The "duplex" or "reversible" machine derives its name from the fact that it prints both sides of the cloth. It consists of two ordinary machines so combined that when the cloth passes, fully printed on one side from the first, its plain side is exposed to the rollers of the second, which prints an exact duplicate of the first impression upon it in such a way that both printings coincide. A pin pushed through the face of the cloth ought to protrude through the corresponding part of the design printed on the back if the two patterns are in good "fit." Lithographic printing is employed with success in the printing of silk handkerchiefs. Haley's process is described in the 1915 Journal of the Society of Dyers and Colourists. In 1920 an Indian named Cooper proposed to avoid engraving rollers by using de signs, cut in paper (stencils) or curtain net, on the bowl and placing the cloth between this and a uniform layer of colour on a roller beneath it. The arrangement was applicable when used with lithographic (oil) inks but was not so successful with calico printers' colours (starch or gum thickening). In 1926 A. Crompton obtained a patent for a machine with rollers arranged side by side but otherwise on a similar principle to that of Cooper and claimed it to be successful in calico printing.
In stencilling colour is brushed through holes cut in stout paper or metal. The Japanese are particularly successful in hand work of this kind for they can by tying parts of the designs with raw silk thread produce almost circular patterns. In England a machine for stencilling has been patented by S. H. Sharp.
Variegated and marbled effects, patented by the Calico Printers' Association, Ltd., E. Schofield and F. Farnworth, are obtained by applying colours to the underside of a horizontally travelling fabric by means of rotating brushes which dip into a colour box beneath, and to the upper side by allowing colour to drop on to the cloth from a series of colour cans provided with taps to regulate the rate of flow. Reversible effects can be ob tained. The rollers are of smooth brass and travel at a higher speed than the cloth ; they pick up the surplus colour and transfer it to doctors which carry it away to a suitable vessel. It may be utilized for a very wide range of styles.
Starch paste is made by mixing 15 lb. of wheat starch with cold water to form a creamy paste ; a little olive oil is then added and sufficient water to bring the whole up to 1 o gallons. The mixture is thickened by being boiled for about an hour and, after cooling, is ready for use. It is the most extensively used of all the thick enings, and is applicable to all but strongly alkaline or strongly acid colours. 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 aluminium and iron mor dants, 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 delicate tints. They are specially useful thickenings for the light ground colours of soft muslins and sateens, on account of the property they possess of dissolving completely out of the fibres of the cloth in the washing process after printing. Starch and artificial gums always leave the cloth somewhat harsh unless they are treated specially and are incapable of yielding the clear and 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 ap parently 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 gallon of water, either by boiling or by standing in the cold.
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 others 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 good thickening with from 2 to 3 lb. of gum to one gallon of water, but the dark est require from 6 to BD lb. per gal. to give a substantial paste, and are very useful for strongly acid colours, and, with the excep tion of gum senegal, are the best thickening agents 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.