An Outline for the Non-Scientist

First, some necessary jargon

When iron (symbol Fe, from the Latin "Ferrum") is chemically combined with other elements, its atoms acquire a positive charge by transferring two or three of their orbiting negative electrons onto atoms of other elements. (It is a law of nature that electric charge has to balance. Electrons are the fundamental particles of negative electricity and form the outer cloud of every atom, surrounding a positively-charged nucleus).

So each iron atom ends up in one of two states:

"ferrous" iron - also called iron(II) or Fe(2+) for short,
"ferric" iron - also called iron(III) or Fe(3+).

This number 2+ or 3+ is called the oxidation state of the iron, and signifies the positive electric charge that the iron atom has acquired in the reaction.

Notation for Chemical Formulae due to ASCII format limitations:
Any number following the symbol for a chemical element, or a bracket, should be subscripted, except those numbers indicating electric charges, written here within brackets, which should be superscripted. Numbers in front of formulae remain on the line.

Oxidation is making an atom or molecule more positive (or less negative) in the electrical sense. So when iron forms compounds, it is oxidized.

Reduction is the converse: making an atom or molecule less positive (or more negative). e.g. oxygen gas, which consists of molecules containing two linked atoms of oxygen, O2, is reduced to form oxides, which contain the O(2-) ion.

Both processes involve the transfer of electrons.
Oxidation is the removal of electrons, reduction is their addition.
Iron is easy to oxidize (think of the problems of rust, Fe2O3!) and consequently is said to be electropositive. In contrast, metals such as platinum, palladium and gold are very resistant to oxidation, don't tarnish or rust, and are described as 'noble'.

All iron imaging systems have the same basis
The key compound is ferric oxalate, formula Fe2(C2O4)3 , which contains ferric iron, Fe(3+), and oxalate, C2O4(2-), chemically bound together. Under the influence of ultra-violet light, these two undergo an internal reduction-oxidation reaction:

light
2Fe(3+) + C2O4(2-) => 2Fe(2+) + 2CO2

or, in words: under the influence of light, Iron(III) plus oxalate ions goes to iron(II) and carbon dioxide gas.

(Notice how electrons have been transferred from oxalate -which is oxidised- to
iron(III) - which is reduced).

This light-induced change occurs in the dry solid ferric oxalate, and in similar compounds such as ammonium ferric oxalate; but there is only a slight color change (from pale yellow-green to pale yellow-brown), and the result is not permanent. To make a satisfactory photographic image, the iron(II) which is formed by the action of light must be reacted with something else. Iron(II) is a reducing agent because it readily gives up an electron and reverts to iron(III); so it can be used to reduce the compounds of a noble metal to the metallic state, as is described next.

The finest example: platinum printing
The traditional sensitizer consists of a mixture of aqueous (i.e. water) solutions of ferric oxalate and potassium chloroplatinite (now called potassium tetrachloroplatinate(II)), which contains platinum in the oxidation state +2. The iron(II) formed by the exposure to light is capable of reducing the platinum(II) to its metallic state (oxidation state 0):

2Fe(2+) + Pt(2+) => 2Fe(3+) + Pt

(This is a simplified version of the chemistry; the actual molecules involved are complex.)

However, this second reaction does not take place in the dry solid, because the molecules cannot encounter one-another; only when they are momentarily dissolved in water do they get the necessary mobility: they can then react to form tiny particles of platinum metal which appear black (or brown, if very small) and, when trapped in the paper fibers, constitute the final image. (The closely-related element, palladium, reacts in an exactly similar way). The traditional "developer" used to dissolve the iron(II), was a solution of the very poisonous salt, potassium oxalate, (although various other substances work, and a better modern reagent is disodium EDTA, which is short for ethylenediaminetetraacetate). This is the normal "development" process of platinum or palladium printing, described in the literature.

An alternative "printing-out" process, which I have researched, can offer some advantages. With this process, the sensitizer is now composed of solutions of ammonium iron(III) oxalate (formula (NH4)3[Fe(C2O4)3].3H2O) and ammonium tetrachloroplatinate(II) (formula (NH4)2[PtCl4]) or the corresponding palladium compound. The sensitized paper is not fully dried but allowed to acquire a controlled degree of humidity prior to exposure. At normal relative humidity - around 70% - paper contains about 8% by weight of water. Under these conditions the platinum or palladium image is formed during the exposure, and requires little or no development afterwards.

In both processes, the next step is to remove the excess unreacted sensitizer and soluble reaction products, thus 'clearing' or 'fixing' the image. The traditional clearing agent was dilute (2%) hydrochloric acid, but this tends to dissolve palladium and weakens the cellulose structure of the paper. Better alternatives are citric acid and/or disodium EDTA, both of which are effective in binding iron(III) strongly and removing it from the paper. Finally a water wash completes the processing, leaving an archivally permanent print.

There are several other iron-based printing processes
As alternatives to platinum and palladium, described above, other noble metals have been used historically, e.g. gold (Chrysotype and Aurotype) or silver (Kallitype and Argentotype) with appropriate changes in the chemistry, although the principles are the same.

Inexpensive methods are offered instead by reacting the iron(II) with ferricyanide (Cyanotype, Ferroprussiate and Blueprint processes) or gallic acid (Ferrogallate 'Ink' process). In these methods, use is made of substances that form highly-coloured insoluble products with iron(II) but not with iron(III).

Ferric oxalate is not the only light-sensitive iron(III) salt that can be used in the sensitizer: other organic salts such as the citrate and tartrate have also been employed, (e.g. in the Van Dyke and Brown print processes).

A comparison with gelatin-silver halide printing
The familiar process of developing a latent silver image in a modern commercial emulsion amplifies the light sensitivity enormously, so that ordinary photographic printing papers are about a million times 'faster' than those based on iron sensitizers. The latter can only receive sufficient exposure by using intense light sources, moreover they are sensitive only to the ultra-violet and blue portions of the spectrum; with the 'iron-based' processes enlargement is therefore not possible, so we are restricted to contact printing only. As compensation, however, a darkroom is not necessary and prints can be made under subdued tungsten lighting.

The intrinsic contrast of the 'iron-based' processes is lower than silver-gelatin; so negatives need to be made very 'contrasty', with a density range of about 1.5 for platinum and 2.0 for palladium printing.


By Mike Ware