1. Lithography

After Nicéphore Niépce, the second application using asphalt for making photogravures dates from 3 July 1852 when Barreswill, Davanne, Lemercier and Lerebours obtained a patent for "an application of photography to lithography and, by extension, to zincography and even to engraving".

This technique is rather similar to Niépee's early work in 1824. However, the powdered asphalt is dissolved in an ether solution that was much more diluted than Niépce's. The layer that was spread on the stone was very thin and because of the granularity of the stone does not form "a glaze, but what engravers call a grain. When one looks at the stone through a magnifying glass, one sees that the whole surface of the layer of varnish has regular cracks and furrows at the points where the stone was laid bar"'.

This figure shows the profile of the layer around one of these grainy points :



When exposing a negative, the more transparent it is the more light the asphalt layer will receive and consequently will harden more deeply. Around each grain the thickness of the insoluble part of the asphalt layer is a function of the amount of light received. During revealing, the area of asphalt that will be dissolved in the hollow spaces between the grains decreases as a function of the light received. When there is a great deal of light the asphalt points are larger than in areas where there was less light.

When the stone is engraved with acid around these points, consequently cleaned of their asphalt, one obtains large points where the negative received a great deal of light (the black parts in the original). The surface of these points will receive the printer's ink transferred from the press to paper with a darker hue than for those zones where the points were small.

In 1947, L. P. Clerc stated that this technique "was used until only recently in various colour lithography plants". What is unusual about this technique is that it works with a dull discontinuous layer of very tightly packed granules rather than with a varnish. This method facilitates the reproduction of continuous shading which correctly apportions the spread of ink as a result of the fineness of the granulation.