Black tea begins with fresh leaves from the Camellia sinensis plant. This plant produces all types of tea - black, green, oolong, white, and yellow - and it is only how the fresh leaves are processed that determines which kind of tea they become.
There are three methods for producing black tea - orthodox, CTC (Crush, Tear, Curl) and LPT (Laurie Tea Processor). CTC and LPT are usually used for tea destined for tea bags while the orthodox processing method is used for loose leaf teas. These loose-leaf teas are usually, but not always, better quality tea than the tea used in tea bags.
No matter which processing method is used, the first step in black tea production is to allow the leaves to wither under controlled conditions. When freshly picked, tea leaves contain about 80% moisture and this must be reduced by 30% to 70% before they proceed to the next stage.
The tea leaves are placed on wire mesh troughs or trays. Circulating air (either fan-forced or natural) removes the moisture over a period of up to 17 hours. The method and amount of withering varies by region and type of tea.
The second stage in black tea production is rolling the leaves. The tea leaves are still pliable after the withering stage, and they are passed through mechanical rollers to extract the sap. This extraction process causes the cells of the leaves to break down, exposing the sap to oxygen which causes the leaves to oxidize (or ferment as it is often called).
The rolling process cause the leaves to break up, so long rolling times produces small-grade teas. After the leaves are rolled for about 30 minutes they are sifted to separate the smaller leaves which are immediately laid out for oxidizing. The remaining larger leaves are re-rolled and re-sifted, and this process may be repeated several times.
The oxidation stage is often referred to as fermentation, but since there is no alcohol being produced, fermentation is not really an accurate term. During this stage the tea leaves are exposed to cool, damp air while the oxygen reacts with the sap to turn the leaves to a dark color. The oxidation process is critical to the quality of the tea and must be halted at precisely the right time to produce the best-quality tea.
When the tea master has decided that the tea leaves have oxidized to their ideal level, the oxidation is halted by drying the leaves with hot air. This brings the moisture level of the tea leaves down to about 3% and the extracted sap dries on the leaves, darkening the color of the leaves to dark brown or black.
Finally, the leaves are sorted to various sizes by sifting them through a sieve. The four tea grades are Leaf, Broken, Fannings, and Dust and refer only to the size of the leaf. Each grade is further broken down according to origin and quality.