I was really excited by the results of my first experiments with liver of sulphur (sulfur). Here's an update.
The ring had a good thick layer on it. I rubbed it away in places when trying to make the non-black parts shinier, but I was happy with the effect. I sprayed the ring with acrylic spray coating (mine is made by Humbrol), applying a total of about 5 thin coats. The ring is shown post-coating in this post.
Unfortunately the thin layer on the fold-formed piece, which had the lovely colours, was not very robust. It soon began to fade, and lost more of its allure when I used Silvo to clean off the ear wire. My plan now is to clean the whole earring and try using the liver of sulphur again. This time if I get a nice effect I shall spray with the acrylic coating at once!