Black tea is fully oxidised, or fermented, tea. The tea leaves are plucked and then laid onto trays where warm air is gently used to dry the leaves for anything up to 20 hours. The dried leaves are then passed through a large rolling machine that breaks up the leaves and allows them to oxidise. Once sufficiently oxidised, the leaves are fired to rapidly reduce the water content ready for packing. Different teas are produced depending on when the leaves are picked, how long they are dried and fired, and according to leaf size.