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First, fill a glass bowl or cup with hydrogen peroxide. Next, drop the jewelry into the bowl. The jewelry should be completely submerged in the liquid. If it's not, add more hydrogen peroxide. Hydrogen peroxide can be found at most grocery stores. Platinum is a strong catalyst for hydrogen peroxide.  If the metal is real platinum, the hydrogen peroxide will begin to bubble almost immediately. Silver is a weaker catalyst. If you don't see any bubbles immediately, allow the metal to sit in the liquid for about a minute and look for small bubbles forming around the jewelry. Hydrogen peroxide won't corrode or damage your jewelry. Rinse the jewelry under cool water to remove the hydrogen peroxide. Plug up your sink or use a sieve while washing to avoid losing your jewelry down the drain. Allow the jewelry to completely air dry before wearing it again.

Summary:
Submerge the jewelry in hydrogen peroxide. Look for a reaction. Rinse the jewelry well.