I haven't actually done this, but it's what I was taught as the
"standard" way to do this. Make a strong solution of NaCl (table
salt) and mix it with your (diluted) acid. It will form silver
chloride which, being insoluble in water will fall out of
solution. Filter and then heat the powder with some ventilation.
The chlorine will be driven off, leaving behind pure silver.
Helen's way will work fine, too. The salt solution will get 100% of
the silver, though. This is from some etching literature I read
somewhere along the way.
John, as others have by now pointed out, this is a bad idea. I tried
it once. Filled a studio with acrid blue chlorine fumes. Not good. By
the way, to properly reduce it, you do more than just heat it (very
hot, by the way), it’s best to also add a reducing flux. I seem to
recall a mix of sodium carbonate and borax, but it’s been 25 years,
so that might be wrong.
However, there’s a better way. Go to I. Shor’s web site and poke
around a bit in the refining info. According to them, after dropping
out the silver with salt solution (or hydrochloric acid if you like),
you use ordinary Karo corn syrup with the still wet silver chloride
mass to reduce it back to silver.
The other classic way to reduce the silver chloride is also done
with the still wet freshly rinsed/washed mass, and that is to add,
with I think a trace of acid, zinc metal. Stirred around, it becomes
zinc chloride, which is soluable, while the silver chloride is
reduced to silver. That needs a bunch of zinc, though, and I suspect
that the corn syrup is cheaper and easier…