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 reëstablishing a vanished design. The second method is good for getting rid of smut. If the smut 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 circumstance 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 reëstablish their interrupted communication with the printing-color. In the second case, the small quantity 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 smutted. 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.
(1) Limestone has countless little pores. These can soak up fatty as well as watery substances.
(2) These can adhere easily to the limestone particles, but are easily separated again, as long as the nature of the stone is not altered. This alteration is produced most readily by sulphuric acid, tartaric acid and phosphoric acid.
(3) Water evaporates from the pores as the stone dries. Gum and other slimy substances do not.