The Invention of Lithography Part 14

Therefore it is best in such cases to polish the ends of the stone with pumice stone till all dirt is gone, and then to etch with diluted acid and prepare anew with gum arabic.

To be sure, it is possible to dip a clean woolen rag in strong, even pure acid, and thus etch dirt away from the ends; but great care is necessary that no drop may touch the design, as the ink that adheres to the latter is not strong enough to resist the acid. In thus cleansing the plate, the roughening is etched away by the violent action, and a new polish is obtained.

Still, in either method of cleaning dirty places, great care must be taken not to touch roughly, press, or rub with dirty fatty rags or with dirty, fatty fingers before the gum arabic is on it. The acid eats away all the previous material used in preparation, and leaves the plate practically in its clean, natural state. Consequently it will take on grease readily, and the application of gum is essential.

It is feasible to mix the gum directly with the acid solution, but this mixture must be made fresh again each day, as otherwise it loses much of its value.

The following points are important:--

_First_: If the grease remain long on a stone that, though prepared, has lost its coating of gum, it will penetrate the surface, and according to its amount and fluidity, will sink more or less into the stone, which will retain its polish on the surface but become more inclined to take dirt. It is better, therefore, to leave a small amount of gum coating on the stone in such cases.

_Second_: As only the extreme outer surface of the stone is prepared by the gum, and this is rubbed away gradually by wiping during the printing, so in the same proportion of wear and tear the original preparation would become lost, if it were not renewed from time to time, that is, if the stone were not again coated with gum. Twice a day, however, is enough.

_Third_: Because of this susceptibility of the surface to injury, a prepared stone must not be rubbed strongly with fatty material, because this damages the surface and the stone would readily soak up the fat.

_Fourth_: If a prepared plate is totally denuded of gum, and has been dry for a time, especially if it has already lost a part of the preparation through printing, it will incline very much to take color and s.m.u.t. Therefore, when it is necessary to stop printing, it is well to coat the plate at once with gum, but only with a very thin coat. If this has not been done, and it is desired to use the plate again, great care must be taken to wet it with the very purest water, or, better still, with diluted etching fluid, for instance one part aquafortis to five hundred parts of water; and then to coat it with gum. To neglect this precaution may cause the total ruin of the plate. For safe-keeping of the plates, if they are to be used again for printing, the coating with gum is, therefore, absolutely necessary.

_Fifth_: Gum can prepare only a thoroughly clean stone or one properly etched. Therefore, if the surface of the stone has even the least trace of grease, it will take color, no matter how thickly it may be coated with gum. On this fact is based the method of transferring copper-plate impressions and other printed subjects, as will be described later.

_Sixth_: If the stone has had fat on its surface, and this fat has been etched away again, the power of the gum a.s.serts itself, and the stone will be thoroughly prepared even if the fat has soaked considerably into the body of the stone.

_Seventh_: Mere grinding of the stone is not sufficient to attain a complete preparation through gum alone. Therefore, if an otherwise clean stone has some places after grinding where the fat has soaked in deeply, and one coats it with gum, the stone will take color after a time on these fatty places, as soon as the inked rag has been rubbed over the stone many times. However, this taking-on of color is only slight if the gum solution is thick, and long-continued cleaning will transform it into complete preparation.

_Eighth_: From both preceding observations we learn:--

Printing forces the color to sink considerably into the stone. If such a stone is required for new designs, it is not practicable to grind it so much that all the fatty places can be ground away. Therefore it is ground only till it is thoroughly even again. Then it must be well etched; otherwise it may happen that in printing the surface will rub away and the entire previous writing or design will appear again, a trouble hardly to be remedied.

If the stone is dirty in the middle, it can be cleaned in many cases by pouring on a few drops of oil of turpentine and the same quant.i.ty of gum solution, and rubbing it clean with a woolen rag. Then it must be washed with a wet sponge, after which it is inked. If it has not lost the s.m.u.t, the only remedy is new preparation. As this must be done differently for each different lithographic style, it will be described in its place.

If fat has soaked well into the stone in places where it is not desired, it is always very difficult to remove it without injuring adjoining parts. Correction of crayon work, if it has been etched and used for printing, is especially difficult. It is true that the defective parts can be cut out easily with a sharp instrument, but then these places must be prepared again. If weak etching fluid is used, it will not suffice. If strong fluid is used, the fine parts are easily attacked, and at the same time the surface will become roughened so that the stone often blackens entirely in the corrected parts. To avoid this trouble, and to facilitate corrections, I made many experiments to discover an acid composition that should prepare a stone anew and perfectly and yet not roughen the surface. I found the best material in phosphoric acid, especially when mixed with finely crushed nutgall.

Water in which phosphorus has been kept a long time becomes acid and etches the stone. The acid can be obtained more quickly by burning the phosphorus and catching the smoke. This method is somewhat expensive, but one does not need much, as it is used only for correcting defects.

If a few drops of aquafortis or other acid are poured on a clean ground stone, it will be etched. Now wipe the etching fluid off clean and coat the plate with soap-water or chemical ink. As soon as it is dry, clean it of the fatty coating with a few drops of oil of turpentine. If it is dampened then with water and inked, it will take color everywhere, even at the etched places. If gum is mixed with the acid, the same result occurs, though the stone has been thoroughly prepared where this mixture touched it. From this it appears that soap-water (and the alkalis in general) can destroy the preparation given to the stone and make it receptive again to fats. It is different if phosphoric acid is used.

This makes a preparation that can be destroyed only by very frequent coating of soapy water.

Still more durable and resistant to soap is the preparation if fine nutgall is mixed with the phosphoric acid and water solution. Nutgall gives even the other acids the property of resisting soap more than ordinarily. The study of this effect led me to invent the method of transforming a relief design into intaglio. Also, it is only by the use of phosphoric acid that one can do thoroughly that style of lithographic work which resembles the sc.r.a.ped style in copper, or the so-called black art.

III

GUM AS THE REAL PREPARATION

If a cleanly polished plate is sprinkled with a few drops of gum arabic dissolved in water, the sprinkled places will take no color so long as they are wet. When they dry, color will adhere, but can be washed away easily with a wet sponge. This shows that the gum alone will prepare the stone. The preparation will become more durable, however, if the stone is etched first.

In both cases, however, the preparation extends only over the outermost surface of the stone, penetrating only slightly, so that the least injury will make it take color as soon as it is dry. On this fact is founded the intaglio style of lithography. Therefore, if a clean ground stone is etched, then prepared with gum and dried clean, it can be coated with printing-ink or other fat substance (excepting soap and all alkaline compositions), and there will be no danger that it will lose its preparation. The thicker the gum coating, the less can the fat penetrate.

In printing, during which the stone must be kept wet, only the original coating of gum is necessary; but as the surface thus prepared soon diminishes under the frequent wiping, it is necessary in some forms of work to mix gum with the printing-color or with the water used to dampen the stone. More of this will be explained in the proper place.

Here I will add only that the domestic gum of cherry and plum trees is good for preparing stones some years and worthless in others, when it cannot be dissolved in water. In possessing the properties for preparing stone, the juice of many plants and fruits, sugar, and most mucous materials of the vegetable and animal kingdom, such as white of egg, approach gum arabic more or less. The latter, however, is to be preferred because of its reliability.

IV

CONCERNING PARTIAL PREPARATION

Here I impart my experiences in regard to an astonis.h.i.+ng phenomenon that occurs often in lithography and gives much trouble, especially to beginners. It is the so-called imperfect or semi-preparation, wherein the stone betrays a strong inclination to take color, and still will not do it or will do it only partially.

(_a_) If a cleanly ground stone is marked with chemical ink, etched, and prepared, the marked places will take the printing-color and produce impressions. If, after the stone is inked, one rubs strongly with the wetted finger, the color can be wiped from the design, especially if it has not been on the stone long and has been standing in a damp place. A place whence the color thus has been removed does not take it readily when the inking-roller is applied again; and the reluctance is the greater in proportion to the length and violence of the rubbing and the toughness of the printing-color. The stone shows clearly the traces of the penetrating fat; indeed, if the stone is rubbed with a wet linen rag that is inky from previous use, the design will reappear in black. But as soon as the roller is used, instead of inking these places, it takes the color off; and whatever means may be tried to make the defective places receptive again to color, it remains difficult, often impossible.

What has happened is that the wet rubbing has cleansed the surface of the stone of all its fat and at the same time has polished it and made it slippery. It is a sort of preparation; and though the fat of the ink has penetrated into the interior of the stone, the accidental preparation still offers an obstacle which prevents the printing-ink from adhering to the fat in those places. As I will show, these places can be prepared again thoroughly.

(_b_) Another case is when the design is too weak, and has been attacked by the etching-fluid too powerfully, though without being destroyed.

Here the printing-color usually is removed by the ink-roller, even though it adheres pretty well when being wiped.

(_c_) A third kind of imperfect preparation is when a stone inclines to take color or s.m.u.t on prepared places. This happens sometimes in part, sometimes over the whole surface, which latter effect is described by saying that the stone has acquired a tone.

The cause of this phenomenon may be one of many. It is either due to the appearance of a fat that has been in the stone, or to the fact that unskilled manipulation has destroyed the preparation partially.

Thence follow several observations again:--

(1) Mere wiping with clean water will give the stone a sort of preparation if the material used for wiping is suitable. This preparation is incomplete, but can be transformed very easily into a complete one. This incomplete preparation is according to the strength with which the rubbing material affects the stone. Linen and cotton stuffs have the least effect. More potent are animal wools and hair, silk or wet leather. The printing-color itself has a preparing property if it is made of very tough varnish or contains much lampblack. This effect is increased if Frankfurter black or powdered charcoal is mixed with the color, and the stone is kept very wet.

(2) The partial preparation is produced more quickly and made more durable if the water contains gum or gummy stuffs.

(3) The operation is still quicker if a weak etching fluid is used. A stronger fluid would make the preparation a complete one, but would also injure the good spots. Then again one must remember that the second etching produces the roughness discussed already.

(4) Grinding with sand, pumice, and other grinding materials also produces partial preparation, which is transformed easily into complete preparation by applying gum. Here, however, the circ.u.mstance is noteworthy that a plate that has been blemished by rubbing can be made to do the reverse, namely, to take color, by means of light grinding with water. a.s.sume, for instance, that a plate designed and prepared in relief style has been spoiled by handling so that the design refuses to take color. It is necessary merely to rub it all over with water and fine sand or to clean it with oil of turpentine so that all printing-color is removed from the surface. Then place it in a receptacle containing a great deal of very clean water. If it is ground delicately then with a very clean pumice stone, without destroying the traces of the fatty material that has soaked in, it can be brought to take color again as well as ever. Take a little of the before-mentioned acid-proof ink, smear it on the color-stone, and apply a clean linen or cotton rag. Wipe the stone that is lying in the water very gently with this rag, and the color will fix itself bit by bit on all parts of the design, even if the entire relief produced by the etching should have been ground away. It is necessary only that the fat shall have soaked in sufficiently; and this usually is produced soon enough by the printing.

After the plate has accepted color completely, it is to be completely prepared by light etching and with gum, and then it will take the color properly from the ink-roller.

If this experiment is to succeed, it is to be noted that in grinding there must be no trace of fat on the stone or the pumice, because the rubbing during grinding might transfer this greasiness to those parts of the stone that are to remain white. Care must be taken, also, not to press too hard in applying the etching color, because the places that have been cleansed of all gum by the water, and thus are inclined to accept color, will s.m.u.t easily. Finally, the stone must not be permitted to dry before it is fully prepared again by etching and gum coating, for it might easily become entirely s.m.u.tted and useless.

This experiment leads to the conclusion, which has been proved correct in many ways, that a soft rubbing in clean water with printing-color, especially if it contains tallow, is very well adapted for transforming the incomplete preparation into a condition of accepting fat perfectly, and of giving injured places new potency. Also, that the contrary effect can be produced by violent rubbing, especially with wool, leather, or tough colors, because this prepares the wet stone and makes it useless for accepting fat. The first method may be used with advantage, therefore, for reestablis.h.i.+ng a vanished design. The second method is good for getting rid of s.m.u.t. If the s.m.u.t has occurred in previously clean and thoroughly prepared places, it can be destroyed entirely. But if it is only that the deeper fat has lost its superficial polish, and has appeared again, the stone will be only partially prepared by this last method and must be newly prepared on the desired places with weak etching-fluid and gum, for durability's sake. It is easy to see how important this circ.u.mstance is. With the one and the same process in various degrees of manipulation, opposite results can be produced; and I may declare that only he is to be termed a perfect lithographer who has exact knowledge of this especial matter.

(5) It has been mentioned already that every sort of preparation can be destroyed by a renewed etching, and particularly with alum and citric acid. The same is caused by soap and alkaline compositions; therefore also by chemical ink if it contains a sufficient amount of alkali.

(6) Simply letting the stone plate rest produces important, often contradictory, phenomena. If smeared parts refuse color, clean water poured over these places runs from them as quickly as it does from the fatty parts. This is the surest sign that they still have fat, though it is not sufficient to attract the color. If such a stone is permitted to lie idle a few days, even if coated with gum, it will often take the color thereafter. On the contrary, if a stone plate has taken on color at the well-prepared places (usually readily removable by wiping with oil of turpentine and gum solution, but generally reappearing), it need merely be inked after such cleansing, coated with gum and left idle, and in a few days it loses the readiness to take dirt.

The cause of both phenomena is that in the first case the fats that lie deep gradually work upward into the partly prepared surface and practically reestablish their interrupted communication with the printing-color. In the second case, the small quant.i.ty of fat that has adhered merely to the surface has penetrated into the stone, so that it loses its effectiveness. Added to this, in the latter case, is the fact that the linseed oil, and the varnish prepared from it, acquire the property of losing their fats when they are dried in the air, and thus will take color poorly or not at all. This observation led to the invention of an artificial stone or stone-paper.

(7) In contrast with preparation by wet wiping there is the wiping with dry and fatty bodies, which produces full acceptance of color on the partially prepared plate, while in the case of the fully prepared plate there occurs at least partial color acceptance or semi-preparation. As every property of the stone can be used for good impressions just as well as it serves in unskilled hands to ruin a design, so in this case; the lost parts can be restored through proper use of rubbing with a dry, fatty substance, and the clean, prepared portions of the plate can be s.m.u.tted. There will be more about this.

V

SHORT REVIEW OF THE PRECEDING

As the entire art of stone-printing depends on proper preparation, it will not be out of place to express my views as to the nature of the process. This will serve also as recapitulation.