Yellow iron stain in cyanotype highlights

Owned by Break/Fix — the cyanotype that went wrong, whose second case reaches this entry’s conclusion by the same route — yellow is stain rather than fog, and a neutral or alkaline wash can manufacture it by hydrolysing iron(III) to its insoluble oxide before it has left the paper — and which names this entry as carrying the stain-removal literature. That page also records what the course looked for and could not find, a published chemical spot test for residual iron in a washed cyanotype, and offers a ferricyanide drop on a sacrificial offcut as its own derivation with the limits stated. This entry was written earlier from Ware’s monograph.

What you see

Yellow, not blue. The highlights and the coated-but-unexposed border carry a yellow or yellow-brown tone, strongest where the coating was heaviest, and it does not look like a weak version of the image.

Ware’s rule is the whole diagnosis and it takes one glance: fog is unwanted image substance and is grey; stain is unwanted other residual chemicals, especially ferric salts, and is yellow. If the tone is blue, this is not your entry — go to blue fog.

Likely causes

1. An inadequate clearing or washing procedure. In Ware’s algorithm, a stain of sensitiser after the wash points at cause 6 and at nothing else.
2. Hydrolysis in the wash. Ware notes that most acids assist clearing because a low pH prevents hydrolysis of the iron(III) to its insoluble yellow hydrated oxide — so a neutral or alkaline wash can convert a soluble salt into an insoluble stain before it has left the paper.
3. Poor absorption. Sensitiser that stayed on the surface or in the coarse pores between fibres is both easily washed out and easily left behind, depending on where it ends up.
4. Contaminated wet chemistry. Ware’s algorithm has a separate branch for this and a separate test: stain in the uncoated areas of the paper.

What is happening: the chemistry and physics

A cyanotype is developed by washing. Everything the light did not use has to leave the paper, and what has to leave is an iron(III) salt of an organic acid — soluble, and therefore removable, as long as it stays an iron(III) salt of an organic acid.

It does not always stay one. In water that is neutral or alkaline, iron(III) hydrolyses, and what it hydrolyses to is a hydrated iron oxide: insoluble, yellow-brown, and inclined to bind to cellulose. Ware makes the point in one clause — a low pH prevents that hydrolysis, which is why acids assist clearing — and the practical consequence is that a wash can create the stain it was supposed to remove.

The same substance appears by a second route. Ware records that hydrolysed Prussian blue leaves behind a yellow-brown stain of iron(III) oxide-hydroxide which may bind quite strongly to paper or fabric — which is why a cyanotype bleached by alkali is yellow as well as faint. Same stain, two histories, and the difference is whether the image is still there.

!The confirming test: three regions on one sheet

Coat a margin, mask part of it during exposure, and leave a strip uncoated. Then read the sheet:

• Yellow in the coated-but-masked border: the clearing is inadequate. This entry.
• Yellow in the uncoated margin as well: the wet chemistry is contaminated with iron, which is a different fault with a different fix — fresh baths and clean trays.
• Blue in the border rather than yellow: fogging, not staining.

Then test the wash rather than the print. Extend the wash on a scrap by a further twenty minutes in fresh water and see whether the yellow lifts. If it does, the wash was short. If it does not, the iron has already hydrolysed, and no amount of water will move it.

Ware makes the same design point for platinotype: his test targets deliberately include areas masked with Rubylith, because that is where the stain will be most evident. A print with no unexposed coated area is a print you cannot assess.

Diagnostic questions

• Where is the yellow — coated areas only, or everywhere? The single question that splits clearing from contamination.
• How long, in how much water, at what temperature? A wash in a static tray reaches equilibrium and stops working; a wash in a changing volume does not.
• What is the pH of the wash water? A hard, alkaline supply is the condition under which the hydrolysis Ware describes happens fastest.
• Was any acid used in the processing? Ware’s observation is that most acids assist clearing, and the cyanotype literature is full of dilute citric, nitric and sulfamic baths for that reason.
• Is the image itself still full strength? If the image has faded as well as the whites yellowing, the fault is alkaline bleaching rather than incomplete clearing.

Corrective action

Wash longer, in more water, and slightly acidified, and try that first because it is free and reversible.

Ware records a treatment for a bound stain and the course records it without prescribing it. For removing iron stains from platinotypes he gives immersion for about an hour at room temperature in an aqueous solution 5 per cent w/v in each of sodium dithionite and tetrasodium EDTA at about pH 9, with sodium sulphite substitutable for the dithionite at some loss of effectiveness.

Two reasons the course does not hand that over as a procedure. Sodium dithionite has no entry in the chemical encyclopaedia and no hazard record anywhere in this corpus — Rule 1 forbids the course to supply the missing assessment itself. And the bath is at pH 9, which is above the pH at which Ware reports Prussian blue being destroyed in minutes, so on a cyanotype it would take the image with the stain. It is recorded here because it exists and because a reader will meet it, not because it is being recommended.

So the working answer on a cyanotype is prevention, and a re-print.

Prevention

Get the sensitiser into the fibres. Ware’s remedy for poor absorption is a surfactant such as Tween 20 — two to five drops of a 20 per cent solution per 10 cc of sensitiser — and it addresses this fault and blotchy coating together.

Acidify the first bath, within the limits the process tolerates: under 1 per cent citric for the Classic and Simple cyanotypes, since stronger acids fog them.

Change the water. A single static tray saturates; several changes, or a slow flow, do not.

Coat a margin and mask a border on every print, so that the stain has somewhere to be seen before it is in the picture.