Pewter is a soft, malleable metal used for fancy metalware, such as mugs, plates, vases and figurines. It is a metal alloy containing up to 99 percent tin, with antimony, bismuth, lead and copper comprising the remaining material. Its softness is the result of a patina, a tarnish that results from the oxidizing on the surface of certain types of metals. The abrasive detergents used in dishwashers can strip this patina away and discolor pewter.
Soak the pewter in an "all-metal" detergent, usually diluted in some amount of water. If the dishwasher has left dark spots on the pewter, this specially formulated detergent should help lift the spots out, as well as light scratches.
Rinse the pewter in warm water, then dry it off with a soft cloth. The dark spots should come off the pewter after several passes by the cloth. If there are noticeable scratches in the pewter, buff the surface lightly with a very fine steel wool to get the scratches out.
Polish with a fine-threaded cloth. Pewter isn't polished with any chemical agents, but exposure to air creates a patina over time. After serving food or drink in a pewter vessel, hand-clean the vessel with a mild detergent and warm water.