A test stamp is a postage stamp which is intended for internal use only, with no face value. Test stamps cannot be affixed to mail as postage. They are used for the purpose of developing new production methods, testing stamp equipment, and training postal workers. Like other stamps, test stamps are also objects of interest to collectors and they are a popular item among some collectors because they can be difficult to obtain and they can also be used to trace the history of new stamp designs and production methods.
In terms of testing stamp equipment, test stamps are used when postal authorities are testing new stamp printing machines, and also in the development of new stamp printing methods. They are also used for experimenting with adhesives and other aspects of stamp production. Machines which affix or dispense stamps are classically tested with test stamps as well, because the post office does not want to waste real stamps on testing, but does want to make sure that these machines will function correctly.
Test stamps are also used for trial color proofs, in which the post office wants to produce proofs or test versions of stamps with new colors and designs. This is done to refine colors before they are applied to real stamps. While most test stamps have relatively simple designs, trial proofs may have elaborate and ornate designs because the test stamp is used to develop new designs and colors for real stamps.
A typical test stamp will be marked in some way to indicate that it is a test stamp and that it is not valid postage. These stamps may also be made specifically in colors not used by the post office so that they can be readily identified as invalid postage if they are affixed to mail. This may happen on accident, as for example if a roll of test stamps is left in a stamp dispensing machine, or on purpose, if someone unscrupulous wants to avoid paying postage and has obtained some test stamps.
Also known as a training stamp or dummy stamp, a test stamp is of the same size, weight, and texture as a regular stamp. It is designed to look, act, and feel like a conventional stamp so that it can stand in for real postage. Using test stamps of different configurations might result in problems with calibrating postal equipment, as changes in the paper, adhesive, or inks used can change the way a stamp behaves on a printing press or in a stamp dispensing machine.