Highlight Loss in Carbon Printing
The loss of highlight detail (i.e. densities less than .20) in con-tone carbon printing is a physical process problem and is not ameliorated by the use of any variations in curve shape, gamma, density. QTR, etc. Charles Lighton first described this in detail in the September 1927 Photographic Journal (pgs 418-419) and other photographic researchers have reported similar findings.
Unlike chemically hardened layers of gelatin which have sharply defined boundaries, light hardened gelatin has a soft and diffuse border between the hardened and unhardened portions that is uneven, variable and easily washed away. Many carbon workers resort to "fogging" (chemical or light ) or multiple-printings to artificially place density in these areas. While this can produce highlight density, it does not provide highlight detail. This can be seen in the milkweed print where the quarter-tone highlights are "mushy" and without definition. Similarly, the fine details of a brides white-on-white wedding gown will be lost in a carbon print made from a con-tone negative.
Sightings of highlight detail in con-tone carbon prints are a lot like sightings of Bigfoot and the Loch Ness Monster - many have claimed to see them but none have been able to offer any (photographic) proof.