Ferric-gum photography - Exploring the role of iron(III) salts in the hardening of gum arabic

摘要

Long before digital imaging was so common, photography relied on light-induced chemical reactions to produce images. For over a century, the photochemical reactions of silver salts dominated this market, but the reactions of other photosensitive salts containing iron or chromium have also been used to generate photographs as far back as the 1840s. These “alternative” processes have been rediscovered by modern photographers who are interested in making one-of-a-kind prints from digitally-captured images.The science of many alternative photographic processes has long been overlooked. The ferric-gum photographic development technique is such a process. It relies on the hardening of the binder in watercolor paint (gum Arabic) when it contacts paper containing a salt called iron(III) chloride. Prior to applying the paint, the paper is saturated with iron(III) chloride, then exposed to ultraviolet light through a transparency of a digitally-recorded image. This produces a faint image due to the bleaching of the iron(III) ions in the paper. Watercolor paint is now applied to the paper and a hardened film is formed where the gum Arabic contacts the unbleached iron(III) chloride. Washing the paper in water removes the unhardened paint, producing an image with much greater contrast. The process that hardens the gum Arabic has been named “crys-gelling” by practitioners, but the nature of the chemical interaction between the iron(III) ions and the molecules in the gum Arabic has not yet been unraveled.To better understand the interaction between iron(III) ions and gum Arabic, we have prepared paper samples saturated with various iron(III) salts, coated them with a suspension of gum Arabic and measured their absorbance of infrared light. This type of analysis, called infrared spectroscopy, provides us with information of the chemical bonding within the gum Arabic and any changes in that bonding produced by the presence of iron(III) ions. We have made measurements of multiple points on each sample and applied a statistical technique called principal component analysis to highlight differences in the measurements between samples. Finally, we correlate these differences to identify changes in the chemical bonding. We have, to date, studied a group of organic salts (iron(III) ammonium oxalate, iron(III) oxalate and iron(III) ammonium citrate) and a group of inorganic salts (iron(III) chloride, iron(III) bromide, iron(III) sulfate and iron(III) nitrate) and will present the results from our analysis of these samples.