This is going to be a short section, but I wanted to include this. Metal antiques should be cleaned with at most a mild detergent and water. Coins should also not be washed, but if you have to, again, mild detergent and water are the most you want to expose coins to. If you have a used stamp, it will be worth the most if you leave it attached to the envelope it was mailed with. If stamps are attached to a book with glue, don't do anything to remove them until you have done research. Of course, there are many types of things I haven't covered, but if you have to clear or alter something, do research before doing it. Cleaning a piece of silver with silver cleaner could cause the piece to be worth half of what it was before, or less, even if it was tarnished.