anyone who is reading this, how old is the oldest piece you have with silver sickness?
it just struck me that none of the really old pieces i have got have this problem.
When I'm faced with a heavily tarnished piece, I usually try Tarn-X (thiourea in sulfamic acid) first, which will reduce silver sulfide to elemental silver. If the black coating is fairly thick, I'll wash it first with hot detergent to make sure that there are no embedded waxes or oils to repel the solution. When this works, thick sulfides are reduced to a whitish chalk-like coating, which I then polish away using Wright's Silver Polish. My feeling is that this takes off somewhat less silver than just using polish alone.
Thicker tarnish usually takes more Tarn-X applications, but there is great variability in susceptibility, unrelated to thickness. Some stains bleach out quickly, others take forever, or just don't yield. The worst case was a c.1870 Vanderslice & Co. coin silver (.900 fine) goblet that appeared to have been in a fire (perhaps "the" fire). Most of the outside surface was a dull coal black, almost like a flat paint, though I made sure it was indeed not an applied coating. Tarn-X had no effect whatsoever, nor did the silver polish. I finally had to go to 600 wet and dry sandpaper, the one and only time (thankfully) I've ever had to get this aggressive. I'm somewhat embarrassed to admit it here, but whatever that oxide/sulfide/??? was, it was like granite.
The next most aggravating piece was a Charles Hougham 1787 London helmet style creamer. It finally yielded to the Tarn-X, but only after repeated applications and rubbing over a couple of hours. But generally, according to my fuzzy anecdotal memory, I seem to recall that I generally have less problem with French (.950) and Brit (.925) pieces, and more trouble with American coin (nominally .900) and Continental silver (.800/.830/.835). So I think there may be some positive correlation to copper content, such as there is with silver scale formation, but this is just a guesstimate.