Georg Fritz

THE half-tone process is used in photo-lithography to reproduce originals which do not contain distinct lines or points, but closed tones, which are executed either in one or several colours. With this process any coloured picture, oil, water, pastel, indian-ink or sepia drawing, as well as any photographs from nature of living, moving scenes, or still life objects can be prepared for reproduction on the printing press.

I will first describe briefly the theory of the process, and then come to the practical part.

As regards the photographic part and the quality of the negatives, in the present state of this method of reproduction the same principles apply as in line reproduction; where they differ I will mention the same.

The duration of printing, the transfer of the image to the stone, as well as the other arrangements will be described in the different processes.

We divide this chapter again into two sections:

1. Methods in which the half-tone is broken up when making the negative.

2. Those methods in which the formation of the grain is effected after the negative has been made by some suitable means.

As every important technical discovery is formed not at once, but must be made by a longer or shorter series of smaller discoveries or improvements in order to attain the hoped-for goal, so was it also with autotypy.

From the work of Paul Pretsch, Mariot, Brown, and Fred. E. Ives the present state of the certain and beautifully-working half-tone process gradually evolved, and the last process, which ought to be considered as the immediate predecessor, is far outshone by autotypy as now practised.

Early attempts were made to obtain a printable image on stone by breaking up the chromated gelatine film, so that an irregular so-called serpentine grain was formed on it, which corresponded {68} fairly well with the theory of lithography, but gave no precise lines and no beautiful gradations of stone. If the grain was somewhat too coarse the effect of the picture was lost, the tone gradations were too far apart, and only light and shadows were given; if the grain was too fine the stone was not printable.

A further experiment was printing a design over the original, the strength of which had to be brought in correct proportion to the original; the impressing of a design was also tried, and when using this method the correct angle of the incident light when making the exposure had to be taken into account.

Further experiments, which were principally carried out by Mariot, Cronenberg, and others, were founded principally on the basis of breaking up the tones in printing. For this a lineature or screen on glass, or a gelatine film, was introduced between the negative and the sensitive film. Others, again, coated the blank glass plate with a lineature, and prepared the plate afterwards with collodion or gelatine emulsion for the exposure.

It will be thus seen how this method of reproduction developed step by step till net-work of silk muslin or woven horsehair, and finally grating images, strongly reduced by photography, were placed in front of the photographic plate.

Thus were efforts made to make half-tone pictures suitable for printing by litho-or typography, and although the end was very nearly attained, yet the crux was not quite solved. These methods did not, however, yield the desired result, namely, a beautiful sharp clear image. There was still required a considerable improvement of the existing methods, and this was made by Meisenbach, of Munich.

The principle by which Meisenbach prepared his images was essentially different from the previously-described results. He broke up the half-tones, also by means of a lineature, into a printable grain, but the process was essentially different from the previous methods, in that Meisenbach used a glass plate on which, on a black ground, a grating was drawn till the glass was laid bare, and in this way prepared a lineature which consisted of clear glass, transparent lines and absolutely opaque lines.

He produced, first, an ordinary negative, from this a positive, and from this, by the interposition of the lineature, the actual half-tone negative for making the printing plate. The lineature or screen was in the second exposure interposed before the sensitive plate, and half the time of exposure given; then the screen was turned till the line first obtained crossed the second at an angle of 90°, and then the exposure was completed. By this means an absolutely certain breaking up of the half-tones into mathematically exact points was attained, and the most important step made in making photography useful for preparing printing plates for the two principal methods, typographic and lithographic. Meisenbach has called his process “autotypy.” It was, indeed, {69} somewhat inconvenient, but had the advantage that on the negative and also on the positive any retouching that was necessary could be done.

I cannot here enter into the numerous simplifications and improvements of Meisenbach’s process which, as well in the photographic process as also in the preparation of the lineature, were in the course of time made partly by him and partly by Carl Angerer, Gillot, Bussod and Valadon, Lefmann, and others, since, on the one hand, it would carry us too far, and, on the other hand, they may be considered as a natural consequence of the discovery. I will only mention that it was found after a short time that the two exposures and the production of the necessary positive could be omitted, and that the lineature could be simply interposed before the sensitive plate in the first exposure, by which an important simplification of the work was attained, and, thanks to the continued perfecting of the processes, no detraction of the good results was thus produced.

I do not consider it superfluous to give a short explanation of the action of the interposed lineature on the photographic plate during exposure.

If we take, for the sake of simplicity, not a picture, but a scale with four or five-tone gradations from light to deep black, the light tones will act more or less on the sensitive photographic plate according to their degree of brightness, but as the rays of light have to pass through the cross-lined screen interposed between the lens and sensitive plate, and as the rays can only pass through the transparent parts and not through the opaque, no homogeneous surface is obtained on the negative, but a tone produced by the cross-lines of the screen. If the tone was very bright, the rays reflected with great intensity on the sensitive plate will completely decompose the silver film. Since they are prevented by the screen from acting with equal power on all places of the surface, these places will show very plainly on the negative, and the result is therefore a darker tone on the same, which is marked with bright fine lines, corresponding in thickness to the screen plate.

If the original tone was darker so many rays of light will not be reflected, and these have thus not the power to impress the screen on the sensitive plate in full intensity; the result is therefore on the negative a tone which does not contain such strong dark lines as the first; the tone appears lighter in the negative, and in the print from the same darker than the first.

In the reproduction of deep black surfaces finally no rays of light are reflected, therefore no rays of light can penetrate through the screen, and because actually here no chemical change of the light sensitive film takes place, we obtain a negative which is clear and transparent in these places, that is to say without any interruption, which gives in the print a full black tone. {70}

In this process there comes, however, to our aid a very important physical law, namely, the diffraction of light. A ray of light passing through a round hole or a slit and falling on to a black surface is represented not the same size as the slit, but more or less broadened the more the receiving surface is moved away, and in the middle the brightest light will be, and this gradually fades off into shadow towards the margin.

If we take now the opposite, and use instead of the hole or slit a black surface or a conglomerate of such in the form of points, squares, or other geometrical figures, and if these be placed before a screen illuminated with a bright light, these figures become somewhat smaller with this bright illumination, whilst with a weaker illumination they are represented in correct strength.

Where the light acts in its full intensity, or in excess, in a manner of speaking it is diffracted or bent over the figures, and proportionately makes them smaller.

This law does good service in autotypic work.

As we have seen, the rays of light must pass through the screen during exposure. This screen consists of equally thick opaque black and transparent white lines. Where the light acts with full intensity it will be more diffracted over the black lines. It makes the lines of the screen broader on the negative, and will, therefore, have more covered places, which in the after printing of the prints cannot be penetrated by the light, and thus represent the light places in the print covered with fine dots.

The darker the tone the less the rays of light reflected, and the less, therefore, can the light be diffracted. As the tones get darker the lines of the screen become of equal value on the negative, till finally, when at the opposite end of the screen, they totally disappear in the deep blacks.

A valuable quality of a correctly-prepared autotype is, therefore, that the covered lines or the covered grain appear of different strengths on the negative. In the high lights they are stronger, in the half-tones weaker, till finally in the black parts they have totally disappeared.

A further not less valuable property is that the grain in the highest lights is not sharply defined, does not appear as a square point, but is rounded by the diffraction of light, which gives a softer, more beautiful appearance and plasticity.

Whilst the light reflected from the brightest parts of the drawing acts with full intensity, and is diffracted over the figures standing in its path and makes the same smaller, and cuts off the sharp corners and gives more covered surfaces to the negative, as it decreases according to the more or less deep shadows of the drawing, and can only act now with less intensity through the open places of the screen, the result is that the points become larger, till finally in the deepest parts it is quite inactive, and the shadows begin to block. The action of the light rays of different {71} strengths reflected on to the negative in accordance with the brighter and darker tones of the drawing, taking into consideration the print, can be graphically represented as a pyramid of which the base represents the deepest shadows and the point the high lights. If we interpose a cross-lined screen, or more correctly called a grain plate, before the sensitive plate, we obtain, graphically considered, grain figures, actually of many more very different sizes, as shown in Fig. 7, 1–6, in which the smallest points represent the high lights of the drawing, and those becoming gradually bigger the gradually increasing shadows.

The original grain plate contained, measured diagonally, five black points, five white interstices to the millimetre. The figures represent enlargements of a negative made with the above-named grain plate, and for this the grain plate was one millimetre distant from the sensitive plate.

The accompanying Figure 8, 1–6, represents very much enlarged the scheme of grain of an autotype from a perfectly-executed wash drawing, with a screen of five opaque and five transparent lines to the millimetre, in equal proportions of black and white, at a distance of one millimetre from the sensitive plate, with equal time of exposure before and after the turning of the screen, and obviously with correct exposure for the whole.

The scheme is only given here in six gradations of tone. As a matter of fact, a good autotype has at least from six to eight times the number of tones, as with a sharp screen and an original cleanly {72} and sharply drawn in many tones, the light reflects the least alteration of tone in the size of the grain.

As previously mentioned, the smallest points represent the highest lights of the drawing; the larger, according to our graphic representation, the darker tones, but a correct reproduction of the grain.

In a similar way obviously the formation of the lines on the negative behave with an interposed but not revolved screen. Here also the lines in the high lights of the negative appear more covered than in the shadows.

The proportion of grain can, if the original requires it, be so far altered by not giving equal exposures for each position of the screen—for instance, in the proportion of two to one or three to one. We obtain then on the given scheme less closed tone gradations, 3, 4, and 5, and just before 3, and between and after 5, tone gradations, no single black and white points, but more or less jagged black or white lines, whilst the open black grain in 1 and 2 becomes more an oval instead of the round form in 6, and beyond the round openings are drawn out into ovals.

We have, however, in autotypy still the physical law, which is very useful to us, and that is the greater or less distance of the point from the light surface. The nearer a point or a figure is brought to the wall on which it should throw the shadow, the greater the size of the original, and so much sharper will be the outlines of the shadows. The more the point or the figure is {73} removed from the wall the less the shadow will correspond to the shape of the original; it will be smaller and less defined. The shape will also frequently change, and a square will become a round. We have then in the near or distant position of the lineature to the sensitive plate, on which it throws its shadow, an important assistance to act on the general tone gradations of the image. Let us remember that in focussing the shadow-forming object on to the surface on which the light falls, we obtain the same formed in all its actuality on the receiving surface; therefore, when this refers to a lineature with equal proportions of black and white, we obtain these in equal ratio. When, however, we remove the lineature from the light-receiving surface, the shadows which it throws contract, the lights predominate, we obtain no longer the ratio of 1:1 between light and shadow, but about 1·5:1 and so on.

By placing the lineature close to the sensitive plate, we obtain more closed tones, and as this gradation of tone is carried naturally throughout the whole of the image, also less high lights, less deep, middle shadows, whilst the deep shadows appear in their natural strength. With the removal of the lineature from the sensitive plate, on the other hand, the ratio of tones for the whole image, as well as for the details, is displaced. In the first place we shall obtain a picture which contains more contrasts of black and white. The light tones become considerably lighter by the reduction of the shadows. The dark parts, on the other hand, attain full vigour. In the first place, therefore, the original has to be considered as regards the distance of the lineature from the sensitive plate. Obviously, therefore, no accurate directions can be given in a case where feeling and experience are required. Next to the original, the number of the lines on the screen has to be considered. As a rule, one uses lineatures with five or six transparent and opaque lines to the millimetre (=125–150 to the inch), and for these a distance of ½ to 1 mm. (=1?/?50–1?/?25th inch) from the sensitive plate is generally sufficient for most cases. With a less number of lines the distance can be increased.

Autotypy is a process in which unprintable half-tones can be broken up by the use of a well-defined physical law into a mathematically determined grain, and the size of the grain can also be regulated to a certain extent with certain precautions.

It is, therefore, easily understood that the autotypic process is now very general and most used, and not only in the limited department of photo-lithography, but in a more extended way, does excellent service for the preparation of illustrations of all kinds, for the simplest picture for the daily paper as for the finely-printed magazines and journals, for simple monochromatic printing, and for the highest colour printing.

With accurate knowledge and command of the technique of {74} photography and lithography, there is required before everything, for successful work, a faultless screen plate. The indispensable requirements which must be found in the screen are absolutely opaque black lines and bare glass in the white lines. The home preparation of such a screen plate is a somewhat difficult matter, and will not be absolutely successful if one has not a faultless ruling machine, a composition which is thoroughly opaque, which adheres well to the glass and gives clean, sharp lines in ruling, and which does not chip, and it will require a tremendous expenditure of patience and perseverance. Glass screens of excellent quality may now be obtained commercially.

For reproduction of the very highest quality, a drawn and etched screen plate is more suitable than one merely ruled on pigment, which never gives such sharp results.

From various firms etched screen plates may now be obtained either as single or crossed-line screens, which fulfil all requirements.

The best screen to use is one with five or six opaque and transparent lines to the millimetre, which corresponds with 2,500 to 3,600 points to the square centimetre. The above number of lines will suffice for nearly all work, and such screens give reproductions in which the grain is no longer visible to the naked eye, but appears as a closed tone. It is not advisable to go beyond this number of lines, except in special cases, when the details of the drawing in the original are specially small. With a smaller number of about three or four lines to the millimetre, too coarse a grain results, of which the individual points become too plainly visible, and these are distracting and act roughly, and do not accurately reproduce the gradation of tone.

The best proportion between opacity and transparency is 1:1. From this is obtained, according to my opinion, the most beautiful and restful tones. It must also be remembered that the opaque spots actually lose considerably in strength in the brighter portions of the picture by the spreading action of the light.

The printing may be done on bichromated gelatine paper, with which, however, extremely accurate inking up of the print is essential, and the prints must not have too much ink and must be very carefully printed. If, however, very good, clear negatives are used, either of the direct printing methods, as already described in Chapter IV., is to be preferred. (See No. 4 Supplement.)

Of the many existing processes by which the breaking-up of the half-tones into printable lines or grain is effected, not in the negative, but subsequently by mechanical and chemical action on the stone or plate or the transfer film, I will only mention those {75} which appear to me to be the most important, the new methods which have already found firm footing in practical work or which appear likely to do so. All the older processes, which were either never actually successful or have been superseded on account of difficulty of working, we may properly pass over.