Before cleaning nickel, make sure you have the right products for the job and are working in a well-ventilated area. Also make sure the nickel has no serious flakes or chips. Cleaning nickel is easy and does not usually require hazardous chemicals, but taking precautions ensures a quality cleaning job.
Before cleaning, make sure the piece of nickel is suitable for the job. While tarnish, dirt and grime are not a problem, chips and flakes are. These allow the cleaning solution to penetrate under the plating, causing serious damage to the finish.
For nickel that is only slightly dirty or tarnished, a simple vinegar and water solution is sufficient. As a starting solution, dilute about 1/4 cup of vinegar in a single cup of water, and use a soft rag to rub the mixture onto the nickel. If a four-to-one solution doesn't work, use progressively larger amounts of vinegar until the tarnish lifts. Rinse the nickel with clean water no more than 30 minutes after applying the solution, as prolonged exposure to vinegar can cause damage.
For very stubborn tarnish, soak the nickel in custodial-grade ammonia for no more than 30 minutes. As with vinegar, prolonged exposure to ammonia causes the nickel plating to flake off, ruining the finish.