The very first tea bags did not exist until tea had been around for about four thousand years. Several tea and coffee merchants in 1903 New York City sold both tea samples and bulk tea wrapped up in hand-sewn silk bags for shipment around the globe. The intention was that people who bought the tea would tear the bag open and pour the contents into a teapot into which they would then pour the hot water. Or, they might use a tea infuser, a perforated globe used by the Victorians to brew tea. No one thought that people would actually dip the silk bags in hot water to make tea. But, as more and more tea was shipped in this way, the more normal it became to brew tea in that way.
In the 1930s, to reduce cost, paper replaced silk, but the shape remained that of a small bag. In the 1940s, Salada Tea came out with a patent for the rectangular tea bag made of paper fibers, heat-sealed with adhesive. Variations of this style are still being used, some with a string attached to the bag, the other end of which has a paper tag, mostly used for advertising or identification of the tea, but also sometimes used for slogans, decorative patterns, or witticisms. In those countries in the world that still use loose tea leaves to brew tea, small tearable bags of paper or foil contain the loose tea, and are also called tea bags.
Basically, a tea bag is a sealed small bag of paper, cloth, or plastic that contains loose tea leaves. Besides allowing the brewing of tea, the bag makes it easy to dispose of the soggy tea leaves when done. Many types of tea are packaged in tea bags, including herbal teas. The tea used for tea bags is usually the fannings, the small pieces of tea leaves left over after the larger tea leaves have been picked out of the mix for sale as loose tea, or, sometimes, as whole-leaf tea.
The paper used in tea bags is the same type found in coffee filters, and is made of a blend of some vegetable and wood fibers, but mostly of the bleached pulp of the abaca hemp, a small tree that grows in both Columbia and in the Philippines. Tea bags are sealed with a heat-sealing thermoplastic, such as polypropylene or PVC (polyvinyl-chloride), set along the edges of the inner surface of the tea bag. Rather than being hand-made, tea bags today are made by machines of German or Argentinian manufacture that can make, fill, and seal as many as 120 rectangular bags per minute. There are machines, though, that can make up to 250 bags per minute, but these are the more unusual pyramidal type of tea bag.
Besides the traditional rectangular tea bag (and that slight variation, the square tea bag), there are also circular and pyramidal tea bags. Manufacturers of those types of tea bags claim that the non-rectangular shapes improve the quality of tea brewed, and this may well be true, because of the greater exposure of the tea leaves within to the hot water. Also, because these shapes use less paper, less adhesive is used, which might improve the taste somewhat. But, such improvement in quality will occur only if the tea is prepared properly. Tea made haphazardly will taste poorly no matter what the shape of the bag. It must be admitted, though, that the non-rectangular bags, because they use less paper and less adhesive, are more economical to make. Consumers can also buy empty tea bags of various shapes that are more of an open-ended pouch that can be filled with the blend of tea leaves of their choice, and then sealed by a flap.
Incidentally, a cold soggy tea bag can be used as a sunburn treatment -- we'll bet you didn't know that!