Article: Cleaning a rare, collectible coin can reduce its value by over 50% due to tiny scratches or the removal of the tarnish. If the gentle cleaning methods above do not work, it's safer to have it cleaned professionally or accept an imperfect appearance. Feel free to use the methods below on common, non-collectible coins if you prefer a brighter appearance for your own collection.  An even gold, grey, or iridescent tarnish (toning) is desirable and increases the coin's value.  A black or blotchy tarnish is not ideal, but harsh cleaning can still lower the value. Continue very carefully or take it to a professional. After washing the coin in soapy water, you may still see dark areas outlining letters or detailed images. Wash again and pick away this dirt slowly and carefully using a toothpick or bamboo skewer. Take care not to scratch the coin, since the pick can scratch away the tarnish. Pointy plastic utensils are slower, but less likely to scratch the coin. Silver polishes clean metal using acid and abrasives, both of which eat away a small layer of metal. This is an easy way to brighten non-collectible coins, but will almost certainly damage the value of collectibles:  Put a coin-sized dollop of polish on a soft cloth. Fold the cloth in half to transfer the polish to two sides. Place the coin between the fold of cloth. Rub together briefly. Wipe off the coin on a clean part of the cloth. This cheaper alternative to silver polish also abrades the silver coin, causing scratches that will be visible under a magnifying glass. Minimize damage by using distilled water and mixing in just a little baking soda to make a watery paste. Dip the entire coin the paste, rub between your fingers for a minute, then rinse with water. Repeat two or three times if necessary. Pat dry. A thick buildup on ancient coins is likely "horn silver" (silver chloride). This may be worth removing, especially if it prevents identification. Submerge the coin in a 10% ammonia solution in a tightly sealed container. Remove it after a few minutes, rinse immediately in water, then brush with a soft toothbrush. Repeat as needed.   Warning: ammonia liquid and fumes are corrosive. Wear neoprene gloves and work in a ventilated area.  Some hard buildups are the result of a different chemical reaction, which may require other treatments. Consult a coin dealer first if the coin might be valuable. The following treatments are especially risky. Without expert supervision, they may remove the tarnish or pit the coin's surface. These are more often used on ancient coins, where heavy buildup may prevent identification or severely lower the value. Relatively modern, machine-made coins should only be treated this way if they have little collector's value and an unattractive tarnish:  Soak the coin in a 50/50 mix of distilled water and lemon juice for one hour.  Soak the coin in 1 tsp (5 mL) lemon juice mixed into 1.5 cups (360 mL) olive oil. Warm this in a heat safe container inside a water bath, but keep it below boiling.  "Dipping" treatments can return a coin to a frosty white silver appearance, which some collectors value. However, this is an extremely risky procedure that can cause blemishes to appear. Hiring a professional is strongly recommended, but you can buy a dipping solution to use at home.
What is a summary of what this article is about?
Use extreme caution with valuable coins. Pick dirt out of small crevices. Rub with silver polish. Wipe with a paste of baking soda and water. Remove hard deposits with ammonium. Use extra-harsh treatments as a last resort.