Cyanotype bleached by alkaline wash water or buffered paper

Owned by Break/Fix — the cyanotype that went wrong, whose third case carries the same numbers from Ware — a buffer at pH 9.4 decolourises Prussian blue in one to ten minutes, and pH 9.4 is the pH of a saturated calcium carbonate solution — and adds the arithmetic that makes a buffered sheet a reagent rather than a background: a 250 g/m² permanent paper carries an eight-fold molar excess of alkaline reserve over the iron a normal coating deposits on it. That page names this entry as carrying the partial-restoration literature. It was written earlier from the Prussian blue page, from the same source.

What you see

A blue image that has gone pale, or gone entirely, leaving a yellow-brown stain where the deepest tones were. In the acute case it happens in the wash, in front of you, in minutes.

The two halves together are the diagnosis. A cyanotype that has merely faded in light goes pale and largely comes back in the dark, because light fading is substantially reversible. A cyanotype hydrolysed by alkali does not come back, and it leaves iron behind.

Likely causes

1. A buffered paper or board. This is the commonest and the cruellest, because the buffer is there on purpose and is sold as archival.
2. Alkaline wash water, or any bath that raised the pH.
3. An alkaline mount, mat or enclosure in contact with the print in storage.
4. A deliberate alkaline treatment, including baths recommended for other processes.

What is happening: the chemistry and physics

Alkali hydrolyses Prussian blue, and the reaction is irreversible. Ware reports Holtzman’s investigation with the number that makes this entry urgent: a buffer at pH 9.4 completely decolourises Prussian blue in one to ten minutes, depending on how the pigment was prepared.

Then he puts that number in context, and the context is the whole problem: pH 9.4 is the pH of a saturated solution of calcium carbonate — the buffer commonly incorporated in archival papers and boards. A stronger alkali is worse: 0.25 molar sodium carbonate, at about pH 10.7, destroys the Prussian blue of a cyanotype in less than half a minute.

Ware’s own comparison at pH 9.4 for ten minutes measured losses across five historic sensitiser formulations and found every one of them lost between half and three-quarters of its shadow density. No variety of the process is safe from this; the differences are of degree.

What is left behind is iron. Hydrolysed Prussian blue leaves a yellow-brown stain of iron(III) oxide-hydroxide which may bind quite strongly to cellulose — which is why the stain of this fault and the stain of incomplete clearing are the same substance arrived at from opposite directions.

!The confirming test: a drop of alkali on a corner, and a pH reading

Test the paper, not the print. Wet a corner of the same paper stock and read it with a pH meter or with indicator paper. Anything alkaline is disqualifying for this process, and a chalk-buffered sheet reads distinctly so.

Then test the causal link on a scrap. Coat, expose and process a test strip; cut it in two; leave one half alone and immerse the other in a mildly alkaline solution for ten minutes. If the immersed half loses density and yellows and the control does not, the diagnosis is complete.

And distinguish it from light fading, which is the fault it is most often confused with. Put the faded print in the dark for some days. Light fading is substantially reversible and regains; alkaline hydrolysis does not regain at all.

Note the warning IPI attach to the whole category: acid-free, archival, museum-quality and conservation board are marketing terms that are neither standardised nor legal. The test that means something is ISO 18916, the Photographic Activity Test — and even a print that passes it may be on a buffered board, because buffering is a feature rather than a defect for every other process.

Diagnostic questions

• What is the paper, and is it buffered? Most fine-art papers are. That is the point of them and it is fatal here.
• What is the wash water’s pH? A hard supply is usually mildly alkaline.
• What is the mount and the mat? An archival board with an alkaline reserve is the standard product.
• Did the loss happen in the wash or over months? Minutes points at a bath; months points at the enclosure.
• Is there a yellow-brown residue where the blue was? That is the hydrolysis product, and it is corroboration.

Corrective action

A freshly bleached cyanotype can sometimes be partly restored, and the course records the limits. Ware reports some evidence that a freshly bleached print can be partially restored by dilute acid — hydrochloric or nitric — with added ferrocyanide; but that over time the iron oxide-hydroxide hydrolysis product becomes insoluble in dilute acid and cannot regenerate Prussian blue, and he calls the area one that needs further research.

So the honest position is: early, partial, uncertain, and using reagents the course would want assessed first. It is recorded because it is what the literature says, not as a procedure.

Get the print off the alkaline material immediately, which is the only action that certainly helps.

Re-print on an unbuffered paper.

Prevention

Unbuffered paper, unbuffered mount, unbuffered enclosure. This is the one photograph in the whole course whose storage requirement is the opposite of the standard archival recommendation, and the Prussian blue page says so in its storage field.

Acidify the sensitiser or pre-treat the paper where the stock demands it; Ware’s simple-cyanotype instructions treat chalk-buffered papers explicitly rather than rejecting them.

Check the wash water, and use a mildly acidified first bath within the limits the process tolerates.

Ware records the nickel(II) protection and the course records its price. Soaking Prussian blue in a nickel(II) salt protects it against pH 9.4 by about an order of magnitude — and in Ware’s own tests shifted the hue towards green and cost 10 to 21 units of mid-tone density to peptisation. A conservation option with a stated cost, not a darkroom step.