If one wished to make an exhaustive list of all the major techniques that involve asphalt, the list would be a long one. Nevertheless, to conclude let me mention zincography, which is extensively developed in graphic printing. As a basis, we will be using the very complete description done by H. Calmels in 1907. Zincography is a 'planographie' method, that is, the image on the metal is neither hollowed out nor in relief. Rather like lithography, there are areas that attract or repel water and repel or retain ink.
The perfectly cleaned zinc plate was at first prepared using a solution of gallic and phosphoric acid in order to create hygroscopic salts on its surface (Figure above). This helped retain water and repel printers ink. Preparing the surface of the metal in this way gave it the same properties as lithographic stone. The surface was covered with an asphalt varnish, exposed under a drawing (a) and then developed, as was done using Niépce's technique (b).
Once exposed, the metal under the lines of the drawing was covered with hygroscopic salts. The plate was then bathed in acetic acid that dissolved the hygroscopic salts that had not been protected by the varnish (c). The plate was then covered with a coloured gum-lacquer varnish called fuchsine (d). It was dried and dipped in benzene. The layer of asphalt disappeared and took with it any coloured varnish overlay (e).
All that was left were the lines of the drawing that had been made by the fuchsine varnish.
Everywhere else the metal was covered by a layer of hygroscopic salts. As in lithography, all that was necessary was to wet the plate, which absorbed water except where there was gum lacquer, and then to ink it. The ink was repelled by wet areas and only adhered where there was coloured varnish, namely, to the lines of the drawing. Paper was then pressed on to the inked plate.
To sum up, the metal plate was prepared in the same way as limestone and then handled like a photograph. The drawing was produced by a varnish that played the same role as the soft lead pencil in lithography. In its final form, the metal plate was totally analogous to a lithographic stone, which is why the technique is called lithography on zinc. Its principal advantage was that zinc was lighter and easier to store than stone.
10. Techniques that Use Asphalt
In 1925, L. R Clerc wrote: "the only technique used at the start of photoengraving, Syrian bitumen, called judean (asphalt), has gradually been abandoned in favour of bichromate albumin. This later technique obtains the same results but takes much less time". Bitumen does not seem to have been used in photomechanical processes after 1930.
The screening of printing plates engraved with photographs was one of the major advances to have been made among the various improvements to the asphalt technique invented by Niépce as applied to plate-making for printing. Thick varnish was no longer necessary to reproduce varying shades of darkness. Instead, the denseness of points did the work. This meant that the varnish could be very thin and consequently exposure time was reduced. By working on selecting different qualities of bitumen, on their purification, and on their enrichment with sulphur, their light sensitivity was considerably increased, resulting in the kind of rapid treatment needed for industrial production. This was not true of Niépce's heliograph.
All this research gave birth to photoengraving and Niépce is undoubtedly its father, Bitumen of Judea was used for many years and its principle is still employed today by microlithography in the manufacturing of electronic microcircuits (bitumen has been replaced by synthetic resins when X- or ultra-violet rays are involved).