In still greater degree does this appear when using citric acid or a solution of alum in water. Take a finely ground stone, pour diluted aquafortis over it, prepare it with the gum solution, and then dry it thoroughly with a clean rag. Now pour a little citric acid or alum solution on parts of it and let it dry. Then paint the parts so treated with a fat or printing-ink. If the color is rubbed off with a wet rag, it will be seen that the stone has become white again in all places except those where the citric acid or alum are. Those parts will have taken the color exactly as if they had been painted with chemical ink. The same occurs when applying other acids, but in a lesser degree. This effect will be mentioned in future for many methods. Here I will remark only:--
It happens often that the stone takes color on places where it should remain clean. This is caused by clumsy handling, unclean rags, etc., and occurs particularly at the ends, because they dry first and are more exposed to careless manipulation. These smutted places usually can be cleansed with a clean woolen rag and gum solution or even with a wetted clean finger. But sometimes the defect will not yield so easily, especially if the printing-color is soft. Then the only remedy is to prepare the stone over again, and that is the time when one must have regard to the roughening that ensues, if the stone is not to be rendered worse instead of better.
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.