British pennies and two pence coins are made of copper or copper-plated steel. After a period of years the copper reacts with oxygen in the atmosphere and becomes oxidised, causing a tarnished appearance. If you wish to clean your copper currency there are a number of ways to do so, and some of the most popular ways involve using a mild household acidic such as vinegar or Coca-Cola.
Choose the full-sugared original variety of Coca Cola rather than Diet Coke or Coke Zero. The reason for this is that the cleaning agent in Coca-Cola is phosphoric acid, which is also used in commercial cleaning products. Diet Coke contains only half the amount of phosphoric acid as does original Coca-Cola, and so is not such an effective cleaning agent.
Place your pennies in a bowl. You may also like to check the date of your currency to find the coin's composition. UK pennies and two pence coins were introduced in 1971 after decimalisation, and up until 1992 were made of 97% copper; after that they were made of copper-plated steel. If your coins were minted after 1992 there is a risk of dissolving all the copper-plate from the coins' surface during cleaning.
Cover the pennies with Coca-Cola, and leave them to soak. After a few minutes the Coca-Cola will begin to attack the oxidised surface of the coins, so that the copper colour is visible. Try not to leave the coins immersed for too long, as the phosphoric acid will eventually have a detrimental effect on the metal itself.
Remove the pennies from the Coca-Cola. Rinse them in clean water, and if you wish you can gently scrub the coin with an old toothbrush. Rinse again, and finally dry the coins with a soft cloth. You will now have clean shiny pennies.
Remember that phosphoric acid in Coca-Cola will corrode copper, so do not clean your coins repeatedly. NEVER use Coca-Cola to clean vintage and antique coins.