_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.