The Leardo Map of the World, 1452 or 1453 / In the Collections of the American Geographical Society

by John Kirtland Wright

THE REPRODUCTION OF THE LEARDO MAP

By A. B. Hoen
A. Hoen & Company, Baltimore, Md.

[One of the first things usually asked in regard to the reproduction or facsimile of an old map is: “How was it made?” To answer this question and to give some idea of the difficult technical problems involved, Mr. Hoen, under whose direction the Society’s reproduction of the Leardo Map was made, has been kind enough to furnish the following note.—J. K. W.]

The Leardo Map is painted on parchment. Some of the colors have faded, and others here and there have separated from the skin, leaving blanks in the painting. The latter defects are especially noticeable in the yellow zones encircling the map proper. To avoid the injection of the personal element into the reproduction, no attempt was made to restore the missing letters or symbols. It is further to be noted that in cases of partial legibility the very palest parts of the faded manuscript may have failed to register in the reproduction, although great care was bestowed on this part of the work.

As a first step in the reproduction of the map, color separation negatives were made on photographic plates sensitized for all the colors. By interposing proper light filters and by making separate exposures for each color, negatives giving red, yellow or green, and blue or purple values were made, together with a fourth negative giving neutral tones—black and grays.

As the last negative comprehends almost the entire base of the map, special attention was devoted to its conversion into a printing plate. The process employed is known in Germany as “Albertype” or “Lichtdruck,” in England as “collotype,” and in America as “heliotype” or “photogelatin.” Of these names, “collotype” seems to be the most fitting. Briefly, this process consists of sensitizing a gelatin film with a chromic salt and exposing it to light under a negative. In proportion to the amount of light passing the negative there will be a reaction in the chromated gelatin. In this reaction the gelatin loses its power of absorbing water and takes on the opposite property of holding “non-watery” substances, such as printing ink. The action of the light is a graded one, varying from full effect under the clear parts of the negative to nil under the very dense parts. A similar gradation in ink-retaining powers is acquired by the exposed gelatin film. Thus, where the light exerts full effect the gelatin will be completely hardened and will hold the ink in its greatest intensity (solid); the parts which receive less light or none at all will hold the ink in attenuated quantity. The lights and shades of the monochrome picture are thus reproduced.

In order that the film may exercise this selective power of taking on or rejecting ink it is necessary that the unaltered parts be kept moist. Therefore, after exposure under the negative, the film is washed to free the gelatin of the unused chromates. While still moist it is rolled with a roller carrying printing ink. This roller will discharge its ink on the hardened parts of the film in proportion to the amount of light that each part has received through the negative. If a sheet of paper is then pressed on the inked film it will lift the ink and the resulting impression will be of the same character as the base color of the Leardo Map.

It is of interest to note that as the light-affected and hardened surface of the film accommodates itself to the unaffected underlying gelatin (as the latter swells in washing) it breaks up into a net of lines. This reticulation is barely perceptible in the high lights of the picture but gradually increases in strength until the mesh fuses into the solid color of the deepest shades.

It will now be apparent that the feasibility of printing these colloid plates hinges on the fact that the graded ink-attracting mesh is separated by inversely graded ink-repelling, interstitial, unaltered, and moist gelatin.

Its mesh not being apparent to the unaided eye, the collotype approaches the fidelity of a true photograph in the rendering of details. For this reason, the collotype process has been selected as best suited for the reproduction of the Leardo base.

The coloring of the map was done by overprinting, in lithography, as many colors as were deemed necessary to convey a fair idea of the original. Lithographic plates were made from the color separation negatives mentioned above. The principles underlying the lithographic process are, broadly, similar to those described for gelatin printing, the essential elements in the process being a water-absorbing ground mass (limestone) in place of the gelatin and a water-repelling and ink-attracting surface affection similar to that created by the action of light on the chromated gelatin film.

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.