Cleaning antique thimbles of an unknown or uncertain age can be tricky. First, you have to figure out what the thimble is made of. Generally, thimbles are made from metals such as silver, brass, iron, steel or even aluminium, but they also can be made of leather, rubber, wood, glass and even china. The oldest thimble ever found was discovered in the ashes of Pompeii. It was a Roman thimble, made of brass, from the first centre A.D. Once you figure out what your thimble is made of, you have to follow instructions for cleaning that particular metal or other material. You also have to decide whether you want to clean the thimble in the first place; some collectors like the old, tarnished look, particularly for old silver thimbles.
Before you can even think about how to clean your antique thimbles of an unknown age, you need to do a little research into what your thimble is made of. Bone china, glass, wooden and leather thimbles are easy enough to identify, but when you get into thimbles that are made of metal, particularly tarnished metal, you may have to do a little digging. Zalkin's Handbook of Sewing Implements, by Estelle Zalkin, 1988, is an excellent reference guide, as is The Story of the Thimble---An Illustrated Guide for Collectors, by Bridget McConnel, 1997. You also should check out eBay's thimble listing under antiques and collectibles. Chances are, you will find a thimble that looks something like yours. The following steps outline proper cleaning techniques for various types of thimbles.
Silver, iron or brass thimbles: Only clean if the thimble has intricate patterns carved into it that are obscured by its natural patina. Keep in mind that when you clean silver, you remove some of the silver. The best way to clean a silver thimble is to simply use a toothbrush with a little warm water and soap. Dry thoroughly. If you really want to use a silver or brass cleaner, use the least abrasive one you can find, but this is not recommended.
Porcelain, glass or bone china thimbles: Wipe the thimble down with a damp cloth. Don't use cleaning agents of any kind. They could destroy the decoration and maybe even damage the porcelain.
Aluminium thimbles: Use warm water and a mild detergent. Stay away from baking soda and other alkalis, as they tend to discolour aluminium.
Leather thimbles: Cleaning leather is risky because the cleaning agent can change the colour or even the texture of leather. It's best to clean leather thimbles with a soft cloth dipped in warm water mixed with a gentle, moisturing soap. Allow to dry thoroughly before returning to display case.
Wood thimbles: Don't try to clean. If you must, use a soft cloth with warm water and gentle soap. Allow to dry thoroughly.
The oldest thimbles you are like to find in an antique store are porcelain thimbles from the 1700s. If your thimble was made in the 19th century, chances are it was made of silver.
Beware of contemporary "copy" thimbles made to look old. They are generally made of porcelain or bone china and decorated with crests, coats of arms or slogans. If you have any doubts, speak with a qualified appraiser.