The product you are talking about is liver of sulfur. It will turn silver instantly black but for a lasting finish leave it overnight. It can be bought online for craft use. If you can buy the hard liver of sulfur crystals. This will save you money in the long run. you only make up what you need. The liquids you buy have a shelf life of about a year unless you pay more for the shelf stable preparations. Anyway blacken it then buff off the highlights. Makes a great patina. I used it on the bracelet in my post in the Identifying section. I don’t think the purists would like someone touching up patina but if it’s messed up already and you need to restore it, Go ahead. It is the same process that the sulfides in the air do to darken it with age only quicker. Funny side note you can do it with boiled eggs too. But I have had weak results with that. Hope this helps, Kyle