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 scraped style in copper, or the so-called black art.
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.