Some colours adhere at once to the stuff, and are called

substantial colours

; while others require that the material to be dyed should undergo some previous preparation in order to render it permanent. The substances used to fix the colouring matters are called

mordants

, which should possess four qualifications:

  1. They should possess an equal affinity for the fibre of the material and the colouring matter.
  1. They should be incapable of injuring or destroying either by prolonged action.
  1. They should form, with the colour, a compound capable of resisting the action of air and water.
  1. They should be capable of readily conforming to the various operations of the dyer.