Gum tragacanth, or "Dragon" is one of the most indispensable thickening agents possessed by the textile printer. It may be mixed in any proportion with starch or flour and is equally use ful for pigment colours and mordant colours. When added to starch paste it increases its penetrating power, adds to its soft ness without diminishing its thickness, makes it easier to wash out of the fabric and produces much more level colours than starch paste alone. Used by itself it is suitable for printing all kinds of dark grounds on goods which are required to retain their softness. A tragacanth mucilage may be made either by allow ing it to stand a day or two in contact with cold water or by soaking it for 24 hours in warm water, and then boiling it up until it is smooth and homogeneous. If bottled under pressure it gives a very fine smooth mucilage (not a solution proper), much thinner than if it had been made in the cold.
Albumen is both a thickening and a fixing agent for insoluble pigments such as chrome yellow, the ochres, vermilion and ultra marine. Albumen is always dissolved in the cold, a process which takes several days when large quantities are required. The usual strength of the solution is 4 lb. per gal. of water for blood albu men, and 6 lb. per gal. for egg albumen. The latter is expensive and only used for the lightest shades. For most purposes one part of albumen solution is mixed with one part of tragacanth mucilage, this proportion of albumen being found amply suf ficient for the fixation of all ordinary pigment colours. In special
instances the blood albumen solution is made as strong as 50% but this is only in cases where very dark colours are required to be fast to washing. After printing, albumen-thickened colours are exposed to hot steam which coagulates the albumen and ef fectually fixes the colours.
Concentrated solutions of the colouring matters and other ad juncts are added to the cold thickenings, of which large quan tities are kept in stock. Colours are reduced in shade by simply adding more starch or other paste. For example, a dark blue con taining 4 oz. of methylene blue per gal. may readily be made into a pale shade by adding to it 3o times its bulk of starch paste or gum, as the case may be. Similarly with other colours. Before printing it is very essential to strain or sieve all colours in order to free them from lumps, fine sand, etc., which would in evitably damage the polished surface of the engraved rollers and result in bad printing, since every scratch on the surface of a roller prints a fine line in the cloth. The straining is usually done by squeezing the colour through fine cotton or silk cloths. Mechanical means are also employed for colours that are used hot or are very strongly alkaline or acid.