Silver refining in acid is very simple, requires almost no investment in time, equipment or supplies and it can be done by almost anybody. However, it requires the use of concentrated acid and the fumes it produces are very corrosive. Therefore, it must be done out-of-doors, protective clothing should be worn and reasonable care must be taken.
It is best for the silver to have as much surface area as possible. That makes for much faster dissolution. So, if possible and convenient, melt your silver and pour it into the form of shot. Open-up granules are best. If gold is mixed with the silver, it is important that the gold be no more than 20% of the total weight. Otherwise the gold will interfere with the dissolving process. If you believe the gold content to be more than 20%, alloy down with copper to reduce the percentage of gold.
Weigh the silver and put it in one or more 5 gallon buckets. Add 150 ml nitric acid for every ounce of metal in the bucket(s). The acid will tend to react violently to the metal, bubbling and fuming. Make sure there is enough extra room in the bucket to accommodate the foaming (2-3 times, or more, the volume taken up by the silver).
When the acid stops foaming and all the metal appears to be dissolved. Pour off the acid ( filtering it if possible ) into another bucket or buckets. Do not allow any solids to be poured off with the acid or they will contaminate the final silver.
Add to the acid 1 ounce of SAC (silver precipitant crystals) for every 40 ounces of silver that is dissolved. As the SAC hits the acid, it will form a white precipitate (silver ) that will sink to the bottom of the acid.
Give the SAC about 30 minutes or more to work and then pour off, neutralize and dispose of the acid. If possible, filter the acid when pouring it off to make sure that no particles of silver are lost.
Wash the silver repeatedly in water to remove any traces of acid. Add a couple of drops of aqua ammonia to the silver after your last rinse to test. If you see any color of blue, rinse some more. Use only a few drops of aqua ammonia. Ammonia not only smells strong, it can dissolves some of the fine particles of silver.
Wash the silver mud thoroughly and repeatedly with tap water, making a final rinsing with distilled water.
Dry and melt.
How to refine silver using baking soda, Karo pancake syrup, table salt, Red Devil drain cleaner and water: First dissolve your silver in nitric acid. You can do this in a plastic bucket but be sure to wear protective clothing like rubber gloves and to do it out of doors. When the silver is all dissolved, pour the acid into another plastic bucket. Be sure not to pour any particles along with the acid. Add ordinary table salt to the acid until the salt stops making white clumps in the acid. Pour off the acid. Add baking soda to the acid to neutralize it. Do not pour off the white "precipitate" that the salt formed. This is pure silver chloride. Rinse the silver chloride with water. To the silver chloride add Red Devil brand drain cleaner (lye) until all the silver chloride has turned black. Rinse with water. Add Karo syrup (dextrose) until all the black material (silver oxide) turns to pure silver.