Prussian blue bleeding into the highlights by peptisation

Owned by Break/Fix — the cyanotype that went wrong, whose fourth case treats bleeding and weak maximum density as one diagnosis read from the wash tray, and by the exposure-scale experiment, which measures what is lost. The break/fix page adds the reason a peptised colloid is not dissolution — the particles were never securely lodged inside the surface cellulose fibres — and names Ware’s remedy, a surfactant and a better-absorbing sheet, against the gross over-exposure the older literature offered. This entry was written earlier from the Prussian blue page.

What you see

Blue where the negative put none, spreading out of dense areas into the highlights next to them. Sometimes it is a soft halo around a shadow; sometimes it is a general blueing of the whole white; and sometimes it is the opposite — a light margin where the image has been eaten away next to a dense area, which Ware calls the edge etch effect.

Likely causes

Ware separates two mechanisms that the register’s title runs together, and the separation is the value of this entry.

1. Bleeding, from excess ferrous iron. His sixth shortcoming of the classic process: in heavily exposed print areas the sensitiser tends to produce excess ferrous iron which diffuses into, and stains, adjacent highlight areas with Prussian blue. That is a diffusion of a reagent during and after exposure.
2. Peptisation, from ionic strength. A quite different thing: Prussian blue itself dispersing as a colloid and being carried about by the water. Ware notes that solutions of high ionic strength tend to promote peptization.
3. Redundant pigment in the bath. Ware points out that wet processing removes large quantities of non-image pigment, and that it may bleed into and stain the picture area on its way out.
4. The paper. Ware found the edge etch effect greatly reduced or absent in papers sized with gelatin, which he suggests acts by inhibiting the mobility of the ions that cause it.

What is happening: the chemistry and physics

Bleeding is a reagent problem and the fix is stoichiometric. Where the sensitiser is not balanced, a heavily exposed area makes more iron(II) than there is hexacyanoferrate beside it to react with. The surplus diffuses, meets ferricyanide in the neighbouring highlight, and makes pigment there. Ware’s remedy in the New process is exactly what that diagnosis implies: mix the two components in chemically equivalent amounts, to avoid excess iron(II) and therefore suppress image bleeding.

Peptisation is a colloid problem and the fix is the water. The Prussian blue page records Ware’s position that all forms of the pigment are highly insoluble and that what looks like a solution is a dispersion of very fine particles. Fine particles in a bath of high ionic strength disperse; dispersed particles travel; travelling particles land somewhere else. Hence Jacquard’s own instruction — use distilled water if your tap water is hard, and wash until the water runs clear, changing it periodically.

The edge etch effect is the most instructive of the three, because Ware demonstrated exactly what it is and what it is not. It is a loss of image substance at the edge of a step next to a region of maximum density. It is invisible immediately after exposure and appears only during wet processing. It is not migration through the processing solution: it still occurs when the adjacent dense region is cut off after exposure and before processing. And — the decisive experiment — it disappears if the print is allowed to rest in the dark at normal humidity for a few hours before wet processing, because the causative ions have by then diffused to a uniform distribution and there is no longer a local effect.

So a mobile by-product of the cyanotype photochemistry promotes peptisation of the image substance. Ware says as much and does not name the ion, and neither does this course.

!The confirming test: rest the print, and change the water

Two tests, each of which changes exactly one thing Ware identified.

Rest it. Expose two sheets identically. Process one immediately and let the other sit in the dark, at normal humidity, for a few hours before processing. If the bleeding or the etch is markedly better on the rested sheet, the cause is the mobile by-product, and resting is both the diagnosis and the remedy.

Change the water. Wash one half of an exposed sheet in tap water and the other in distilled or deionised water. If the bleeding follows the tap water, it is peptisation driven by ionic strength.

And read the direction. Blue added to a highlight beside a shadow is bleeding. Blue removed from the edge of a step beside a shadow is edge etch. They have the same geometry and opposite signs, and Ware’s own tests found the etch far worse with the brown variety of the citrate than with the green.

Diagnostic questions

• Which variety of the salt? Ware’s comparison table gives the brown a distinctive edge etch and the green none.
• How hard is the water? Jacquard make this an instruction rather than a refinement.
• Was the print processed immediately after exposure? The resting experiment turns on it.
• Is the paper gelatin-sized? Ware found the effect greatly reduced or absent when it is.
• Is the wash water still blue? Redundant pigment leaving the sheet is pigment that can land on the picture, and Jacquard’s instruction to wash until the water runs clear addresses it directly.
• Is there a masked border? Ware makes the point that the unexposed but coated border is the tell-tale check on whether clearing has worked — and that if you expose the whole coated area you will never know.

Corrective action

Wash more, in cleaner water, with changes. Redundant pigment that is still in the sheet is pigment that can still move.

Rest the exposed sheet before processing if the fault is edge etch. It is free and Ware’s own experiment says it works.

Ware records a reagent that removes blue from fogged highlights and the course records its cost. Sodium hexametaphosphate at 5 to 10 per cent will strip the blue from the highlights of a fogged cyanotype — by promoting peptisation, which is the very mechanism this entry is about, so it takes density from everywhere else at the same time. Sodium hexametaphosphate has an encyclopaedia entry; the treatment is recorded, not recommended.

Prevention

Balance the sensitiser, which is Ware’s own structural answer to bleeding.

Use a gelatin-sized paper where the process allows it.

Finish the wash in distilled or deionised water, which addresses this fault and weak maximum density at once.

Coat a margin and mask a border, so that clearing can be judged off the picture.