Ferric Gum Photography: The Role of Ferric Ions in Crosslinking Through the Process of Carboxylate Complexation

Indianapolis, IN

Gum printing is an alternative process for reproducing photographs without using silver halides; instead naturally-occurring biopolymers are mixed with light-sensitive compounds (sensitizers) that can lower their aqueous solubility. The chemistry of this process originates in the 19th century as competing innovations sought to advance the commercial viability of photography. The most common type of sensitizer, salts containing the dichromate ion, was first reported by Mungo Ponton in 1839 and later used by Henry Fox Talbot to gel colloids of gelatin and gum Arabic. Michael Andrews more recently established a new method based on ferric chloride as the sensitizer; he produced photographs using both colloids of gum Arabic and gelatin, and referred to the interaction between the colloids and the ferric chloride as “crys-gelling”. To date, no scientific investigation of this phenomenon has been undertaken; however, one possible explanation is complexation between the ferric ions and the acidic moieties in the biopolymers. To probe this possibility we have synthesized ferric salts of glucuronic acid (the acid-containing monosaccharide in gum Arabic), and glutamic acid and aspartic acid (the amino acid residues within gelatin containing acidic side-chains). We have compared the infrared spectra of these salts to those of their parent acids to identify vibrational bands associated with the complexation of the ferric ion.