Add ordinary table salt to the used etchant, and it reacts with the
silver nitrate to form silver chloride, which then precipitates out,
and the white powdered silver chloride can be filtered from the
solution. The silver chloride can then be sold to your refiner (not
all of them take it, though), or with additional steps, it can be
reduced back to silver. Of course, you can also recover the silver
from your nitrate solution by electroplating it out, once excess
acid has been neutralized, which might be simpler, should you be
bothering to try and recover the dissolved silver at all…