A natural and safe way to darken new pewter is to handle and use the pewter piece frequently. The more the usage, the quicker the piece will turn dark. For example, if the piece is a key, you might keep it close by on a keychain, and use it repeatedly a couple of times every day, to speed up the process of creating a patina. If it is a cup or a mug, you can keep it on the kitchen counter, and drink from it a couple of times each day. This is a safe and easy, albeit slow, method for darkening new pewter.
If you are looking to develop a patina more rapidly, then you can use a dishwasher for the purpose – however, before you do this, you will need to check the label on your pewter piece, to ensure that it is dishwasher-friendly. If the object can safely be put into a dishwasher, proceed to do so - put it on a rack so that it is secure and does not fall or move around. However, make sure you keep it away from the detergent dispenser, the water sprayers, and the direct drying heat – since pewter is generally soft, these components of the dishwasher could easily damage your piece.
Then, fill your dishwasher up with the usual dirty dishes – load them into the racks as you normally would, but make sure you do not put anything on top of your pewter piece. Then, add in the amount of regular dish-washing detergent you use for a full load. Put it through a normal cycle, and the heat inside the appliance will cause a patina to develop on the pewter piece.