Georg Fritz

1.—The theory of printing from stone.

He who would study or practise photo-lithography must first become acquainted with the fundamental principles on which lithography rests. I find it therefore necessary to first give in concise form a small treatise on the theory of this process as well as on the most important materials which are necessary in the practice of the same.

The theoretical principle of lithography is purely chemical, and is based principally on the repulsion of water and other substances by fatty bodies, and the alteration of these greasy bodies by acids. As the support for this chemical opposition a stone or zinc plate is used, which are sufficiently porous to allow grease, water, acid, and certain resinous solutions to penetrate to a certain degree.

The fatty substances which are transferred to the stone by writing, drawing, or transfer, are autographic ink, lithographic ink, and chalk, the so-called transfer and photo-lithographic colours. There are also other substances, which are used especially in photo-lithography, which do not evidently penetrate the stone and which cannot be considered as greasy substances, but which, as we shall see later on, have exactly the same action.

The above-mentioned fatty substances actually penetrate the porous stone and make it in places suitable to take more printing ink. By the so-called etching—dilute nitric acid mixed with gum water—two effects are produced. On the one hand the fatty bodies are altered in chemical composition, being decomposed into the fatty acids which combine intimately with the material of the stone—calcium carbonate. On the other hand, the surface of the stone not covered with the fatty substances is chemically changed, since the carbonate of lime is converted into nitrate, which has now the property of keeping moderately damp when moistened with water, and thus repelling grease. If such a transfer of greasy bodies on to the stone is allowed to act for some hours, and the fatty substances are removed with turpentine, it will be seen that these places have a lighter colour than the other parts of the surface of the stone, and if the stone be rolled up with a leather roller, charged with a greasy ink, after the stone {4} has been damped, only these places will take the greasy ink. By means of a litho steam or hand press prints can then be prepared from this.

It is frequently necessary to make corrections or additions to the drawing on the stone; if the stone has been already etched some means must be used which will remove the action of the etching, or else the stone will not take the greasy matter, or at least not so readily as is desirable. Very weak acetic acid, about 1–2 per cent., will do this. If the stone is flowed over with this dilute acetic acid or dilute table vinegar, the action of the stronger nitric acid and the gum is stopped, and the surface of the stone will again take fatty matter.

Before the penetration of the grease the stone is prepared by polishing with a concentrated solution of oxalic acid. Oxalic acid poured on to the stone would exert an etching action, but make the stone to a certain degree capable of taking the ink. If this liquid is rubbed on the stone with a cloth or linen pad with moderate pressure, the stone receives a high polish—especially the hard kinds—and in this condition the stone takes the ink well, like any flat object, but will not retain it. If a transfer is made on to such a stone, and the same etched, the image will completely disappear the first time the roller passes over it. This polish can be removed with 1 or 2 per cent. acetic acid, and the stone then has its primitive qualities. This method is used to reverse the image, which will be described later on.

The commercial litho stones are of different colours, from whitish yellow to greenish shining grey. The latter has a finer and more even grain, and is considerably harder than the other kind. These two kinds are briefly designated in the trade as “yellow” and “blue,” and the brighter kinds are, as a rule, softer and cheaper than the darker.

The faults which the stones sometimes show differ considerably, and these produce certain inconveniences in continuous work. Many stones have dark and light spots, and as these spots have, as a rule, different densities, the etching solution cannot exert the {5} same action over the whole of the stone, and consequently in fine drawings, autotypes, etc., there are formed bright and dark spots. This also occurs with calcareous, speckled, and marbly stones. The very bright stones have less density, and therefore suck up the damping solution very quickly, and begin with repeated printing to become tinted. Stones with calcareous, rusty or other veins split easily, especially in the large sizes. Lime spots or lime veins, which are easily recognized by their brighter colour, take in graining a very unequal grain. The drawing is darker on these places, since the grain is greater according to the softness of the stone. Such stones are unsuitable for those photo-lithographic processes in which graining is used, such as the processes of Orell and FÜssli and BartÖs. The so-called glass veins and glass spots are crystallizations consisting of silicon, and are harder than the other parts of the stone. On these places the fatty inks do not penetrate deep enough, and the drawing does not adhere well, and can be easily removed by the rollers.

The hard stones receive a much finer surface than the soft when polished with pumice stone; they also reproduce every point, line, and detail of the drawing much more delicately, finer, and sharper; and, lastly, they have the advantage that they etch much more evenly, and re-etching is easy if moderate care be taken.

For the finer photo-lithographic work, if quality and continuous working is desired, only the good “blue” stones should be used; for cheaper line drawings “yellow” stones free from faults may be used.

Zincographic processes differ essentially in the preparation of the plates. In the one, the plate is given a coating which has a similar composition to a litho stone; in the other, the plate is roughened and given a fine grain by a mechanical method, such as the sandblast, in order that it may be then oxidized by a chemical process, with acids or alkalies. All these processes are more or less advantageous. The zinc plates cost about one-tenth to one-twentieth of the cost of the stones, and this may well be sufficient grounds for the proprietors of large establishments being interested in zinc plates, as the stones are rather costly.

Excellent zinc plates, coated with a film or merely oxidized, can be procured commercially in Germany and Austria. For those {6} who wish to prepare their own plates the following tested formulÆ are given:—

A zinc plate from 1 to 1·5 mm. thick is roughened by means of the sandblast till it has a fine, even grain and appears smooth, but clean. The following solution is prepared:—

or

The roughened plate is first flowed over with water, and immediately afterwards, with one of the above solutions, in the same way as a plate, is covered with a developer. The plate is oxidized by this, and there is formed on the surface a white precipitate. It is then washed thoroughly under a rose tap and dried, and the plate may be either coated with a light sensitive substance for printing or a transfer be made on to it.

After the development or the transfer, the plate is gummed and then etched with an etching solution consisting of—

this being allowed to act for 30 or 40 seconds.

For repeated printing one to two per cent. of gallic acid is added to the damping water.

If the plate has been correctly treated the print or the photo-litho transfer will furnish thousands of good impressions, just like a stone. Further details as to the printing on the zinc plates, or the transfer of the chromated gelatine prints on to the same, will be found in Chapter IV.

The two first are used in the liquid state, the chalk, however, in solid form. The ink can be obtained in a liquid form and the two others in solid state. All three materials consist principally of soap, tallow, wax, resin, and soot. Soap and tallow give the necessary grease, wax and resin give hardness and consistency, and soot the colour. The proportion of grease in the drawing materials must be so great that even the finest lines or points of a drawing can be well transferred to the stone, so that they may not be attacked by the proper etching solution, and do not break away from the stone by continuous printing. As much black as possible in the drawing materials is pleasant for the draughtsman, but is not actually necessary for this particular purpose. {7}

The tusch must dissolve well in distilled water and flow fine and clean from the pen. It should be tolerably brittle, and the fractured surfaces should be shiny. The photo-lithographer will frequently require the lithographic tusch for additions or for corrections. The best is the so-called Lemercier’s tusch, which is used in nearly all works. It consists of—

These ingredients should be melted together by boiling.

The autographic ink is, as a rule, used for drawing or writing on paper, and the result is then transferred to the stone. Instead of this, any litho tusch can be used, but for some work the autographic ink is more advantageous. This ink must be thin, run easily, but not patchily, from the pen, and must keep for a long time equal in quality and action. It is very advantageous that specially-prepared paper is not required, since this always makes the work more difficult; firm, hard writing-paper is quite good enough for this work. The commercial inks generally possess these good qualities, and if much is not required one will hardly care to prepare it oneself. A good successful autographic ink which draws in brown may be obtained from the following formula:—

Originals prepared with this ink transfer as well immediately as after several months, and ordinary well-sized writing paper can be used for drawing or writing on.

Lithographic chalk is only prepared in the solid form for drawing on paper direct or on stone. According to the nature of the work, it is harder or softer, fatter or leaner. The hard kinds, which are also usually leaner, contain more resin and less fat; the soft, on the contrary, more grease.

Fatty chalk is composed of—

Lean chalk is composed of—

For lithographic work a greasy ink is required in contra­dis­tinc­tion to ordinary printing ink, which consists of linseed oil and lamp­black, which would be called “lean.” For inking-up a photo-lith­o­graphic chromated gelatine print such an ink cannot be used. In order to obtain a good result in trans­ferring, this ink must contain grease, soap, and resin.

Any good transfer ink can be used for a developing ink, as this is applied with a roller. It usually consists of equal parts of tallow, wax, soap, some resin, and as much litho ink as all the other ingredients put together.

That prepared by C. Kampmann, Technical Instructor in the K.K. Lehr-und Versuchsanstalt at Vienna, contains, besides the above-named ingredients, asphalt and gum elemi, and works well in practice.

Thin developing inks, which are distributed with a broad brush or a pad, can be prepared by diluting good transfer ink with equal quantities of wax, benzine, and turpentine.

A good developing ink must have the following properties: When spread upon the print must give a clean, sharp, sufficiently dense impression, which can be easily transferred to the plate or stone; on the other hand, the drawing ought not to spread. The transfer should possess such resistance that, without rolling up, it should resist a slight etching.

The following are principal conclusions to be drawn from my experiments:—

(a.) The exposed asphalt, whether this exposure is made by printing or whether a drawing is first made and then exposed to the light, adheres more firmly to the stone or zinc plate than if it was not exposed. An asphalt drawing prepared in the dark and inked up does not adhere to the stone, and this may be explained in that the grease cannot act through the asphalt film. Insensitive asphalt behaves in the same way. With correct exposure under a clear negative of about 20 degrees Vogel, and subsequent development and auxiliary exposure for about half to one hour, the drawing adheres firmly to the stone. If the image is to be washed off, the turpentine must act for five to ten minutes on the film, till the exposed asphalt has dissolved. If, after washing off, it is inked up, all the places take the ink well, and the image appears clear and plain. The image can only be destroyed by strong acids or alkalies, which actually dissolve the stone itself.

From this it is clear that the prints neither require inking up nor strengthening in any other way, and my experiments have proved that in all cases the prints are, by this treatment, clogged up, and neither gain as regards a longer run nor in withstanding etching.

(b.) Solution of asphalt, to which oil has not been added, behaves as described above; if, however, oil is added to it, it acts still better as regards the keeping of the drawing, and the prints need no subsequent exposure. Too large an addition of oil has the disadvantage that, in developing with turpentine, the asphalt dissolves too quickly. The images wash off and one hardly has time to examine. The exposed asphalt, to which oil has been added, also dissolves very readily. For the preparation of colour plates this point is of some importance.

Of the acids the most important is nitric acid, the purpose of which is to convert the surface of the stone into a nitrate, so that the grease cannot penetrate. This acid acts even when diluted very energetically on the stone, and produces a fine grain.

Hydrochloric acid exerts a less energetic action; it leaves the stone smoother and does not attack it so evenly as nitric acid, but has the same result finally.

Sulphuric acid ought not to be used for etching stone. It forms calcium sulphate on the surface of the stone, and this separates from the stone in printing and spoils the pulls. {10}

Phosphoric acid may be used for etching as well as for preparing the stone; it exerts a very faint etching action and is seldom used for this purpose, but principally for making corrections.

Acetic acid plays a very important part in lithography. It possesses the property, even in very dilute condition, of stopping the action of the gum and oxalic acid, and, indeed, even that of the stronger nitric, sulphuric, and hydrochloric acids. It finds, therefore, considerable use in negative printing, in making corrections, etc. It converts the stone into its original condition and makes it suitable again for taking ink.

Citric acid fulfils the same purpose, but is, however, much higher in price.

Oxalic acid is not used for etching, but only for preparation of the stone. By rubbing the stone with a solution of oxalic acid with a pad of cloth it takes a mirror-like surface or polish, into which the fatty ink cannot penetrate. It is used for preparing the stone for lithogravure, negative drawing, and negative transfer, etc.

Tincture of galls, or instead of this, what is simpler, gallic acid, is used in lithography. It contains plenty of tannin, and is one of the most efficacious means of preventing the combination of the stone or zinc plate with fatty substances.

Gum arabic or some other vegetable gum is indispensable for litho and zincography. A 10 per cent. solution of gum is generally used. It penetrates all the pores of the stone and the plate, and gives the same a thin but very firm film, and prevents the ink taking on these places. If it has become slightly acid it acts as a weak etcher, and as an addition to nitric acid it makes the same of thicker consistence and more easily distributable. The action of gum can be removed with acetic or citric acid; with water alone this cannot be perfectly effected.

The ordinary etching chiefly causes the fatty substances to assume a new chemical condition, since in combination with the stone they form insoluble fatty salts of lime, which represent the printing surfaces. It has, however, also the property of cleaning the blank stone surfaces from any adherent grease, and makes the blank parts of the stone incapable of taking ink; also it makes the drawing clearer and more precise.

The strength of the first etching may differ, according to the following circumstances:—

(a.) The stone; for hard stones stand a stronger, and soft stones a weaker etching. {11}

(b.) The printing image; fine drawings must at first be etched with a weaker solution; coarser drawings, on the other hand, will stand a fairly strong etcher.

(c.) The material with which the print or drawing has been prepared; very fatty developing inks and asphalt stand well even with strong etching, whereas with lean or hard inks only a weak etching should be used.

An etching solution which is at the limit of strength, but still suitable for a hard stone or a very coarse drawing, would irretrievably ruin a very fine drawing on a soft stone.

Commercial acids differ considerably in strength, and it is, therefore, better to use a hydrometer than a measure. It is also always more satisfactory not to use the acid by itself, but in conjunction with gum. A solution of gum of eight to ten degrees strength is diluted with nitric acid till it measures 10 to 12 degrees on the hydrometer. This solution may be safely used for every case, and it is only necessary to allow it to act longer for coarser drawings; it should be evenly distributed over the surface of the stone with a clean soft pad. In very hot weather it acts more energetically, and then it should be used weaker for fine work. In any case it is advisable to let the stone or plate stand for some hours before etching, so that the ink may thoroughly combine. The etching of zinc plates will be more fully described later on; in other respects the treatment is the same as for stone.

There are two deep-etching processes which are chiefly used, one in which the resin is melted with a spirit flame, the other in which the same thing is done by ether vapour.

The procedure in each case is extremely simple. In the so-called burnt-etching process, which has been perfected by Eberle, of Vienna, the already etched stone ready for printing is rolled up with a very stiff printing ink, not transfer ink, and then the drawing dusted with the finest powdered resin. The stone is then gone over with a cotton-wool pad and plenty of talc or French chalk, and then well dusted off with a broad camel’s hair brush in order to remove all traces of resin from the blank stone. To melt the resin a spirit lamp with a side flame is used. The flame is passed over the drawing, and ought only to be allowed to act sufficiently long to make the resin combine with the ink, which can be recognized by the drawing looking shiny. If it has been melted too long the sharpness of the drawing is damaged. After this the etching may be at once proceeded with, and this is effected with a solution of gum to which 8 to 10 per cent. by measure of 44 per cent. nitric acid has been added. For fine drawings 4 to {12} 6 per cent. of acid should be used. In using this a fairly strong effervescence takes place, which should not frighten anyone.

In the ether or cold-melting process exactly the same procedure takes place, only that instead of melting with an open flame this is effected by ether vapour.

For this work a wooden stick of about 2 mm. thick is laid on two edges of the stone. On another ruler, which should be large enough to cover the whole of the stone, and which should be covered with flannel or stout cloth, ether is poured; this ruler is laid on the others, so that it forms a right angle with them, and is slowly drawn over the surface of the stone. The ether vapour falls on to the surface of the stone and dissolves the resin which thus forms the protective film for the deep etching, which is done precisely as described above.

Another melting process recommended by Scamoni consists in pouring alcohol on the stone and then setting fire to it.

Typographic prints are distinguished by the lines, figures, etc., being somewhat pressed into the paper, and show raised up on the back, which is technically termed “impression.” When this “impression” is very plain, that is to say when the printing has gone pretty deeply into the paper, it is advisable before reproduction to remove this by strong pressure.

The second method of printing is exactly the opposite of the first, and is founded on the principle of printing from intaglio. The lines and points which should print are cut mechanically (by engraving or piercing) or chemically (by etching) into the stone. The etched or engraved intaglio plate represents a negative, or a reversed wood-cut, only with the difference that the lines do not {13} lay in the same plane. In order to obtain an impression the ink is pressed into the cut-out parts, and is wiped off from the surface of the plate. By stronger or weaker wiping the print receives a stronger or weaker local tone, which is of great advantage from an artistic point of view, but which forms no small obstacle to the reproduction.

The impression is represented in relief on the paper. According to the depth of the lines they take more or less ink, and appear therefore raised up in the impression according to the amount of their depth in the plate. Reproduction photography has to reckon on this disadvantage, for with a side illumination the raised up lines cast shadows, and a correct negative does not follow.

By the third method, lithography, of which we have already spoken, the impression lies generally flat on the paper like a drawing, if a very strong deep etching was not used, in which case it is also slightly raised. In printing from the graver or from stone etching a faint relief is generally noticeable, which is never so high as with copperplate printing, and which is no important obstacle to making a photographic negative.