The Importance of Humidity to Image Formation

The formation of a permanent image by the iron-based photographic printing processes takes place in two stages of chemical reaction:

First is the photochemical step, in which the UV light transforms the iron(III) carboxylate (citrate, oxalate, tartrate, etc) in the sensitizer into an iron(II) carboxylate. The visible effect of this is slight; only the shadow tones of the image become apparent, and at most the colour may change from pale green to pale yellow, or pale yellow to pale brown. The iron(II) photoproduct is not stable indefinitely, either, and the second stage of transforming it into a permanent image must follow fairly promptly if the pale image is not to be re-oxidised by air back to iron(III) and therefore lost.
The second step - a redox (red uction-ox idation) reaction- is the formation of the final image in precious metal by the reaction between the iron(II) photoproduct and the noble metal compound incorporated in the sensitizer - usually a compound of platinum, palladium, silver, or gold. The reaction between these two cannot occur in totally dry paper, however, because the chemicals are in the form of solids, static, and 'locked' into the cellulose fibres. The presence of some water is necessary to dissolve and locally mobilise the iron(II) and noble metal salts (provided they are soluble), and help them to diffuse the short distances needed to make contact with one-another within the fibres, when reaction to form the image can occur.

To provide this water, there are two main options:

to use an aqueous processing bath to 'develop' the image after the exposure
to ensure that sufficient moisture is present in the paper fibres in the first place, to enable 'print-out' during the exposure.

The meaning of 'development' here is rather different from the significance that it carries in conventional silver-gelatin processing. There is no amplification of the effects of the light, and no binder is present, the reaction occurs in solution, not to a latent image in the solid halide.

The traditional iron-based processes employing ferric oxalate (iron(III) oxalate) must be wet-developed, because the photoproduct from this salt is insoluble, ferrous oxalate (iron(II) oxalate), which needs a strong solution of oxalate ions, or other complexing agent, to solubilise it for reaction. In contrast, the print-out processes employ a salt of the ferrioxalate anion (e.g. ammonium iron(III) oxalate). The photoproduct in this case is a soluble iron(II) oxalato-complex, which is reactive enough to reduce the noble metal salt directly, without a 'developer' solution, provided that there is sufficient water present. Many printers find this a more satisfactory way to proceed, because it confers all the benefits of a print-out image, which will be considered below.

It should be realised that, in order to provide sufficient water molecules within the fibres of the paper sheet for this short-range diffusion of ions to occur, the paper does not have to be 'wet', or even perceptibly damp. Cellulose fibres naturally absorb a certain amount of water from a normal humid atmosphere. For instance, paper that has been conditioned in an environment with a moisture content of ca. 70% relative humidity (R.H.) will contain an amount of water in its fibres ca. 8% by weight, which is quite sufficient to facilitate the image-forming redox reaction, yet the sheet still 'rattles' and is, to all appearances, dry.

The benefit of allowing the image to form by print-out is that no chemicals are lost from the paper at this important stage, and a very complete and permanent printed-out image can be obtained. On the other hand, as soon as an exposed paper is immersed in a wet-processing bath - a water solution intended to 'develop' the image and 'clear' the excess chemicals - then the soluble constituents of the exposed sensitizer begin to leach out of the paper fibres into the bulk of the processing solution, where they are diluted and lost for imaging purposes. If the reaction between the photochemicals to form the image substance takes place as slowly as this washing-out of them, then the image will obviously be weak and incomplete. This is the besetting problem of the traditional platinotype process, where the chemistry of the redox reaction is intrinsically slow, and requires a developing agent to solubilise the iron(II) photoproduct (ferrous oxalate); however, as soon as the print is immersed in an aqueous developer bath, the image-forming chemicals begin to dissolve out before they can fully react, leaving a weak and fibrous or 'grainy' image. Four devices have been tried to counter this problem:

the redox reaction is speeded up by using hot developer (Willis's 'hot-bath' platinotype);
the exposed sensitizer is induced to reside longer within the fibres and resist leaching-out, by a judicious choice of sizing agents (Willis's use of alum-rosin sized paper);
a viscous but inert liquid, such as glycerin, is coated on the exposed paper to physically inhibit the leaching-out of the salts as the developer is applied (the 'glycerine development' method used by Keighley and Stieglitz, but first devised by Willis);
substances are added to the sensitizer, such as salts of mercury or lead, which enhance the reactivity of the platinum salts, but tend to change the image colour (Willis's 'Sepia' paper).

Unreacted chemicals extracted into the wet development bath may also flow back into the paper and cause problems by reacting elsewhere to stain the high values, or 'bleed' at the edges of regions of high density. For a good quality of image, making the fullest use of the chemistry of the sensitizer, it is essential to ensure that there is sufficient water in the paper fibres either before or after exposure. This may be simply introduced from the vapour by hydrating the paper sheet in an atmosphere of controlled relative humidity.

If hydration is performed before exposure, then a substantial degree of print-out results, with several attendant advantages and characteristics:

correct exposure can be found by inspection,
the process is self-masking and so accommodates a wide range of exposure without 'blocking up' the shadow tones,
negative contrast does not have to be controlled too precisely, because dense highlights can be 'printed down', accommodating a range of contrast in the negative which enables very delicate gradation in the high values,
there is no need to impose a strict method of 'contrast control' on the composition of the sensitizer, which is often deleterious to the image quality; for instance, the addition of potassium chlorate or dichromate to the platino-palladiotype can cause 'graining' and truncation of the tonal scale.

If vapour hydration is performed after exposure (but before wet-bath processing) and if the chemistry is sufficiently reactive, the image develops as the water vapour is absorbed, with the advantages of no diffusion, no loss or staining of the image, and smooth tones. The process may be continuously inspected, and the print 'pulled' when it has gone far enough. This is also what occurs in the 'breath-developed' processes.

The quantity of water present in the cellulose fibres will also have an important governing effect on the particle size of the image substance, which is a colloidal metal, and hence on the colour of the image. A large 'reservoir' of fibre water will permit greater amounts of substances to react locally and therefore make it possible for the particles of metal to grow larger. Such particles will appear more neutral in hue. A very restricted 'pool' of fibre water will constrain the chemistry to producing small metal particles only, which can show quite marked colours - brown or sepia for silver and palladium, rather than black, and even pink, magenta and blue in the case of gold. Thus, regulating the hydration of the paper controls the colour of the image.

Cellulose itself absorbs water to a degree that is well-documented, as indicated above, but the concentration of water in the paper fibres will also depend on the other chemicals present in the sensitizer, especially on the cations used for the complex salts of iron and the noble metal. A hygroscopic cation, which attracts water molecules to itself, binding them electrostatically in a primary hydration sphere, will not immediately benefit the image print-out process, because none of the water so absorbed is available initially as a 'solvent' for the iron(II) and noble metal anions. The worst possible cation in this respect is lithium, Li+. Although lithium salts are extremely hygroscopic and often deliquescent, they have to take up and bind a large quantity of water in the secondary hydration sphere, before sufficient can be usefully liberated to mobilise the other ions, which are less attracted to water. The resulting sensitizer layer has to be brought to a state almost of 'wetness', which is uncontrolled and liable to damage negatives in contact, unless a thin plastic film is interposed between them. This, of course, can degrade the image sharpness.

Lithium - and sodium to a lesser degree - are known in aqueous solution chemistry as 'structure-making' cations, whereas what is really needed here to promote aqueous diffusion is the converse: a 'structure-breaking' cation. This property can be provided by ammonium, which is similar in its molecular structure to water, and assists the absorption of water in a disordered state without binding it strongly. In consequence, only a relatively small amount of water need be absorbed in the presence of ammonium cations to provide a print-out process.

The difference in hydration behaviour between lithium and ammonium cations can be readily experienced in practice: when lithium chloride is dissolved in water, the mixture becomes hot. The dissolution reaction is exothermic, driven by the spontaneous evolution of heat energy (enthalpy) due to the 'structure making' of bonds between lithium ions and water molecules. In contrast, when ammonium chloride is dissolved in water the mixture becomes cold; here the endothermic reaction proceeds spontaneously, in spite of the need to absorb heat from the surroundings, because it also entails a large, but unseen and unfelt, increase in the entropy (disorder) of the system, due to the 'structure-breaking' effect. Deeper discussion of these factors requires some acquaintance with the concepts of the Second Law of Thermodynamics and the Gibbs Free Energy function.

The inappropriateness of lithium as a cation for these processes becomes obvious if it is added to a pure platinotype sensitizer, where it will totally inhibit the print-out process. Sodium as cation also works very poorly for print-out platinotype. Only with the more energetic and facile metal redox processes - palladium and gold - can these cations be used successfully, but high degrees of hydration may be called for, with increased risk to the negative. Moreover, using these cations at low R.H. can cause the image quality to deteriorate, and give rise to 'solarization' on development - a reversal of the tonal scale in regions of maximum exposure.

It should be noted that the larger alkali metal cations, potassium, rubidium and caesium, while less hygroscopic than lithium or sodium, are also unsuited to the iron-based processes, but for a different reason: they tend to form salts of low solubility with either the ferrioxalate anion or the complex tetrachlorometallates of platinum, palladium, and gold. Low solubilities mean low sensitizer coating weights and consequently weak images, unless double coating is employed, which often proves counter-productive. If the inconvenience of hot, or supersaturated solutions is resorted to, in an attempt to overcome this disadvantage, then crystallization is likely to prove a problem.

Calcium is not a good choice for cation either, because of the low solubility of calcium oxalate (one of the causes of gall and kidney stones!). Its presence in the sensitizer will tend to promote the hydrolysis of the ferrioxalate anion, with the formation of 'iron stains' of ferric hydroxide.

To sum up: there are very good scientific reasons why ammonium should be the cation of choice in the print-out iron-based processes, if the quality of the image and the ease of working are held to be of paramount importance.