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PREPARATION OF
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PREPARATION OF See also:COLOURS The See also:art of making colours for textile-See also:printing demands both chemical knowledge and extensive technical experience, for their ingredients must not only be properly proportioned to each other, but they must be specially chosen and compounded for the particular See also:style of See also:work in See also:hand. For a See also:pattern containing only one See also:colour any mixture whatever may be used so See also:long as it fulfils all conditions as to shade, quality and fastness; but where two or more colours are associated in the same See also:design each must be capableof undergoing without injury the various operations necessary for the development and fixation of the others. All printing pastes whether containing colouring See also:matter or not are known technically as " colours," and are referred to as such in the sequence. Colours vary considerably in See also:composition. The greater number of them contain all the elements necessary for the See also:direct See also:production and fixation of the colour-See also:lake. Some few contain the colouring matter alone and require various after-treatments for its fixation; and others again are simply " mordants " thickened. A See also:mordant is the metallic See also:salt or other substance which combines with the colouring principle to See also:form an insoluble colour-lake, either directly by steaming, or indirectly by See also:dyeing. All printing colours require to be thickened, for the twofold See also:object of enabling them to be transferred from colour-See also:box to See also:cloth Without loss and to prevent them from " See also:running " or spreading beyond the limits of the pattern. Thickening Agents.—The thickening agents in most See also:general use as vehicles in printing, are See also:starch, See also:flour, See also:gum arabic, gum See also:senegal and gum tragacanth, See also:British gum or See also:dextrine and albumen. With the exception of albumen all these are made into pastes, or dissolved, by boiling in See also:double or " jacketed " pans, between the inner and See also:outer casings of which either See also:steam or See also:water may be made to circulate, for boiling and cooling purposes. See also:Mechanical See also:agitators are also fitted in these pans to mix the various ingredients together, and to prevent the formation of lumps by keeping the contents thoroughly stirred up during the whole See also:time they are being boiled and cooled. Starch See also:Paste.—This is made by mixing 15 lb of See also:wheat starch with a little See also:cold water to a smooth creamy paste; a little See also:olive oil is then added and sufficient water to bring the whole up to Io gallons. The mixture is then thickened by being boiled for about an See also:hour and, after cooling, is ready for use. Starch is the most extensively used of all the thickenings. It is applicable to all but strongly alkaline or strongly See also:acid colours. With the former it thickens up to a stiff unworkable jelly, while See also:mineral acids or acid salts convert it into dextrine, thus diminishing its thickening See also:power. Acetic and formic acids have no See also:action on it even at the See also:boil. Flour paste is made in a similar way to starch paste. At the See also:present time it is rarely used for anything but the thickening of See also:aluminium and See also:iron mordants, for which it is eminently adapted. Gum arabic and gum senegal are both very old thickenings, but their expense prevents them from being used for any but See also:pale delicate tints. They are especially useful thickenings for the See also:light ground colours of soft muslins and sateens on See also:account of the See also:property they possess of dissolving completely out of the See also:fibres of the cloth in the washing See also:process after printing. Starch and artificial gums always leave the cloth somewhat harsh in " feel " unless they are treated specially, and are moreover incapable of yielding the beautifully clear and perfectly even tints resulting from the use of natural gums. Very dark colours cannot well be obtained with gum senegal or gum arabic thickenings; they come away too much in washing, the gum apparently preventing them from combining fully with the fibres. Stock solutions of these two gums are usually made by dissolving 6 or 8 lb of either in one See also:gallon of water, either by boiling or in the cold by See also:standing. British gum or dextrine is prepared by See also:heating starch. It varies considerably in composition—sometimes being only slightly roasted and consequently only partly converted into dextrine, and at other times being highly torrefied, and almost completely soluble in cold water and very dark in colour. Its thickening power decreases and its " gummy " nature increases as the temperature at which it is roasted is raised. The lighter coloured gums or dextrines will make a See also:good thickening with from 2 to 3 lb of gum to one gallon of water, but the darkest and most highly calcined require from 6 to Io lb per gallon to give a substantial paste. Between these limits all qualities are obtainable. The darkest qualities are very useful for strongly acid colours, and with the exception of gum senegal, are the best for strongly alkaline colours and discharges. Like the natural gums, neither light nor dark British gums penetrate into the fibre of the cloth so deeply as pure starch or flour, and are therefore unsuitable for very dark strong colours. Gum tragacanth, or " See also: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 useful for pigment colours and mordant colours. When added to starch paste it increases its penetrative power, adds to its softness without diminishing its thickness, makes it easier to See also: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 soft clothy feel. A tragacanth See also:mucilage may be made either by allowing it to stand a See also:day or two in contact with cold water or by soaking it for twenty-four See also:hours in warm water and then boiling it up until it is perfectly smooth and homogeneous. If boiled under pressure it gives a very See also:fine smooth mucilage (not a See also:solution proper), much thinner than if made in the cold. Albumen.--Albumen is both a thickening and a fixing See also:agent for TECHNOLOGY] insoluble See also:pigments such as chrome yellow, the See also:ochres, See also:vermilion and See also:ultramarine. 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 gallon of water for See also:blood albumen, and 6 lb per gallon for See also:egg albumen. The latter is expensive and only used for the lightest shades. For most purposes one See also:part of albumen solution is mixed with one part of tragacanth mucilage, this proportion of albumen being found amply sufficient for the fixation of all See also:ordinary pigment colours. In See also:special instances the blood albumen solution is made as strong as 5o per cent., but this is only in cases where very dark colours are required to be absolutely fast to washing. After printing, albumen-thickened colours are exposed to hot steam, which coagulates the albumen and effectually fixes the colours. Formerly colours were always prepared for printing by boiling the thickening agent, the colouring matter and solvents, &c., together, then cooling and adding the various fixing agents. At the present time, however, concentrated solutions of the colouring matters and other adjuncts are often simply added to the cold thickenings, of which large quantities are kept in stock. Colours are reduced in shade by simply adding more starch or other paste. For example, a dark See also:blue containing 4 oz. of methylene blue per gallon may readily be made into a pale shade by adding to it See also:thirty times its bulk of starch paste or gum, as the See also:case may be. Similarly with other colours. Before printing it is very essential to See also:strain or See also:sieve all colours in See also:order to See also:free them from lumps, fine See also:sand, &c., which would inevitably damage the highly polished See also:surface of the engraved rollers and result in See also:bad printing. Every scratch on the surface of a See also:roller prints a fine See also:line in the cloth, and too much care, therefore, cannot be taken to remove, as far as possible, all grit and other hard particles from every colour. The straining is usually done by squeezing the colour through fine See also:cotton or See also:silk cloths. Mechanical means are also employed for colours that are used hot or are very strongly alkaline or acid. 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