Unlike some of the early photographic processes, gum bichromate does not have a clear point at which it was brought into being. It should rightly be considered as one of a family of colloidal pigment processes, in which colloids of large molecules are made light sensitive by the addition of a dichromate (or bichromate as it would have been known). Large molecules such as: egg albumen, gelatin and, the subject here, acacia gum (or gum arabic). These are colourless, or nearly so, and any colour is added by pigment either as a powder or as paint (normally watercolour paint).
Several later inventors of colloidal processes (such as Henry Fox Talbot, Edmond Becquerel, Alphonse Poitevin, John Pouncy and Joseph Swan), acknowledged their thinking was at least influenced by a wonderfully-named amateur Scottish scientist, Mungo Ponton. Although Ponton did not patent his technique he did publish it in Edinburgh New Philosophical Journal in 1839, which was widely circulated. Ponton observed that paper soaked in potassium dichromate was light-sensitive with gradations of tone “according to the greater or less degree of transparency in the different parts of the object.” It is not clear whether or not he understood that the size on the paper was a critical part of this effect.
There is work that predates Ponton by the French chemist, Louis-Nicolas Vauquelin (1763-1829), at the tail end of the 1700s, who wrote regarding the effect of light on bichromates. However, Vauquelin was prolific in his scientific writing and his research likely did not lead directly to photographic inventions, though Ponton would have known about it.
A few years after Ponton’s publication William Henry Fox Talbot in 1854 began work the hardening effect potassium dichromate colloidal gelatin and its direct proportionality to the amount of UV light received. Building on Fox Talbot's moderate success
Alphonse Louis Poitevin (1819-1882) continued the study of how bichromates rendered gelatin insoluble upon exposure to light. In 1856, Poitevin won the prestigious Honore d'Albert Due de Luynes; a prize of 10,00francs for the first person to describe a permamant photographic process. In fact Poitevin won the prize for two processes: the Carbon Print and the Collotype, both of which a colloidal pigment processes. Pointevin is quite rightly considered to be the father of the gum print
The gum bichromate process came into its own and was embraced by the Pictorialists somewhat later. It was viewed by its greatest proponents, such as Robert Demachy and Alfred Maskell as a process that helped add artistic texture to a print; it can give the effect of a painting or of a charcoal drawing. Demachy and Maskell wrote a book together that compared this process to an aquatint, which is an artistic etching process that allows an artist to add tone and not just line to an etched image. They coined the term Photo-Aquatint reinforcing the interrelation between their photography and art.
The attraction to the Pictorialists in the gum bichromate process is captured in the these two quotes:
‘Now the principal charm of the method of printing which we have described lies in the peculiarly soft and delicate character of the image produced .’ Maskell & Demachy Photo-Aquatint 3rd Edition 1901
‘The artist has a medium that permits the production of any effect desired. These effects are so "unphotographic" in the popular sense of that word as to be described as illegitimate by those ignorant of the method of producing them. In this process the photographer prepares his own paper, using any kind of surface most suited to the result wanted, from the even-surfaced plate paper to rough drawing parchment; he is also at liberty to select the color in which he wishes to finish his picture, and can produce at will in india-ink, red-chalk or any other color desired. The print having been made he moistens it, and with a spray of water or brush can thin-out, shade, or remove any portion of its surface. Besides this, by a system of re-coating, printing-over, etc., he can combine almost any tone color- effect.’ Pictorial Photography, Alfred Stieglitz, Scribener’s Magazine 26(5) 1899
The gum bichromate process has a strong base in current practitiioners and remains one of my favourite processes. During the printing stage you have a great deal of freedom that allows you to explore the image much more expressively on the paper, during the exposure and in the development (which is essentailly washing in water).
Photo-aquatint, or, The gum-bichromate process: a practical treatise on a new process of printing in pigment especially suitable for pictorial workers by Alfred Maskell and Robert Demachy 3rd Edition (1901) Download a PDF copy here
Pictorial Photography, Alfred Stieglitz, Scribener’s Magazine 26(5) (1899) Download a PDF copy here
Encyclopedia of the 19th Century Photography John Hannavy (Editor), Routledge (2008)
The Book of Alternative Photographic Processes by Christopher James (3rd Edition), Cengage Learning (2016)