There are a few different ways to rust metal BUT this is the best way I know of without worrying about the disposal of harsh chemicals:
I have tried this process many times and with trial and error I have found that the expensive bells, safety pins, and stick pins DO NOT RUST, the more expensive products have a shellac coating on them and will not rust unless you sand each of them first, so the best thing to do in my opinion is to buy the cheapest product to save time and money!!
First, take 1c. bleach and put it into a container, I use a old pickle jar and it works perfectly. Then put your jingle bells, liberty bells, cow bells, safety pins or what have you into the jar. Make sure not to add too many, you want them all to be covered.
Wait 24 hours, after the 24 hours has elapsed drain the bleach into the toilet....now you have a cleaner too!!
Next add 1c. white vinegar, again make sure all of your items are covered. Now add a healthy does of salt (just plain salt will do) . Again wait 24 hours.
Now, drain the vinegar/salt mixture down the kitchen sink.....yep another cleaner. Vinegar will eliminate any lingering odors, so let the mixtures sit in the sink for a while.
The Final step is to line a baking sheet or pizza pan with wax paper. Lay your bells, pins, ect on the paper and take them out into the sun. The metal will rust as they dry............leave them there for several hours for the best appearance, the longer the better.
Ta Da! YOUR DONE...............Now you have avoided the worry of disposing of a harmful and to say the least smelly chemical., and also got several uses out of the mixture.
Have fun and have fun rusting...................
Information and Technique gathered from "Old World Primitives"