Photo-Trichromatic Printing. Part IV. Half-Tone and Photochromic Printing Inks.

Photo-Trichromatic Printing
C. G. Zander
Published by Raithby, Lawrence & Co., Ld., Leicester
Although it is intended to deal principally with photochromic printing-inks, a few remarks on half-tone inks in general may be useful. Of late years so-called "art shades," i.e., reds, greens, blues, violets, and purples, subdued or saddened with a small percentage of black, have deservedly come into favour alike with the public and the printers, and particularly so since some of the illustrated papers issued supplements of reproductions of Royal Academy pictures, and other subjects, printed in these subdued colours. These art shades look very effective if used in the production of artistic commercial stationery, illustrated catalogues, menus, etc. The effect, if these broken colours are judiciously selected, is very pleasing, and many of them give half-tone printings the appearance of two shades, as if produced by two printings. Tints of various colours may with great effect be combined with these saddened hues, and I have endeavoured to give a few hints for such combinations in Part II., when speaking of the harmony and contrast of colours and the combination of triads.

For half-tone work only the very best inks should be used. The blocks on account of their flat surface take very little ink in comparison with woodcuts or type, and therefore unless the ink is made of strong pigments, or the finest carbon black, the prints will turn out flat and look washed out. Natural earth colours, such as umbers, siennas, ochres, terra-verte, etc., should be rigidly rejected, for no amount of grinding will alter their hard and gritty nature, or make them fit for the production of delicate half-tones. They also cause unnecessary wear to the fine grain of the blocks, and are only desirable for ordinary letterpress or litho-printing on account of their cheapness and the variety of browns they comprise. For halftone work all the hues of the earth colours may be produced from more suitable sources, and the printer who is desirous of turning out good work, should not mind paying a little more for better inks. Any shade of brown may be made of black and red (madder being the best on account of its great strength and permanency) with or without the addition of yellow or blue. In this way an excellent brown, imitating silver prints, may be mixed. Green-blacks, olivegreens, nut-browns, and other shades, may be produced from madder, Prussian blue, and black, without havmg recourse to earth colours, to which white should be added for diluted colours. The theory of pigmentary mixtures has been dealt with in Part II., and the chromatic clock-dial may be consulted with advantage. The mixture of such art shades should however only be undertaken by those who are well acquainted with the idiosyncrasies of the pigments they are using, otherwise they will only waste time and materials and temper. It will in all cases be found more satisfactory to entrust the printing ink maker with the matching of the pattern.

The proportion of black necessary for the production of subdued colours is very small on account of the fine division and consequent colouring power of the carbon. It will probably not exceed 5% to 10%, varying with the tintorial value of the other pigments. The latter being in great preponderance should be of the best possible quality. Good lakes, i.e., dyes precipitated on an earthy base, such as hydrate of alumina, are most suitable for the purpose. These distribute well, print even, and do not fill up the interstices between the dots forming the half-tones of the block. On account of their strength and bulky nature, such lakes will be found cheaper in the end in spite of their higher price than earth colours. They will save labour m doing away with frequent washing up, and they will not wear the blocks. Excellent lakes of all colours are now produced from aniline dyes, and may with advantage be used where permanency on exposure to light is no object. But if permanency be desired, alizarine lakes made of alizarine, which, like aniline, is a coal-tar product, should be employed. It is also found in nature, as the colouring principle of madder-root, which used to be extensively cultivated in the south of France. Our modern artificial madders are perfectly permanent even in tints, and can now be produced in all shades of red, from scarlet to purple, and in excellent imitations of carmine and cochineal crimson and scarlet lakes. They form a very desirable base for half-tone inks, and should be used by the conscientious printing ink maker for the red in photochromic three-colour printing. More about this later on. Ultramarine is a most undesirable pigment for half-tone inks, as it does not print flat, and it is particularly unsuitable for photochromic printing on account of its opacity.

With these few introductory remarks upon the quality of process printing-inks, I now come to deal with photochromic inks used for three-colour work. The success of photochromic work depends on the blocks, on the inks, on the prmter, and in a certain degree also on the paper. I am going to speak of the ink only, and therefore take it for granted that the blocks are produced by colour-filters constructed on scientific principle, i.e., with a thorough knowledge of spectroscopy and photography, and not chosen in an arbitrary way. I have dealt with this subject in the preceding parts of this book. Further, I must take it for granted that the printer does his part ot the work not in a purely mechanical style, but that he has at least some knowledge of the principles of three-colour printing, the lack of knowledge of which is at present a serious stumbling block in the way of the success of photochromic printing.

One of the crucial tests of good photochromic work is the production not only of a correct rendering of the colouring of the original, but the production of neutral blacks and greys wherever they occur in the original, be it a painting or still-life object. If the colour-filters are correct representatives of the primary colour-sensations of the spectrum, the three negatives or chromograms will be monochrome representatives of the excitations caused on the end organs of our eyes by each of the respective primary colour-sensations reflected from the object. It is necessary in order to obtain a correct rendering of the tintorial representation in print that the positives, i.e., the blocks, should be printed in inks which are complementary colours of the colour-filters used. It is obvious, therefore, that as the colours of the three selective screens, if scientifically constructed, are—if I may use the term—a fixture, so the three pigmentary colours used in printing are also a fixture, and cannot be arbitrarily selected. Here the carelessness, indifference, and ignorance of most printing ink makers has been and is still causing great mischief and bringing ridicule upon one of the most interesting achievements of modern science. I have from time to time examined samples of photochromic inks of various makers, and found they all differ more or less, not only in the shades, but in the strength of the pigments used in their manufacture. Some makers unscrupulously use fugitive aniline lakes for the red, which, after a few days' exposure to light will fade and render the colouring of the whole picture incorrect. The three pigments which alone produce a correct colouring of a picture produced by the photochromic process, are a pure red pigment, one that is neither a purple nor an orange, but is the primary red of the artist, i.e., the combination of fundamental red and blue-violet of the spectrum, as explained before. The yellow ink must be a pure yellow, not inclined either to orange or green, i.e., about the shade of sulphur, or what artists' colourmen call "lemon yellow." The third ink, the blue, must be cyan-blue, somewhat similar to a greenish cobalt blue. Neither the violet nor the green should, however, preponderate in this blue. If these three inks are correctly made, it will, by their mixture, be possible to produce every colour, including tints, saddened hues, and dense blacks.

The tintorial mixture in a photochromic print will be two-fold, optical and pigmentary. Those acquainted with half-tone work know that the shades, tones, and half-tones in a picture are produced by dots of various sizes, the smaller producing the lighter parts of the picture, and the larger the shades and outlines. Now in a photochromic picture, the various colours are produced by the superposition of yellow, blue, and red dots of various sizes. Wl]ere these dots cover each other they produce a pigmentary mixture, almost identically as if the pigments had been mixed by a palette knife previous to being printed. Where these dots lie next to each other they produce an optical mixture, that is, the eye will record two adjoining dots simultaneously, for instance, red and blue appear as violet; blue and yellow as green; red, yellow, and blue, i.e., the three colours combined, as black (or grey if the dots are small and allow the paper to reflect white light through between the interstices).

These remarks now lead us to the second essential quality of the photochromic inks, viz.—transparency. Unless the pigments used are transparent, the pigmentary mixture just alluded to cannot take place. Wherever, for instance, an opaque red dot should cover a yellow one, instead of producing an orange or scarlet it would only show the colour last printed, but if the red is transparent it will combine with the yellow to form orange. It is not very difficult to find a red that answers not only to the required shade but possesses transparency; we find it in madder lake, struck on a transparent base such as hydrate of alumina. This pigment possesses another valuable property, that of absolute permanency when exposed to 46 sunlight. The blue pigment is more difficult to produce. The best is a cyanide blue, which can be made of the requisite shade, and is transparent. It cannot be called absolutely permanent, but the fading when printed full strength is so slight that it need not be taken into consideration. Artists do not hesitate to use this blue in the most valuable pictures. Ultramarine, as I have stated before, must be rejected on account of its opacity, and aniline blues are much too fugitive. The most serious difficulty presents itself in the selection of the yellow and only very recently after a great many experiments I have found a transparent yellow lake which promises well for permanency. It is of the requisite shade and perfectly transparent. Up to now this non-success of producing a permanent transparent yellow necessitated the use of an opaque pigment and printing the yellow first. If that is done it does not matter if an opaque yellow pigment is used so long as it is permanent and of the requisite shade. It is also advisable to print the blue last on account of its possessing the smallest luminosity. But for these two reasons, it would not matter in what order the colours are printed. So it is necessary to print them in the order of yellow, red, and blue. I need hardly mention that it is also of great importance that the pigments should be well proportionate as regards their colouring power. If that is not so, it will be found that the strongest pigment causes the picture to be coloured with a preponderance of that particular colour, which is generally the red. Placed in Lovibond's tintometer it will be found that the yellow and blue pigments are of about equal strength (about seventeen units each), whilst the red pigment, if madder, will measure probably thirty-four units, or about double the strength. It is, therefore, necessary the printing ink maker should proportion the strength of the pigments if correct colouring of the picture is to be expected. This is a matter which I find is almost always ignored.

What I have said about photochromic printing inks will be sufficient to prove that great attention to details is required in the manufacture of these inks. The selection of pigments, suitable not only as to shade but also as far as their permanency, transparency, and tintorial strength is concerned, must be a matter of great care and experience. They require far more care in grinding than ordinary inks, as from this cause variations in shade would cause serious differences in the colouring of the prints.

Although not falling under the heading of "Photochromic Printing Inks," I feel bound to make a few observations about the paper. If you take a pigment — madder lake, for instance — and rub it up either in plain water or gum water, and paint it on a sheet of hard, well-sized, and glazed paper, you will get a bright red, owing to the smooth surface of the paper reflecting some white light with the pigment, making the latter appear bright. Now take a sheet of white blotting paper and paint it with the same colour, the result will be a kind of dirty maroon or claret colour. The blotting paper absorbs some of the light which the glazed paper reflects. The lesson is obvious—use good paper only, hard, well-sized, and glazed, and in printing use hard packing, eight or ten sheets of cream wove paper. It will then not be difficult to print from blocks of very fine grain, and the colours will appear much cleaner and brighter. No pains should be spared, either, in the making ready of the blocks.

Thus with blocks made from scientifically constructed colourfilters, inks of correct hue, good paper, a knowledge of the principles of photochromic printing, combined with a little enthusiasm, which this new scientific way of colour-printing well deserves, we may expect to obtain as good results as are possible at the present state of photo-trichromatic printing.

View of one of the grinding sheds at Caroline Part Works, Edinburgh.

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