dyed. The waxed areas do not accept the dye, leaving uncoloured patterns against a coloured ground. Discharge printing, in which a bleaching agent is printed onto previously dyed fabrics to remove some or all of the colour. Resist and discharge techniques were particularly fashionable in the 19th century, as were combination techniques in which indigo resist was used to create blue backgrounds prior to block-printing of other colours. Modern industrial printing mainly uses direct printing techniques. The printing process does involve several stages in order to prepare the fabric and printing paste, and to fix the impression permanently on the fabric: pre-treatment of fabric, preparation of colours, preparation of printing paste, impression of paste on fabric using printing methods, drying of fabric, fixing the printing with steam or hot air (for pigments), after process treatments. Preparation of cloth for printing Cloth is prepared by washing and bleaching. For a coloured ground it is then dyed. The cloth has always to be brushed, to free it from loose nap, flocks and dust that it picks up whilst stored. Frequently, too, it has to be sheared by being passed over rapidly revolving knives arranged spirally round an axle, which rapidly and effectually cuts off all filaments and knots, leaving the cloth perfectly smooth and clean and in a condition fit to receive impressions of the most delicate engraving. Some fabrics require very careful stretching and straightening on a stenter before they are wound around hollow wooden or iron centers into rolls of convenient size for mounting on the printing machines. Preparation of colours The art of making colours for textile printing demands both chemical knowledge and extensive technical experience, for their ingredients must not only be in proper proportion to each other, but also specially chosen and compounded for the particular style of work in hand. A colour must comply to conditions such as shade, quality and fastness; where more colours are associated in the same design each must be capable of withstanding the various operations necessary for the development and fixation of the others. All printing pastes whether containing colouring matter or not are known technically as colours. Colours vary considerably in composition. Most of them contain all the elements necessary for direct production and fixation. Some, however, contain the colouring matter alone and require various after-treatments; and others again are simply thickened mordants. A mordant is a metallic salt or