Indigo is a vat dye. Vat dyes are in pigment form which turns into water-soluble dyes by going through a chemical reduction using a reducing agent and caustic soda. The most common reducing agent is hydrosulphite. Only the reduced form of indigo can bind to the cotton fibre. The indigo dyeing process is unique, combining different dyeing techniques, exhaust and continuous dyeing. To ensure an effective dyeing process other chemicals, wetting, dispersing and complexing agents, and physical parameters, such as temperature and pressure, are considered in the dying process. Typically, the yarn is dyed in a continuous process, using a sequence of multiple dye baths, where the yarn is dipped into the dye bath and the indigo dye is being exhausted, followed by the exposure to air to undergo oxidation, which allows the indigo dye to be fixed as a pigment. The number of dye baths (dips) and the concentration of the chemicals in the dye baths determine the contrast and colour of the denim fabric. Utilising one to thirty dye baths is possible, but typically the warp yarn goes through an average of six to eight dyeing baths. Innovative dyeing equipment, like modified dye boxes, foam dyeing and spray dyeing, has emerged in recent years to reduce the energy, water and chemical consumption, improving the sustainability of the denim dyeing.