Philately is the study and collecting of revenue and postage stamps. This includes the design, production and uses of stamps after they are authorized for issue, usually by government officials such as postal authorities. Philately is the distinct activity of studying stamps, which may or may not include stamp collecting. For instance, some philatelists will study extremely rare stamps without expecting to own copies of them, whether because of cost, or because the sole survivors are in museums. Conversely, stamp collecting itself is the acquisition of stamps, at times without regard for origin or usage.
The origin of philately is in the observation that in a pile of stamps all appearing to be the same type, closer examination may reveal different kinds of paper, different watermarks embedded in the paper, variations in color shades, different perforations, and other kinds of differences. Comparison with records of postal authorities may or may not show that the variations were intentional, which leads to further inquiry as to how the changes could have happened, and why. To make things more interesting, thousands of forgeries have been produced over the years, some of them very good, and only a thorough knowledge of philately gives any hope of detecting the fakes.
One explanation for all the variation is that stamp printing was among the early attempts at large-scale mass production activity by postal authorities. Even in the 19th century, stamps were being issued by the billions, more than any other kind of manufactured object at the time.
Topical, also known as Thematic, philately is the study of what is depicted on the stamps. There are hundreds of popular subjects, such as birds, insects, sports, maps, and so forth. Interesting aspects of topical philately include design mistakes (such as use of the wrong picture on a US stamp honoring Bill Pickett), design alterations (for instance, the recent editing out of cigarettes from the pictures used for US stamps), and the stories of how particular images came to be used (one US stamp from the 1920s shows a Viking ship apparently flying an American flag, but this was not a mistake; the stamp depicted a modern replica).
Postal history concentrates on the use of stamps on mail. It includes the study of postmarks, post offices, postal authorities and the process by which letters are moved from sender to recipient, including routes and choice of conveyance. A classic example is the Pony Express, which was the fastest way to send letters across the United States during the few months that it operated. Covers that can be proved to have been sent by the Pony Express are highly prized by collectors.
The results of philatelic study have been extensively documented by the philatelic literature, which includes many books and nearly 15,000 different periodical titles.
Philately is basically an activity of reading and study, but the human senses typically need augmentation. The stamps themselves are handled with stamp tongs or tweezers so as to preserve them from large, clumsy, and possibly greasy fingers. A strong magnifier reveals details of paper and printing, while the odontometer or perforation gauge helps distinguish a "perf 12" from a "perf 13".
While many watermarks can be detected merely by turning the stamp over, or holding it up to the light, others require the services of watermark fluid, such as benzine (not to be confused with benzene, which is toxic), carbon tetrachloride or trichloro-trifluoro-ethane that "wets" the stamp without dissolving gum or ink. Other techniques, such as using coloured light filters have been attempted in an effort to avoid the use of toxic substances.
Experts evaluating the authenticity of the rarest stamps use additional equipment such as fluoroscopes. Some stamps are printed with ink which fluoresces when exposed to ultraviolet light. Ultraviolet light sources are also used to examine stamps and postal history for signs of repairs or various types of faults.