Black tea is a variety of tea that is more oxidized than the oolong, green, and whiteCamellia 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 sinensis), also used for green and white teas, and the large-leaved Assamese plant (C. sinensis assamica), which was traditionally only used for black tea, although in recent years some green has been produced.

In Chinese and Chinese influenced languages, black tea is known as "crimson tea" (紅茶, Mandarin Chinese hóngchá; Japanese kōcha; Korean hongcha), perhaps a more accurate description of the colour of the liquid. The name black tea, however, could alternatively refer 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. However, 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.  It was known since the Tang Dynasty that black tea steeped in hot water could also serve as a passable cloth dye for the lower classes that could not afford the better quality clothing colours of the time.  However, far from being a mark of shame, the "brown star" mark of the dyeing process was seen as much better than plain cloth and held some importance as a mark of the lower merchant classes through the Ming Dynasty. The tea originally imported to Europe was either green or semi-oxidized. Only in the 19th century did black tea surpass green in popularity. 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 Manufacturing

The freshly picked leaves are withered for 16 - 18 hours. The leaves are laid on wire mesh in troughs and warm air is blown through them until the leaf moisture content drops from 78 - 82% down to 63 - 73%.

The withered leaf can be processed as Orthodox large leaf tea, or as CTC (crush, tear, curl) tea with tiny leaf particles suitable for tea bags.

Orthodox tea is rolled slowly using traditional rolling tables that mimic the action of hand rolling. This unhurried disruption of the leaf cells causes a slow oxidation of the catechins, due to the realease of a natural leaf enzyme that causes the green catechins to change to orange red polyphenols. Slow oxidation favours aroma and flavour at the expense of intense colour.

CTC processing chops the withered leaf in seconds and initiates such rapid oxidation that the leaf heats up. The CTC process gives the intense colour that teabag brewing requires.

During oxidation, often incorrectly referred to as fermentation, the leaf colour changes from green to coppery red. It takes up to four hours during orthodox manufacture, but only 90 minutes for CTC.

When oxidation is complete, it is arrested by applying heat: this process is often called firing. The oxidised leaf is passed into a hot air dryer for twenty minutes whie the moisture content is reduced to 3% and the familiar black colour of the tea develops. Care has to be taken to maintain the dryer temperature within close limits so that unwanted aromas are not formed.