It depends on what your wanting to clean A Electrolysis machine can help clean caked metal, or soak it in WD-40 for a bit and then give a brushing with a soft brush.
Coins have to be cleaned with care or they will scratch silver is a soft metal and even rubbing them will cause scratches you won't be able to see but the jewelers loupes can.
Silver, copper, and nickel-clad coins can be soaked in distilled water or, to remove tough stains, white vinegar. A 6-minute soak in lemon juice may also be used on silver coins. This will not only protect the coin but it will help give the coin a finish at the end of the cleaning.
Lightly brush the coin. If dirt remains, lightly scrub the coin with a soft toothbrush and dish soap (toothbrush bristles come with varying degrees of firmness — you want one labeled as soft or extra-soft). Run the toothbrush under warm water to further soften the bristles, and rinse the brush and the coin frequently to avoid scratching the coin with the loosened dirt. Do not apply much pressure, and focus on brushing only the soiled area of the coin.
Dry the coin. Pat the coin dry with a soft, lint-free cloth, and then lay the coin on a soft, dry cloth to finish drying. Do not rub the coin dry.
Spot-clean encrusted dirt. Especially on ancient coins, there may be dirt that remains after soaking and brushing. To remove this, use a small, sharp implement such as a toothpick, precision screwdriver. Carefully pry and chip off the encrusted dirt, and try not to scratch the surface of the coin. You can spot-clean more precisely if you work under good lighting and use a microscope or magnifying glass.
Or if you want, you could clean pennies with white vinegar. If you use 1/4 cup of it and 1 teaspoon of salt in a clear shallow bowl that's not metal, use a paper towel and spoon and your pennies will become clean.
Commercial coin polish can be purchased from coin dealers or hobby shops. While this may bring out the luster in your coin, it is still not advisable to clean a valuable coin.
Do not mix types of coins when soaking. Copper especially will sometimes discolor other coins.