Until fairly recently it was only possible to obtain complete fastness to a boiling wash by using colorants known as vat dyes. These are actually pigments; they are insoluble in water. In order to apply them they must first be subjected to a process of chemical reduction known as ‘vatting’ (hence their name), which makes them soluble. They can then be applied to the fabric by a fairly normal dyeing process. After this, the fabric is exposed to the air or treated with an oxidising compound, whereby the reduced soluble form of the dye is re-converted to its original insoluble pigment from in the fibre and so is not liable to be removed by washing. But vat dyes are expensive and rather tricky to apply, and in the 1960s the situation was eased by the introduction of a new class of dye known as ‘reactive’. These dyes have the ability to combine chemically with, and so form an actual part of, the molecules of the fibre to which they are applied, so they are, of course, very resistant to detergents.They are also cheaper and easier to apply than vat dyes, and they are much brighter.