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 smut. 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 asserts 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 quantity 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 smut, 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.