http://www.bostick-sullivan.com/articles/silvernitrate.html
Stains can be removed from some materials with a permanganate and bisulfite treatment. It will vary in effectiveness depending on the material, of course. Wash the material in potassium permanganate, of 1 teaspoon to a pint of water. The material will turn dark red brown. This can be a little unsettling. Using 1 teaspoon of sodium bisulfite to a pint of water, rinse the material in this and the red color will immediately and completely disappear. There may be a slight release of sulfur dioxide gas, which is pungent, so this should be done in a ventilated area. I probably could have removed the stain in our sink had I know about this treatment.
Potassium permanganate can be purchased at DIY/home improvement stores in the water softener section (it's used for cleaning them). Check with your preferred first. Sodium bisulfite can be bought at home brew shops -- it's used to sanitize brew and wine tanks.
Time is not on your side here, I am sorry to say.
Another suggestion from wet etch glass plate photographers is:
Try a little tincture of iodine on the stain first.
Then use thiosulfate or cyanide fixer on the stain. (Photography shops may still carry, but be warned -- poison.)
The iodine converts the silver to silver iodide making it soluble in your fixing solution. Works on hand stains also.
ETA: found one more reference:
I want to thank you for suggesting Jeweler's Rouge because it led me to experiment by applying regular silver polish (Wright's Silver Cream) to the silver nitrate stains. This worked well, removing 90% or more of the visible stain on first application. The product MSDS lists only two components: microcrystalline silica and sodium carbonate [soda ash!]
There are probably other ways to lift silver nitrate stains from hard surfaces. I didn't think to try a powdered cleanser, such as Bon Ami, before the experiment, so am unable to judge its effectiveness; nor was iron oxide paste (Jeweller's Rouge) tried. The sodium hypochlorite, surfactants and carboxylic acids in the dishwashing liquid, bleach and spray cleaner had no obvious effect, but constituent strength (household concentrations) and duration of contact (< 1 min.) preclude negative inference.
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