CHAPTER V.

Chapter 612,301 wordsPublic domain

THE HALF-TONE PROCESS FOR PHOTO-LITHOGRAPHY.

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.

1. BREAKING UP THE HALF-TONE ON THE NEGATIVE (AUTOTYPY).

Of the various methods by which a breaking-up of the half-tones can be obtained, the so-called “autotypy” is the one most used practically. With this process the half-tones are broken up in the negative.

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.)

2. PHOTO-LITHOGRAPHIC PROCESSES IN WHICH THE GRAIN IS NOT FORMED DURING THE EXPOSURE.

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.

(A.) THE PATENT ASPHALT PROCESS OF ORELL, FÜSSLI, AND CO. USED FOR THE PREPARATION OF COLOUR PRINTS.

I have already mentioned that the use of light sensitive asphalt for the preparation of printing plates for the various branches is by no means new.

The fundamental principles of the well-known processes we find also used in the process of Orell, Füssli, and Co.

The grained stone is coated with a thin film of light-sensitive asphalt, the composition of which we have previously learnt, and exposed under a reversed or stripped negative. The image after correct exposure is developed, and then, according to the construction of the asphalt, in combination with the graining of the stone and the developer, there is formed a finer or coarser grain, or a closed or open image.

The development is effected with petroleum, to which one-fifth or one-sixth of its volume of benzine is added, or with turpentine alone. Development takes place rather rapidly, and after a short time the image appears on the stone. The print is then treated as was suggested in asphalt printing; the stone is first gummed, and then by rolling up with ink and etching rendered suitable for printing. The pictures produced by this have a very fine grain and are in appearance very much like good collotypes; still two or more tones are necessary to obtain an absolutely closed and vigorous action.

Very similar to Orel and Füssli’s process is the following, also patented.

(B.) THE COLOUR PRINTING PROCESS OF WEZEL AND NAUMANN OF REUDNITZ, LEIPZIG.

In this process also all the colour stones of a chromo picture are produced from a single stripped and reversed negative which is taken of the original. For the preparation of any colour stone there are three distinct stages. The first stage is the exposure, the second the development, and the third the development continued still further, and any necessary small help by the lithographer. In each stage a special mode of treatment is required, according to whether the stone in question is to be used for a light, a more vigorous, or full colour. The stones for grey and the light blue or light red inks—thus those for many surfaces, but with few details—are treated differently to those for yellow, the second red {76} or blue; and these, again, differently to those for brown or quite dark colours.

Everyone only slightly conversant with colour printing knows that the first plates are used for the general tones of the picture, and contain, therefore, few details, and with the exception of the highest lights are tolerably full; the latter plates, on the other hand, which are devoted to the darker colours, contain less toned surfaces, but, on the contrary, more details of the drawings. In order to attain this correctly the three stages of the production of the plates are carried out with this end in view.

It should still be mentioned that the more or less fine graining of the stone determines the general ratio of the grain of the picture.

The first stage is the exposure of grained stone coated with asphalt. With longer or shorter exposure it is possible to make the asphalt film more or less soluble, that is to say, to make the same more or less capable of resisting ethereal oils.

It is therefore possible at this stage, by long exposure of the asphalt film, to give to the colour stone for bright colours more tone and less details. The stones for the darker inks are printed for a shorter time, and the asphalt film is less capable of withstanding the solvents, and the resulting plate contains less tones and more details. Finally, the plates for the deep colours are normally printed; the asphalt film is still less resistant, and consequently the stone will only have tone and details on those places which in printing ought to be the darkest.

It is possible, therefore, by correct manipulation of this operation, to obtain approximately the correct colour plate.

There is, however, in the second process a corrector for the first. The development may be so performed that by the longer or shorter use of strong solvents, Hungarian or Russian turpentine, more or less of the asphalt will be dissolved. Thus if a slight error is made in printing it can be made good in developing. As a rule, the stones for bright colours, which consist of many tones, are not developed so long as those for the stronger colours. The actual stones for the deep colours are very strongly developed, so that almost all the delicate parts of the picture disappear, and only the dense strong parts remain on the stone.

Still the fine details which are to be found in the vigorous parts, and which cannot be recognized, must be brought out. The fine parts of the drawing must be quite clear if the details are to be recognized, or the picture will be a fuzzy copy of the original, wanting in structure.

This fault is remedied in the third stage of the treatment, in which a strong development of the picture is effected with strong acting oils or benzole and turpentine. As this operation, however, would also wash away such delicate places which ought to remain on the colour stone, they must be covered with solution of gum and allowed to dry. Then those parts not covered with gum, {77} in which the details of the image cannot be seen, are painted for a longer or shorter time, as may be required, with a brush dipped in the developing solution, and when correctly developed may be covered up. The lighter places are treated for a shorter time, the very dark places for a longer time.

By this treatment the particular parts of the picture become lighter and lighter, the details appear more and more; and, finally, the whole picture shows as it should do. Then the gum is washed off and the stone subjected to the already described treatment, so as to make it suitable for printing in quantities. Finally, the lithographer can make corrections with a needle or tusch.

This process may be used, therefore, to prepare colour-printing plates from a negative obtained in the camera from an oil painting or any coloured original.

The negative for this process must contain all the details very clear, must be soft, and not be too dense.

The different colour plates are thus in this process obtained by long or short printing, by strong or weak developing, and finally the details in the dark plates are obtained by separate development and covering up the other places with gum.

The breaking up of the half-tones into printable grain is effected by graining the stone, before coating with the light sensitive asphalt film. On the finer or coarser graining of the stone depends also the structure of the grain of the image.

This process yields the most beautiful results with correct manipulation, and in the hands of an expert photographer and lithographer is a valuable aid to the preparation of colour plates. The process is patented.

(C.) J. BARTÖS’ PROCESS FOR PREPARING PHOTO-LITHOGRAPHS AND PHOTOTYPES IN GRAIN AND HALF-TONE.

Bartös’ process belongs to those in which the breaking up of the half-tones into a printable grain is effected subsequently on the plate or stone by mechanical means. This is attained in a very original manner by means of a sandblast, which makes the half-tone printable. We have here not the breaking up of the half-tones in a mathematically regular manner, dependent on optical laws, as is the case with autotypy, but a method similar to that of Pretsch, which obtains the printable grain by reticulation of the chromated gelatine film, or other discoverers who attain the same result by asphalt dusting or some other means.

For this process a well-polished litho stone or zinc plate is coated with a thin film of varnish, composed of—

Chloroform 300 g. Mastic 5 g. Asphalt 10 g. Benzole 300 g. Linseed oil 2 g.

{78}

On the stone or plate thus prepared is transferred a carbon image in half-tone.

This pigment film, when completely dry, is flowed over with a mixture of—

Glycerine 35 g. Water 25 g. Alum 2 g.

and this should be allowed to act for about five minutes and then removed with blotting-paper. After this operation the pigment image can be easily destroyed. The pigment image is now exposed to the action of a weak sandblast. The sand acts on the relief image and gradually destroys it, and so that the film of varnish underneath is gradually grained.

After the pigment film has been removed the picture shows in all its details in grain on the varnish of the stone or plate. The image is now visible on the film of varnish, according to the strength of the tones in partial graining, and can be etched into the stone or plate.

This is effected by etching with phosphoric acid—

Water 100 g. Phosphoric acid 4 g.

When the etching solution has acted for about two minutes, the stone is washed and coated with a solution of gum—

Water 100 g. Gum arabic 5 g.

and allowed to dry.

Then the film of varnish is removed with a pad of cotton wool damped with turpentine, the dry stone rolled up with litho ink, damped, and the excess of ink rolled off. The picture now shows with all its finest details on the stone, and can be printed.

In preparing a relief printing plate the film of varnish forms the resist for the first etching. It is then rolled up and etched in the usual way.

For preparing larger pictures on stone or metal plates, in which the grain must be coarser than with small pictures, the fully-developed and still damp pigment image is dusted with powdered resin and the film allowed to dry with the powder adhering to it. The pigment image thus dusted is treated, as previously described, with alum and glycerine, exposed to the action of the sandblast, and then etched with phosphoric or nitric acid.

As the soft pigment film receives a grain by the dusting, this is reproduced of the same size by the action of the sandblast on the film of varnish on the stone or plate.

The principal points of this process of preparing photo-lithographs or phototypes in half-tone, with a grain, are as follows:—

1. From the photographic negative a pigment image is taken {79} and transferred to a litho stone or zinc plate, which has been previously coated with varnish.

2. The pigment image is then exposed to the action of the sandblast; and

3. Then the pigment image is removed, and the image obtained on the film of varnish is etched.

In the K.K. Hof und Staatsdruckerei photo-lithographs as well as phototypes have been prepared with excellent results by this process. The former in result are similar to the productions of the Orell and Füssli process, and the latter were noticeable for good gradation. On the other hand, it cannot be denied that this process, which is patented, has been already surpassed by autotypy, which is far simpler and easier to work, and is founded on a scientific basis.

(D.) LITHO-HELIOGRAVURE.

This process, invented by Chas. Eckstein, of Hague, the general director of the Topographical Bureau of the Royal Netherland General Staff, also belongs to those processes of photo-lithography by means of which any picture in half-tone can be reproduced. The breaking up of the half-tones is effected, as with the autotypic methods, by the use of a lineature, but instead of this being done in the photographic exposure, it is effected direct on the stone before the transfer of the photographic image. In this particular process the stone itself is not coated with a light sensitive film and printed on direct, but the ruling is made on the stone, and then a negative pigment image—a diapositive—is transferred to it exactly the same as with photogravure on copper. A further feature is that the result is not an ordinary transfer stone result, but an intaglio etched stone, which is printed from the depressions.

For this original process the basis is a ruled pattern stone, prepared with great care and accuracy. For preparing this a grey stone of the best quality should be taken, and one without any chalky spots, crystals, or holes. This should be polished with a coarse cloth and oxalic acid solution till it has a very high polish. On the highly-polished stone surface is spread a very thin but even film of asphalt.

The solution is composed of—

Asphalt 5 parts. White wax 6 parts. Stearic acid 6 parts.

This mixture is boiled, and to it whilst boiling is added in drops a solution of two parts of soda. When cold this mixture, which is fairly hard, is dissolved in about an equal volume of turpentine, filtered and kept in bottles for use.

A sufficient quantity of this asphalt solution, which must not be sensitive to light, is poured into the middle of a levelled stone and {80} distributed by means of a lithographic ink roller, which must not be used for any other purpose but this, till the coating is absolutely even and shows a light brown colour.

When this film has become quite hard, fine parallel lines are drawn over the whole surface of the stone with a very accurate working ruling machine, by means of which from five to ten lines per millimetre can be ruled.

When the ruling is finished the stone is given a margin of wax, and quickly and evenly flooded with the following etching fluid—

Pure nitric acid 0·16 parts. Alcohol 36 per cent. 0·60 parts. Water 35 parts.

And, watch in hand, is etched for about half-a-minute.

Instead of the above a solution of—

Glacial acetic acid 2 g. Water 100 g.

may be used and allowed to act for half-a-minute. The stone is then well washed under a strong stream of water, allowed to dry, and covered with linseed oil. After about five minutes the whole of the stone may be washed with turpentine. Then it should be inked up with a pad, care being taken that all lines take the ink well and evenly. This pattern stone serves as the original plate for the subsequent ruled transfer.

From the original ruled stone can be made, according to the nature of the original, a single, double, triple, or four-fold transfer.

For preparing a stone with a ruled transfer, a smoothly ground stone well polished with oxalic acid is evenly coated with sensitive asphalt solution. On this, when dry, the desired single or crossed transfer from the original stone is made, and dusted with bronze powder. This adheres to the lines of the pattern, and in combination with the printing ink protects the asphalt film underneath from the action of light. During the subsequent exposure, which, according to the sensitiveness of the asphalt and intensity of the light, can last from a half to two hours or more, the particles of asphalt not covered by the bronze powder become insoluble, whilst the protected parts preserve their solubility.

After correct exposure the surface of the stone is carefully washed with a tuft of cotton wool soaked in oil of turpentine, when the particles of asphalt underlying the bronzed lines dissolve. The stone at these parts is laid quite bare, and therefore made capable of being subsequently etched. The deep etching of the pattern is effected in the same way as was suggested in the preparation of the mother pattern stone.

By this method a positive pattern is obtained, that is to say, the lines print, and the impression shows therefore a black network.

A second method of making a pattern transfer consists in making a pull from the original stone on to chalk transfer paper. {81} This pull is then damped on the back, and when just properly damp is laid on the polished stone (without any asphalt) and the transfer made. In order to remove the chalk paper without damaging the pattern the stone is covered with warm water, when the lines will dissolve. The simple pattern will now be found on the stone, which should now be well washed, then dusted with resin powder, which should be melted on with ether vapour. Then if the direction of the first lines was vertical, the second transfer is arranged with the direction of the lines horizontal to the first, and there is thus obtained a simple crossed pattern. With four-fold crossed patterns the subsequent pulls are made in opposite diagonal directions. After each transfer the stone is well washed, allowed to dry, then dusted with resin powder, and with ether melted on to the pattern.

The stone is now deep etched with dilute nitric or acetic acid and covered with linseed oil.

By this method a grain instead of lines is obtained. By polishing the stone and transferring on to it, as well as by etching the places not covered by the ink, we have the pattern reversed, that is to say, it is converted into grain.

For further operations of litho-heliogravure a very clear and soft negative is required, from which a glass transparency is taken. Under this transparency pigment paper sensitized on a 20 per cent. solution of potassium bichromate is printed to 8 or 10 degrees Vogel. Printing must be very carefully performed, as with under-printing too many details are lost. The exposed pigmented paper is washed in cold water, laid film side down on the previously prepared stone, and pressed into contact with an india-rubber squeegee. It should then be allowed to dry in a not too dry or too warm place.

The stone should now be placed in a trough with warm water of about 40 to 45° C., and care should be taken to obtain water of as even a temperature as possible. After about five minutes the paper will be free from the stone, which should remain in the bath and be developed so long by shaking in order that the water may flow evenly over it till the image appears quite distinct; a negative image is thus obtained on the stone. The film is then allowed to dry spontaneously for four or five hours till the pigment image has become quite hard.

For etching a solution of perchloride of iron of various strengths is used, 40, 37, 33, and 30° Beaumé. With subjects very rich in tones all four will be used; with simpler subjects one or the other may be omitted.

The etching will begin, as a rule, with the 40° solution. The ferric chloride will first dissolve the thin, hardened pigment film and then go through the open lines of the pattern, where it will attack the stone and etch the deepest parts.

Slowly the ferric chloride dissolves also the thicker pigment {82} film—begins also to etch the stone there, but less deeply. To tell when the four solutions should be used in turn, as well as the correct moment to interrupt the etching, requires considerable experience. The 30° solution, the weakest of the ferric chloride solutions, is used last, and in consequence of the greater proportion of water possesses a solvent power for the whole of the light-hardened pigment film. Accurate directions as to how long the etching with the different solutions is to be continued cannot be given, as this depends upon the character of the subject as well as upon the thickness of the pigment image. It may be always assumed, however, that any individual etching ought to last under no circumstances longer than three to four minutes. With well-covered stones the first etching should not be allowed to act so long as the last, and _vice-versâ_ with less covered stones.

As soon as the image shows the correct character through the etching solution the stone should be brought under the tap in the trough and quickly washed with clean cold water. The surface of the image should then be flowed over with oil of turpentine in order to remove the asphalt and gelatine film, and finally well washed with the rose and allowed to dry. The picture will be found deeply etched into the stone. The stone is now smeared over with linseed oil, inked up, and further treated as in ordinary lithogravure.

In this process the negative pigment image is transferred to the stone and the printing image prepared by etching; the formation of the grain is produced by the pattern printed either on a polished stone, when it is converted into points, or the transfer may be effected on to an asphalted polished stone, when a network of lines will be obtained.

For dark subjects the latter process is better, for lighter subjects the grain.

Printing from such a plate has more similarities with etching printing than from stone; the effect of the ink is very good and expressive.

Various pictures executed by Eckstein and in our establishment in one or more colours have proved the value of this process in practice.

The foregoing are the different photo-lithographic methods for obtaining printing images in half-tone, and although in no way exhaustive, yet it appears to me that they are practically the most important.

{83}

APPENDIX.

1. NEGATIVE TRANSFER WITH BICHROMATED GELATINE PAPER.

The properties of bichromated gelatine paper can also be very well used for the so-called negative printing, and this method offers many advantages.

In order to convert a positive printing image into a negative various methods may be used; the most convenient for the lithographer is the following:—A good litho stone is first ground in the ordinary way and then the surface rubbed with a pad and oxalic acid solution till it has a high glaze or polish. On to this stone is then transferred a sharp impression in a non-greasy ink from the positive printing image. According to the destination of the stone for large or small editions one of two processes may be adopted. If a thousand or more pulls are required from the stone the transfer may be dusted with resin powder, and this melted with the burning, heating, or ether process and the drawing etched in relief with 8 to 12° nitric acid and gum. Then the stone may be well washed with water and the acid removed with one to two per cent. acetic acid.

For small runs the first etching is omitted, and the stone treated with acetic acid. In both cases the stone should be well washed, dried, and then coated with dissolved lithographic tusch or autographic ink. The greasy substances of these materials penetrate into the surfaces of the stone that are laid bare, and firmly adhere. When the tusch or ink is completely dry the stone is washed with turpentine, inked up, and one now has a negative image from the subject in question, which shows all the original printing places in white and all white places black.

The process of preparing such negative prints by the use of bichromated gelatine papers is much simpler.

A sheet of gelatinized paper is sensitized for this purpose on a 5 per cent. bath of potassium bichromate, squeegeed to a sheet of plate-glass and well dried, and on this is made with black ink, which is very opaque, in the dark or in gaslight, a sharp clean print. This print is then laid on a flat cable, under a sheet of plate-glass, so that it cannot roll up, and exposed to the light for from five to thirty minutes. A long exposure rarely does harm in such cases, as the good opaque black ink allows no rays of light to pass through to those places which later should print white.

The next manipulation is the development and inking up, which is done as in ordinary photo-lithography. In this operation all {84} those places which were not covered up by the printing will take the developing ink; on the other hand, those places where the impression was will wash out clean and white. The print has assumed the appearance of a photographic positive. The print is now treated like any other photo-lithographic print, and finally transferred to a stone or zinc plate. This method is distinguished by its simplicity and safety; it has also the advantage that any desired printing subject can be directly transferred in negative form to a zinc plate for relief etching, which otherwise could only be done by preliminary transfer to stone and subsequent transfer to zinc.

2. PHOTO-AUTOGRAPHY.

DRAWING WITH AUTOGRAPHIC INK OR LITHOGRAPHIC CHALK ON A CYANOTYPE, SALTED OR OTHER PRINT FROM A PHOTOGRAPHIC NEGATIVE OR POSITIVE.

By autography we understand that method of reproduction by which any writing or drawing is made on a prepared or not prepared paper with greasy ink, tusch or chalk, which is subsequently transferred to stone or zinc for printing, or to zinc plates for relief etching.

Autographic drawing papers differ for the different purposes, and for writing and simple line drawings smooth hard post paper is used, and lined transfer paper for fine pen or chalk drawings. The latter are prepared commercially by Angerer and Göschl. Very beautiful grained drawings can also be made for transfer on pyramidal grain paper. The proof for such drawings is usually done with lead pencil or red chalk, or it may also be made on a gelatine proof.

In order to obtain fully and completely the character and correct reproduction of the drawing, without special expense of artistic help, it is advisable to use a photographic print as proof. It should be noted that for pen drawing on smooth paper with greasy tusch or ink, for chalk drawings on a prepared grained paper done as previously mentioned, with greasy lithographic chalk, the drawing should be transferred direct to zinc or stone in order to save having to make subsequently a photographic print.

If a pencil, red crayon, or other proof is used for an autographic drawing on paper with the idea of transferring it direct to stone or zinc, a suitable photographic print may also be used on suitable paper without any disadvantages.

For the prints all photographic papers without glaze, such as ordinary plain salted paper, Eastman’s positive bromide paper, and others, may generally be used. Glossy or albumenized papers are not suitable for this, as the ink runs, and does not form solid clean lines. {85}

The best of all, however, is cyanotype paper, as even with very deep printing it keeps the details in the shadows open, and on account of its blue tones is more suitable for the subsequent drawing with black tusch. When, however, prussiate paper is not handy, and subjects with fewer shadows and less details are to be treated, any of the other papers above-mentioned may be used.

If ferro-prussiate paper is to be used, the same may be prepared as recommended on p. 23.

The so-called salted or plain paper is prepared as follows:—A sheet of well-sized post paper is first bathed in a solution of—

Ordinary salt 1 part Water 25 parts

and then sensitized in a solution of silver nitrate 1 : 12. It is printed till the print has assumed a fairly full brown tone, and all the details in the shadows are well printed out. It should then be toned in the following toning bath:—

Distilled water 1000 g. Sodium acetate (twice fused) 15 g. Solution of gold chloride (1 : 100) 25 g.

till the print has assumed a deep dark brown tone. It should then be fixed in a solution of sodium hyposulphite, 1 : 15. It should be printed rather deep, as in toning and fixing the image loses in depth. It is moreover unnecessary in this case to make a beautiful print; the chief thing is only that all details of the image should be clearly seen. The print is then well washed in frequent changes of water and then dried.

The print on Eastman’s bromide paper is prepared as follows:—

Red light must be used. It should be printed by an ordinary gas flame at a distance of about 50–80 cm. (= 24 to 36 ins.) for 1 to 20 seconds, according to the density of the negative. Here again a technically perfect print need not be arrived at, but one which contains all the details very clear. Under a good transparent negative a useful image for the artist will be obtained with about two seconds’ exposure.

The developer is composed of two solutions:—

(A.) Water 500 g. Neutral oxalate of potash 165 g. (B.) Water 50 cm. Sulphate of iron 15 cm. Concentrated sulphuric acid 2 drops Or glacial acetic acid 5 drops

Three parts of solution A and one part of solution B should be {86} mixed together. As soon as the shadows have attained the desired tone, the prints should be placed in several fresh baths of

Water 200 ccm. Acetic acid 1 drachm. Sat. sol. alum 50 g.

and then washed and fixed in a bath of

Water 50 ccm. Sodium hyposulphite 10 g.

for about ten minutes. Then well washed and dried.

For pen drawings all not gelatinized or albumenized printing papers may be used as already mentioned. The chief thing is that the paper should be well-sized and of good firm texture. If prepared grain paper is used, which is provided with a film of chalk, kaolin, or any other white colouring matter combined with gelatine, and on which film the grain is impressed, only prussiate prints can be used. The paper is sensitized as previously described, only it is advisable not to leave it too long in the solutions, or else the film may become softened. The film will also dissolve in hot water at about 80° C.

The prints as soon as dry can be used for drawing on. On the grain papers the outline may be drawn with the pen and be shaded with chalk, or it may be drawn with chalk only. With unprepared papers it is advisable to coat them with a thin solution of boiled starch, as then the transfer to zinc or stone is most exact. The drawing may also be left a long time without spoiling. The coating is performed with a pad dipped into the liquid, which consists of one part of fine bookbinder’s starch and four or five parts of water, and passed several times over the print.

The drawings ought only to be done with greasy lithographic tusch or chalk or autographic ink, which consists principally of greasy materials and lampblack. For pen drawings hard pens should be used, and care must be taken to make thick strokes. Only a little tusch or ink ought to be taken into the pen, so that it does not run out, and thus clean, sharp lines be obtained. If this be observed very close shadings and cross lines can be produced clean and neat.

When the drawing is finished and the tusch or ink has become dry the same should be laid between damp blotting-paper, and meanwhile a clean ground, and dry pumiced stone worked in the press and the pressure correctly adjusted. As soon as the drawing has become moderately damp right through, and the paper feels soft without being wet, it should be laid with the image downwards on the stone and transferred to the same with strong pressure. After repeated working the paper will adhere firmly to the stone. In order to loosen it the stone should be covered with hot water of about 80° C., when the film will dissolve and the paper become quite free. Any exertion of force must in this {87} operation be quite avoided, and the paper must not be pulled off with violence. All, even the finest lines, will have been transferred to the stone. The stone is now gummed, if possible allowed to stand for some hours, and then etched with a gum etching solution of two degrees’ acid strength, or later etched in relief, and is ready for printing. If the transfer is made to zinc, for printing from this the plate is treated as suggested on p. 6. If, however, a relief etching for the typographic press is to be prepared of the subject, the transfer is made on to a smooth polished zinc plate.

3. LITHOGRAVURE.

With this particular process an intaglio printing stone or intaglio printing plate is prepared by chemico-physical means.

This process was discovered and brought to great perfection by Chas. Eckstein, the general director of the Typographical Bureau at the Hague, and offers especially many advantages for the reproduction of maps.

A map engraved, drawn, or transferred on stone, or written in on copper, in originally one colour, can by this process in a comparatively easy way be converted into a many-coloured print; this process can also be used very advantageously for changing the names for the places—rivers, mountains, etc. In the first place intaglio original printing plates in all the colours can be prepared which can then be printed for further reproduction; in the second case it is necessary to re-engrave the network, hydrography, etc.

When it is desired to convert a monochrome image into a multi-coloured map, as many stones as there are colours to be used must be first ground and polished with oxalic acid till they have a high polish.

In the meantime an asphalt or other light-sensitive solution should be prepared, with which the stone is coated in the dark very evenly, and somewhat more than is done for a photo-lithographic print.

The asphalt solution is composed of—

Syrian asphalt 20 g. Chloroform 300 g. Benzole 100 g. Oil of lavender 20 drops.

After it has been dried in the dark the impression from the black plate, which should be previously made on transfer paper, is transferred to the asphalt film as previously laid down, and before it is dry dusted with bronze powder. All lines, titles, or figures which should not appear on that particular colour-plate must be then covered with the above asphalt solution, and only those places, for example, on the one plate all titles, on another all the roads, on a third the river courses, and so on, should be left, all of which later should appear in printing. {88}

The stone is now, according to the sensitiveness of the film and the strength of the light, exposed for from fifteen minutes to two hours. Over-printing, if it is not very exorbitant, does not do any harm with the stronger letters or lines on account of the good covering. With very fine lines, however, more care must be taken. If there is any doubt as to the length of the exposure a test may be made at one corner of the plate with a pad dipped in rectified turpentine. When the asphalt will no longer dissolve the exposure may be discontinued. With some experience the photometer may also be used.

Then the whole surface should be washed with rectified turpentine and a pad of cotton wool, when the ink lying under the bronze, that is, the lines where the asphalt has been protected from the action of light dissolve, and the surface of the stone appears. The rest of the asphalt film has become insoluble from the action of light, and now forms the etching-ground for the reversed negative image on the surface of the stone.

When the stone has been well washed with water it may be etched, which is done with glacial acetic acid two parts, water 100. When there are any fine lines in the drawing the etching is interrupted at the end of two minutes, and they should then be covered with tusch, and then when the tusch is dry the other parts are further etched for a longer or shorter time according to the degree of fineness. In any case the time of etching should not exceed five minutes. The stone is allowed to dry and then coated with a solution of tusch or linseed oil, which penetrates into the etched parts and makes them capable of taking ink. After about ten minutes the whole stone should be washed with turpentine or benzole, by which the etching ground is removed, when the stone is inked up like an ordinary gravure.

In this way is obtained a stone with deep etched drawing, which according to the blocking contains the titles, the rivers or roadways, and from which subsequently the necessary gravures can be taken. This stone can either be used for direct printing or it may be used as an original stone from which the necessary transfer can be made.

The same process may also be made for intaglio printing plates on zinc or copper, only then the etching must be done with nitric acid or ferric chloride.

Instead of the transfer on the light sensitive film, a drawing on transparent paper may be used, or a photographic transparency. The exposure in this case, as the opaque ink and bronze powder are not used, must be very carefully estimated.

The advantage of this process is that an intaglio printing surface may be obtained so that engraving is entirely or partially avoided, and that this printing surface may be used as an original from which the reproductions are made.

{89}

SCRAPER BOARDS, MANUFACTURED BY AUGERER & GÖSCHL, VIENNA.

No. 6.—A white scraper board printed with black lines. There are 40 black and 40 white lines to the centimetre; the ratio of black to white is as 1 : 2. At right angles to the black lines impressed lines are arranged, of which 37 are depressed and 37 raised up to the square centimetre.

No. 7.—This is practically the same as No. 6, only that the lines are slightly less in number, namely, 35 black and 35 white and 26 impressed lines in equal areas to No. 6.

No. 8.—This contains dots instead of lines, both black and impressed; there are 1,156 black dots at regular intervals, and 961 impressed dots to the square centimetre.

No. 9 is a smooth scraper board.

Nos. 10 and 11.—These are impressed with straight lines in the proportion of 1 : 1. With No. 10 there are 27 lines, in No. 11, 37 lines to the centimetre.

Nos. 12 and 13.—These are impressed with lines at right angles to one another, so that regular squares are formed. No. 11 contains 730, No. 12, 1,370 squares in every square centimetre.

INDEX.

Acid, Acetic, 10

― Citric, 10

― Gallic, 10

― Hydrochloric, 9

― Nitric, 9

― Oxalic, 10

― Phosphoric, 10

― Sulphuric, 9

Action of Lineature, 69

Albert’s Photo-litho Paper, 56

Albumen Process, 64

Angerer & Göschl, 25

Asphalt, 17

― Process, 60

― ― Husnik’s, 60

― ― Valenta’s, 61

Autographic Ink, 7

Autotypy, 67

― Negative, 41

Bartös’ Process, 20, 77

Behaviour of Asphalt on Stone, 8

Cadmium Intensification, 40

Chromate Salts, 48

Coating the Stone, 62

Cold Melting Process, 12

Correcting Negatives, 35

Cyanotype Paper, 24

― ― Bleaching, 24

Dark Room, The, 34

Deep-etching Process, 11

Developer, 37

Developing Ink, 8

Direct Printing, 60

― Transfer, 15

Distance of Lineature, 73

Drawing on Blue Prints, 23

― ― Prepared Paper, 25

Eberle’s Process, 11

Eckstein’s Process, 20

Electric Lamps, 31

Etching the Stone, 10

Failures in Transfers, 58

Fixing, 38

Fox Talbot, 17

Franz’s Paper, 55

Galls, Tincture of, 10

Gelatinizing Paper, 47

General Notes on Photo-lithography, 49

Gold and Mercury Intensifier, 38

Greasy Drawing Materials, 6

Gum Arabic, 10

Half-tone, Breaking up, 20

― Photo-lithography, 67

Hebensperger’s Process, 66

Hübl’s Hydroquinone Intensifier, 38

― Iso Emulsion, 43

Husnik’s Asphalt, 17

― Photo-litho Paper, 53

Hydroquinone Developer, 42

Indirect Transfer, 16

Inking up and Developing, 51

Intensification, 37

Intensifier, Lead, 40

― Mercury, 40

― Metol, 39

― Silver, 40

― Uranium, 40

Iodized Collodion, 37

Isochromatic Emulsion, Albert, 41

― ― Hubl, 43

Lemercier and Lerebours, 16

Line Drawings, 21

Litho Chalk, Hard, 7

― ― Soft, 8

― Stone, 4

Lithogravure, 87

Litho-Heliogravure, 79

Lithography, Theory of, 3

Making Direct Negatives, 36

Mungo, Ponton, 17

Negative for Photo-litho, 34

― Transfer with Paper, 83

Negré, 17

Nicephore, Niépce, 16

Orell and Füssli Process, 20, 75

Photo-Autography, 84

Photo-Litho Transfer, 57

Photometer, 51

Poitevin, 17

Prism, 44

Printing, 62

― on Bichromated Gelatine, 50

Pyramidal Grain Paper, 26

Reproduction of Prints, 27

― Size of, 28

Reversed Negatives, 44

Sacher’s Fish-glue Process, 66

Scamoni’s Process, 12

Scraper Boards, 25

― ― Schäuffelen & Co., 26

Sensitizing Paper, 49

Silver Bath, 37

Stripping Negatives, 45

Studio, The, 30

Tusch, 7

Valenta’s Asphalt, 17

Waterhouse’s Arrowroot Process, 66

Wezel & Naumann, 75

Zinc Plates, 5

― ― Roughened, 6

― ― Oxidized, 6