The thickening agents in most general use as vehicles in printing, are starch, flour, gum arabic, gum senegal and gum tragacanth, British gum or dextrine and albumen.
With the exception of albumen all these are made into pastes, or dissolved, by boiling in double or jacketed pans, between the inner and outer casings of which either steam or water may be made to circulate, for boiling and cooling purposes. Mechanical 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 time they are being boiled and cooled.
This is made by mixing 15 lb of wheat starch with a little cold water to a smooth creamy paste; a little olive oil is then added and sufficient water to bring the whole up to 10 gallons. The mixture is then thickened by being boiled for about an 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 acid colors. With the former it thickens up to a stiff unworkable jelly, while mineral acids or acid salts convert it into dextrine, thus diminishing its thickening power. Acetic and formic acids have no action on it even at the boil.
Flour paste is made in a similar way to starch paste. At the present time it is rarely used for anything but the thickening of aluminum and 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 pale delicate tints. They are especially useful thickenings for the light ground colors of soft muslins and sateens on account of the property they possess of dissolving completely out of the fibers of the cloth in the washing 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 colors 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 fibers. Stock solutions of these two gums are usually made by dissolving 6 or 8 lb of either in one gallon of water, either by boiling or in the cold by standing.
British gum or dextrine is prepared by 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 color. Its thickening power decreases and its gummy nature increases as the temperature at which it is roasted is raised. The lighter colored gums or dextrines will make a 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 lb per gallon to give a substantial paste. Between these limits all qualities are obtainable. The darkest qualities are very useful for strongly acid colors, and with the exception of gum senegal, are the best for strongly alkaline colors and discharges.
Like the natural gums, neither light nor dark British gums penetrate into the fiber of the cloth so deeply as pure starch or flour, and are therefore unsuitable for very dark strong colors.
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 useful for pigment colors and mordant colors. When added to starch paste it increases its penetrative power, adds to its softness without diminishing its thickness, makes it easier to wash Out of the fabric and produces much more level colors than starch paste alone. Used by itself it is suitable for printing all kinds of dark grounds on goods that are required to retain their soft clothy feel. A tragacanth mucilage may be made either by allowing it to stand a day or two in contact with cold water or by soaking it for twenty-four hours in warm water and then boiling it up until it is perfectly smooth and homogeneous. If boiled under pressure it gives a very fine, smooth mucilage (not a solution proper), much thinner than if made in the cold.