Lithographic stone is an amorphous carbonate of lime of fine, close texture. It has an affinity for water—that is, it is easily kept damp. This affinity may be destroyed by changing the carbonate of lime to some water-resisting salt, such as the oleate, or by adding to the surface of the stone a film having the same power. Both of these methods were utilized in making the color plates of the Leardo Map.
A number of lithographic stones were properly surfaced and this surface covered with very thin, light-sensitive, colloid films. The color separation negatives were exposed over these sensitive films and the resulting photographs on stone gave the red, yellow, blue, and other values of the original as they had been analyzed by the light filters.
No color separation process, however, can eliminate from the areal coloring the black and grays of the base. Similarly, the colors themselves absorb a certain amount of white light so that the effect of the areal coloring is also felt in the monochrome reproduction of the base map (e. g., gray lettering is lost in heavily colored areas). For this reason, it is necessary to correct by hand the unnatural effect produced by the overprinting of all the color plates in the darker portions of the picture. Lithography is best suited for the control of these difficulties, and for this reason the color plates were made on stone.
The mechanical printing of the edition from gelatin or stone embraces three essential operations: (1) moistening the plate by damping rollers; (2) inking the plate by inking rollers; (3) pressing of suitable paper on the inked plate. After the base is printed, the base plate is taken from the press, another plate, carrying one of the map colors, is placed in position, and the proper color put on the inking rollers. The printing of the second color is then done as was that of the base. Similar changes of the printing plates and colored inks follow in order for each of the colors which make up the complete map.
Eight color printings in addition to the base color were found necessary for the proper rendering of the Leardo painting. One of these, a light gray-buff, covers the area of the parchment and serves to bring it out from the white paper background.
In selecting a suitable paper for this reproduction, certain qualities had to be considered. Among these were good printing surface, durability, and as much strength as could be had along with the above essentials. A chart plate paper of high rag content was made especially for the work.
KEY MAPS
Fig. 4—General key map. The numbers correspond to those of the main center heads in the commentary on pp. 32-60.
