The leaves are thinly spread to wither either naturally (where the climate is suitable) or by means of heated air forced over the withering racks. The object is to evaporate as much of the tea leaf's water content so that the leaf becomes pliable like soft leather gloves.

Rolling

From the withering-racks the soft, green leaf passes to the rolling machinery where it is twisted and rolled to break up the leaf cells and release the juices which give the tea its flavor. The first important chemical change starts here when the juices which remain on the leaf are exposed to the air and development of the essential oil begins.

From the roller the tea emerges as twisted lumps which are broken up by coarse mesh sieves or roll-breakers. The fine leaf which falls through are taken to the fermenting rooms, while the coarse leaf is returned for further rolling.

Oxidation

The oxidation which started in the rollers is completed in the oxidation room. Here the tea leaves are spread on cement or tile floors (sometimes glass or cement tables) in a cool, damp atmosphere. The leaves undergo further chemical change through the absorption of oxygen, and turn bright copper color, like a new penny. It is this process of oxidation which distinguishes the black teas, almost universally drunk in the United States today, from what are known as green teas.

The purpose of this is to arrest further oxidation, and to dry the leaf evenly and thoroughly without scorching it. The automatic tea drier consists of a large iron box inside which the leaves, spread on trays, travel slowly from top to bottom while a continuous blast of hot dry air is forced into the box. Careful regulation of the temperature and of the speed at which the trays move is the main factor in successful firing.