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 scraped 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 astonishing 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.