Postage Stamp Gum

In philately, gum is the substance applied to the back of a postage stamp to enable it to adhere to a letter or other mailed item. The term is generic, and applies both to traditional types such as gum arabic and to synthetic modern formulations.
The use of gum was part of the original proposal by Rowland Hill, and has been universal from the beginning. There have been a number of stamp types that were issued ungummed, typically due to emergency situations when gum was not available, such as Italy in 1944, Kraków issue of Poland in 1919, Latvia in 1919. Other reasons have included lack of access to gum (the typewritten "Cowries" of 1895 Uganda), extreme tropical climate (1873 Curaçao and Suriname), and intent to sell only to collectors (as with the US "Farley's Follies" souvenir sheets of 1933). The manual gluing-on of postage is such an extreme consumption of time (and "time is money" to businesses with a lot of mail) that these situations are always temporary.
Originally, gumming took place after printing and before perforation, usually because the paper had to be damp for printing to work well, but in modern times most stamp printing is done dry on pregummed paper. There have been a couple of historical instances where stamps were regummed after being perforated, but these were unusual situations.
On early issues, gum was applied by hand, using a brush or roller, but in 1880 De La Rue came up with a machine gumming process using a printing press, and gum is now always applied by machine. The gum is universally spread as uniformly as possible, but a 1946 local issue by the town of Finsterwalde in Germany used an economy process where the back of the stamp had a regular pattern of circular bare patches.
The greatest manufacturing problem of the gumming process is its tendency to make the stamps curl, due to the different reaction of paper and gum to varying moisture levels. In the most extreme cases, the stamp will spontaneously roll up into a small tube. Various schemes have been tried, but the problem persists to this day. In Swiss stamps of the 1930s, Courvoisier used a gum-breaking machine that pressed a pattern of small squares into the gum, resulting in so-called grilled gum. Another scheme has been to slice the gum with knives after it has been applied. In some cases the gum solves the problem itself by becoming "crackly" when it dries.