First of all you need to confirm that the object you are going to age is made of brass. This step is very important because if you apply chemicals on some other surface then it will have a different reaction. This can be very dangerous and may harm you as well. In order to confirm the substance is brass you can take it to an antique dealer.
After confirming the object is brass you need to clean it properly. You should remove all the dust particles present on it along with grease and debris. Use warm and soapy water to clean the brass object and then dry it with help of a soft cloth.
Next you need to remove the lacquer finish or varnish from the brass object. You can do this by applying acetone nail polish and then placing it in boil water. You can also leave the brass object for several hours inside lacquer thinner.
Apply salt and vinegar
You need to make a mixture of salt and vinegar. Take 2 tablespoons of salt and mix it in a cup of vinegar. Then apply the mixture on the brass object with help of a paint brush. This will lead to oxidisation of brass.
Leave the mixture overnight
After applying the salt and vinegar mixture on the brass object, you should leave it overnight. Clean it afterwards with help of a clean and soft cloth. In case you do not get the desired look then you can repeat the process.
In order to give an antique look to your brass, you can apply ammonia. Soak a rag in ammonia and place it in one end of plastic container. Then place the item on the other end and close the lid. Do not let the vapours escape and you can do this by closing the lid. This will give an antique look to your brass item.