In the 4th stage - Fermentation, of tea manufacturing processing - the green leaf is converted to black tea. When the leaf comes out of the orthodox, rotorvane or CTC machines, it is spread out on ceramic, concrete or aluminium slabs in cool, humid air to ‘oxidise’ or, in old-fashioned tea vocabulary, to ‘ferment’. This can take as little as 15 minutes in high grown regions during the peak quality season, but may last from 30 minutes to 3 hours out of season when the air is cooler. It is at this stage that a chemical reaction takes place between the oxygen in the air and the contents of the leaf; the leaf changes gradually from green to amber to dark coppery brown and the air is filled with the tea aroma we all recognize when we brew a pot of black tea. The temperature of the leaf is maintained at around 25-30 degrees where mild acidification of the oxidised tea increases the levels of the tea theaflavins.