INDIGO.

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fibrous mass) containing the blue could drive the film of indigo aside at thevery moment when the pure colour beneath could enter into the cloth andunite with the fibre; but in either block or roller printing* the cloth and designare perpendicular to each other, and the face of the colour comes in flat con-tad with the cloth : the insoluble particles, being deposited first, hinder andprevent the fixation of the others. Many ingenious construdions of reservoirsand sieves for block printing have been made specially for this colour, and,with the exercise of great care, it has been possible to print with some ofthem, and obtain tolerably uniform results; but they have not admitted ofgeneral application.

There is less especial difficulty in printing pencil-blue with the cylinder.Thousands of pieces are weekly printed in America, and a considerable numberhere. The apparatus used is a “ dodor-box.” The blue oxidised layer ofcolour is removed, and the engraved portions of the cylinder work in the greensolution last, and go thence at once to the cloth.

As an instance of the mode of making up a mixture for pencil-blue, wequote the following for dark blue :—10 gals, indigo-pulp, containing 40 lbs.indigo; 40 lbs. yellow orpiment; nj gals, soda-lye, at 70° Tw.; i8| gals,water; 4 lbs. lime. Boil till yellow when spread on glass; let settle, andthicken the clear with 120 lbs. gum Senegal.

We have already referred to Bennet Woodcroft’s process, bearing upon theprodudion of a good dark blue by means of the rollers. The patentee, adingunder the knowledge that it was the oxygen of the air which—both on theroller and on the colour—was the obstacle to neat and proper printing, pro-posed to construd covered apparatus to surround the colour-box and roller,and to receive the piece when printed ; and in all these spaces where the airwas in contad with the colour, he proposed to expel it by introducing commoncoal-gas from the gas-mains, since, as a rule, this gas does not contain anyoxygen, or at least only a very small percentage,f it could not ad upon thecolour, which it was supposed would then have a fair chance of entering the

* The difference between block printing and roller or cylinder printing consists in the fadthat while the block (generally a square piece of wood, upon one surface of which thedesired pattern is either cut en bas-relief , or is executed in a peculiar style by means ofcopper wires projecting very slightly from the surface of the block) not only deposits the colourupon the cloth, but to a greater or less extent forces it in, the cloth in cylinder printing (thecylinders are made of copper, upon which the pattern is engraved) has to absorb the colour,partly by capillary attraction and partly by the pressure of the lapping. The same colours donot answer indifferently for block and roller styles. Block-colours can be worked usuallythinner (i. e require less thickening—the reader should also bear in mind that the term colourts often used in cases where really only a mordant is printed) than cylinder, or so-calledmachine-colours, and it is possible to apply colours by block which could not well be appliedby cylinder. Among such colours are those containing insoluble matters, like pipe-clay,sulphate of lead, &c. For heavy shades upon woollen cloth (flannel printing) the block has un-doubtedly the advantage over the cylinder, because not only does wool demand much morecolouring matter than cotton to produce a shade of equal depth, but it does not absorb it soquickly, since its fibres are less readily wetted than those of cotton, and it consequently doesnot receive colour from the engraving in sufficient quantity. Dark blues, chocolate-browns,greens, &c., on the finest class of French woollen cloth, require to be blocked two or threetimes to receive sufficient colour for full heavy shades.

* It is not the place here to enter into details on this matter; but the fadt is that, unlessunder very exceptional conditions, gas manufactured on the large scale is never quite free fromsome air.

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