The chemical reaction between the silver sulfide and the aluminum only occurs when the two are in contact while in the baking soda solution, so the silver has to touch the aluminum foil. The silver sulfide is converted back to silver and the sulfur is deposited on the aluminum either as yellow flecks of sulfur or as aluminum sulfide. Warming the water speeds up the reaction and the solution carries the sulfur from the silver to the aluminum. Tarnish will begin to disappear within a few minutes. You may need to repeat the process for badly tarnished pieces, reheating the baking soda and water mixture.
You will need a container lined with aluminum foil. The container should be large enough to totally immerse the silver you want to clean. Heat the water and mix in the baking soda at the rate of a cup of baking soda to a gallon of water (adjust the amount of water to the size of your container and silver piece, then add the correct amount of baking soda for the amount of water used). Place the jewelry directly on top of the aluminum foil and add the water and baking soda solution. Some bubbling may occur during this reaction, so you might want to place your container in a larger pan or somewhere where some overflow won’t matter.
Caution is needed when cleaning jewelry set with stones. Some stones are porous and should not be immersed in water or any cleaner. Examples are opals, turquoise, malachite, lapis, or pearls.