Do you know any other compound (than sulphur) that produces the
"oxidizing" (what term do you think would be more accurate?)
effect on silver? And thank you for your so very clear explanation
of what happens when the silver turns dark.
Actual pure sulphur is usually not used. Not effective. Mostly, we
use various compounds of sulphur. There are several. "Liver of
Sulphur" is a sort of impure mix of several potassium/sulphur related
compounds, and it's fairly cheap. But other sources of the sulphur
can be found. Pollution does it with sulphur dioxide gas in the air,
and egg yolks have enough sulphur in them to do it.
Commercially, there are a number of silver oxidizers that blacken
silver by related, but not quite the same means. Some of these
proprietary mixes are quite dangerous, including hydrochloric acid,
teluric acid, and other nasty stuff. I don't know if these actually
produce a sulphide, but I suspect it may not be, since the colors are
slightly different. Not sure what they actually produce. And there
are a range of plating solutions that can give a black finish. Black
nickle and black rhodium are the most commonly seen. They work on
silver, gold, platinum, etc.
On silver, of the simple chemicals used to antique items, the only
other common agent that can be of use that I'm aware of is household
bleach. This does not produce a black color, but instead, can take a
nice clean silver surface, and give it a grey dingy smutty corroded
sort of look. I personally don't like the effect, but some folks find
it useful. There may be many other simple ways to chemically patina
silver, and I know there are many other more complex mixed chemicals
and procedures that can be useful, but I'm not familiar with them.
You can find in various book offerings a number of fine books on
patinas. Many of these will work on silver as well as the more
commonly described copper based alloys.