There is another kind of plate engraving, called “mezzo-tinto.” This is done by first covering the whole plate with a rough granulated surface--called a “mezzo-tint ground,”--by means of a sort of notched chisel, called a “cradle” or “grounding-tool,” which is rocked to and fro over the surface in every possible direction, till the whole is covered with minute dots; so that if it were then used to print from, it would produce an even dark surface. On this ground the engraver works with a “scraper,” scraping gradually away all those parts which are to appear as lights in the finished picture, and the more the ground is scraped away the lighter will be the tint, till, finally, those parts which are to be quite white are scraped smooth and burnished, so that they shall hold no ink at all. In this way the engraver proceeds till his design is finished, trying the effect from time to time. This kind of engraving is often combined with etching, producing greater sharpness of outline than mezzo-tint alone.
This is the art of producing designs upon stone in such a manner that impressions may be printed from them. It is often miscalled “engraving on stone;” properly speaking, it is not engraving, but a process depending on the want of affinity between watery and greasy matters, the design being made with a greasy substance, either in the form of ink or crayon called “lithographic chalk,” as follows. A close-grained stone called lithographic stone (a kind of magnesian limestone) is ground perfectly even on the surface, and (if for being written upon) polished with pumice-stone, or (if for being drawn upon with chalk) grained with fine sand; the design is drawn with chalk or ink, each of which is made up of greasy materials--wax, soap, and asphaltum, colored with lamp-black. When the design is made, the stone is etched--washed over with a weak mixture of nitric acid and water--which answers three purposes; it very slightly bites away the stone in all places except where it is covered with the ink or chalk, it smoothes the grain in the same places, and it converts the soap of the ink or chalk into grease. We have now the surface of the stone quite wet, except where the design is, which, being greasy, keeps quite dry. The stone, placed in a press, is rolled over with a roller smeared with greasy printing ink, which will not adhere to the wet stone, but it readily does to all parts of the design; a piece of paper is placed over the stone, and a flap of leather covers it; along this leather the scraper of the press is brought very forcibly when it is worked, and on removing the paper an impression is found to be transferred to it from the stone. This process is repeated again and again, many hundred times, without much injury to the original design.