I've used this product for years and it's shelf life is much longer that the liver of sulfur crystals jewelry supply houses sell. A few points to note. You are correct about the surface needing to be clean, but just wiping is insufficient and your poor results on some silver suggest this is your problem. Scrubbing the piece with non-scratch bleach cleanser such as Ajax is necessary. You can tell if your piece is properly clean if the water does not bead on the surface. Next it greatly helps to have the liver solution as hot as possible. Also heating the piece with a hair drier will speed the process like instantaneously. Stirring the piece also helps. Be sure when you submerge the piece that it is completely dry especially its nooks and crannies because water will insulate the very recesses you want to darken, Once the piece has blackened, rinse in tap water while lightly brushing the piece with a fine brass brush that has been dunked in soapy detergent water. This removes the black film that lies on the piece's surface but has not bonded with it. You want to get rid of that. The brushing will leave a grey silver finish but also prep the piece for successive liver treatments, bushing with the brass brush after each. Do not omit the soap on the brush as this will remove the desirable liver effect. Once the desired finish is achieved, rinse in the backing soda mix and dry with a hair drier. Very fine steel wool can be used to remove the dark grey from raised areas, but will leave a matte finish which I like better than high polish. Rio Grande also sells foam pads that work very well for removing the liver while not dulling the polish. Finally to preserve the piece's polish and liver treatment, rub the piece with a tiny amount of micro crystalline wax and buff. If you wish to treat brass, heat it and pickle. This will cover it with a tenacious coat of copper which will respond very well to liver of sulfur.
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I've used this product for years and it's shelf life is much longer that the liver of sulfur crystals jewelry supply houses sell. A few points to note. You are correct about the surface needing to be clean, but just wiping is insufficient and your poor results on some silver suggest this is your problem. Scrubbing the piece with non-scratch bleach cleanser such as Ajax is necessary. You can tell if your piece is properly clean if the water does not bead on the surface. Next it greatly helps to have the liver solution as hot as possible. Also heating the piece with a hair drier will speed the process like instantaneously. Stirring the piece also helps. Be sure when you submerge the piece that it is completely dry especially its nooks and crannies because water will insulate the very recesses you want to darken, Once the piece has blackened, rinse in tap water while lightly brushing the piece with a fine brass brush that has been dunked in soapy detergent water. This removes the black film that lies on the piece's surface but has not bonded with it. You want to get rid of that. The brushing will leave a grey silver finish but also prep the piece for successive liver treatments, bushing with the brass brush after each. Do not omit the soap on the brush as this will remove the desirable liver effect. Once the desired finish is achieved, rinse in the backing soda mix and dry with a hair drier. Very fine steel wool can be used to remove the dark grey from raised areas, but will leave a matte finish which I like better than high polish. Rio Grande also sells foam pads that work very well for removing the liver while not dulling the polish. Finally to preserve the piece's polish and liver treatment, rub the piece with a tiny amount of micro crystalline wax and buff. If you wish to treat brass, heat it and pickle. This will cover it with a tenacious coat of copper which will respond very well to liver of sulfur.