“RECENTLY I observed a peculiar behavior of a film of bichromated gelatine after it had been exposed to the action of light for a short time and then immersed into an aqueous solution of picric acid or sodium picrate and dried.
“Those parts of the gelatine film which have been exposed to light are found to be raised and if the exposure has been made under a negative the image will stand out in relief. This behavior of a gelatine film, when treated as above described, may possibly be utilized in some branch of photography.
“I will briefly describe the manner in which one of these relief images may be obtained. An ordinary dry plate composed of a good thick coating of gelatine emulsion is sensitized in a solution containing about thirty grains of potassium bichromate to a pint of water and dried in a dark place.
“After drying, the plate is exposed to light beneath a negative for two or three minutes, then removed from the printing frame, and then washed in cold water to remove all soluble chromium salts. The plate is then put in a saturated aqueous solution of picric acid. This solution should be at a temperature of about 75 F and the plate should remain in same for at least five minutes.
“On examining the plate after its removal from the solution, aside from its yellow colour, it will present the same appearance as if it had been treated with plain water, those parts of the film which have been protected from the light being higher than the other portions. On drying the condition of the film is exactly the reverse: those parts that were protected from the light are in reality depressed. This depression of the film is proportional to the strength of the light acting on said film.
They specialized in unusual photographs, many of which depicted historic and religious themes. Their best known photograph was the “Knaffl Madonna” (1890), which won praise at the Photographer’s Association of American Art Convention in Lake Chatauqua, NY, in 1899.
James Henry Brakebill partnered briefly with photographer William J. McCoy at the turn of the 20th century in Knoxville. But by the end of that first decade he merged his business with the Knaffl brothers to form Knaffl & Brakebill.
Knaffl & Brakebill went on to receive widespread acclaim for their photographic work in the early decades of this century.
“My explanation of this phenomenon is that the picric acid forms a compound with the gelatine, which when dry is much more dense and compact than normal gelatine. That this is the correct explanation, I believe, and is borne out by the fact that if the gelatine film has been exposed beneath a positive, and afterward treated with the picric acid solution, on drying the image will be represented by a depression in the gelatine.
“The exposed portions of the film have become more or less insoluble, and the solution of picric acid is therefore unable to penetrate the film as freely as the unexposed parts and form the compound mentioned above. It appears possible to utilize a relief image of this nature for obtaining prints photo mechanically. The plate in this case should be exposed under a grained negative instead of an ordinary one, and then the image would be composed of numerous small raised dots of very hard gelatine, which should make a good printing surface retaining ink readily.”
sources: “Relief Images,” by Dr. Braxton D. Avis, in ‘The American annual of photography, Volume 34,’ 1920, edited by Percy Y. Howe, publ. by The American Annual of Photography, Inc., New York