The more lampblack is mixed in, and the more thoroughly they are combined by rubbing down, the better will be the color. But lampblack must not be added in such quantities that the color becomes dough-like. In describing the various styles of printing I will describe the best printing-inks also. I will merely make the general note here that designs on stone take the ink best when it is thin and fluid, but that there is less danger of off-set on the parts of the stone that are to remain white, if the ink is tougher or contains more lampblack. Too much lampblack and too tough a varnish endanger the finer strokes and dots, however, so that they will not take ink, being, as lithographers say, rubbed out. The rubbing or grinding effect of too tough an ink is like that of pumice or other grinding material. With tougher varnish, clearer imprints can be made and they do not become yellow easily. But the inking is more difficult and demands greater skill, as well as heavier pressure in the press. The varnish can be mixed not only with lampblack but with many other colors, which will be described when I reach color printing in this essay. Sometimes black lacquer is used with advantage instead of lampblack; and Frankfurter black is successful in the intaglio and aquatint methods. X RUBBING-UP INK It happens often that weak parts of a design cannot withstand the etching fluid and are cut away; also, that fine lines are rubbed away through unskilled treatment during printing. Then frequently a very simple remedy is to ink the plate with the so-called rubbing-up ink. This color consists of a thin varnish in which a portion of litharge of silver or mennig or white lead has been dissolved thoroughly over the fire, and a proper amount of lampblack added. Often it is good to add some finely powdered sand or powdered pumice stone. To prepare this, a portion of the thinnest varnish is heated in a pan till it burns. Then about an ounce of finely powdered mennig (or another lead oxide) is stirred in to each sixteen ounces of varnish till all is thoroughly mixed. A rubbing-up ink can be made also by mixing common printer's ink with vegetable oil, tallow, and a very little soap. Each of these colors adheres to all those places that have a trace of fat and thus gradually makes faint places in a design receptive again. Later I will describe how to use care in applying this color, so that the entire stone shall not be smutted and spoiled. CHAPTER III CONCERNING ACIDS AND OTHER MATERIALS I GENERAL PROPERTIES OF ACIDS