Black tea is a variety of tea that is more oxidized than the oolong, green, and white varieties. All four varieties are made from leaves of the shrub (or small tree) Camellia sinensis. Black tea is generally stronger in flavor and contains more caffeine than the less oxidized teas. Two principal varieties of the species are used, the small-leaved Chinese variety plant (C. sinensis subsp. sinensis), also used for green and white teas, and the large-leaved Assamese plant (C. sinensis subsp. assamica), which was traditionally only used for black tea, although in recent years some green has been produced.
In Chinese languages and neighboring countries, black tea is known as “red tea” (??, Mandarin Chinese hongcha; Japanese kocha; ??, Korean hongcha), a description of the colour of the liquid; the term black tea refers to the colour of the oxidized leaves. In Chinese, “black tea” is a commonly used classification for post-fermented teas, such as Pu-erh tea; in the Western world, “red tea” more commonly refers to rooibos, a South African tisane.
While green tea usually loses its flavor within a year, black tea retains its flavour for several years. For this reason, it has long been an article of trade, and compressed bricks of black tea even served as a form of de facto currency in Mongolia, Tibet, and Siberia into the 19th century. Although green tea has recently seen a revival due to its purported health benefits, black tea still accounts for over ninety percent of all tea sold in the West.
Black tea is usually graded on one of four scales of quality. Whole leaf teas are highest quality followed by broken leaves, fannings, and dusts. Whole leaf teas are produced with little or no alteration to the tea leaf. This results in a finished product with a coarser texture than that of bagged teas. Whole leaf teas are widely considered the most valuable, especially if they contain leaf tips. Broken leaves are commonly sold as medium grade loose teas. Smaller broken varieties may be included in tea bags. Fannings are usually small particles of tea leftover from the production of larger tea varieties, but are occasionally manufactured specifically for use in bagged teas. Dusts are the finest particles of tea leftover from production of the above varieties, and are often used for tea bags with very fast, very harsh brews. Fannings and dust are useful in bagged teas because the greater surface area of the many particles allows for a fast, complete diffusion of the tea into the water. Fannings and dusts usually have a darker colour, lack of sweetness, and stronger flavor when brewed.