(4) Fats soak into the stone more and more. There is no means of destroying them except to remove the limestone itself by grinding or etching.
(5) Printing-color cannot adhere to the stone so long as a proper amount of moisture forms a wall between it and the stone. Under any circumstances it adheres only poorly to the lime particles, and assumes great power of adherence only when the pores of the stone are filled with fat, which are pinched in them, so to speak, and with which the printing-color strives to unite because of mutual affinity.
(6) This stronger adherence (or complete color reception) thus happens only when the outer color can reach and touch the inner fat. If the latter is deep in the stone, so that the communication is broken, it becomes difficult and the communication must be restored.
(7) This interruption occurs either if the color is rubbed away by force and with help of moisture, or if a substance that closes the pores unites with the stone.
(8) The rougher, sharper, and more angular the pores are, the more readily does the color find adhering points. It adheres at first to the surface by virtue of merely mechanical conditions. But when the moisture which hinders a complete union and greater penetration has dried, the color begins to penetrate deeper into the stone and to fill its pores. The most color will always adhere to rough spots. Therefore, it happens often, in some styles of work, that a stone too highly polished will seem perfectly black when inked, and still fail to yield a strong impression. For the same reason the impressions from soft stones usually are the stronger, especially if the mode of printing demands the use of thin printing-color.
(9) The effect of the etching fluid is in part a greater polishing of the surface, in part a filling of pores. Both make the stone reluctant to take color.
(10) If the stone has been prepared and polished already, it can be made rough again and receptive to color by being reëtched. At the same time the prepared surface can be destroyed by etching, and a communication established with the fat lying in the interior. The result is according to the manipulation.
So much in general. In describing the various styles I will make everything clearer.