Indigo exists in two forms; a blue form which is insoluble, and a yellow form which is soluble.  For dyeing, indigo must first be dissolved in water.  The process is one of chemical reduction in the absence of air.  The traditional method involved fermenting for several weeks in a vat alkalinised with urine.  Wool was dipped into the heated vat and exposed to the air.  The yellow soluble form oxidises spontaneously to the blue insoluble form which is deposited on the surface of the fibres in the form of a protective coating.  Wool dyed with indigo is more resistant to wear and tear than undyed wool.  In theory it can be rubbed off, but in practice indigo-dyed woollen textiles do not develop the well-used look of cotton clothing dyed with indigo.  The saturation or depth of colour depends on the amount of indigo deposited on the fibres, so a dark blue can be expensive.  The lavish use of midnight blue is a popular extravagance in Persian rugs.