Iron-blue toned silver gelatin print

Blue toning makes a cyanotype’s pigment inside a silver print, and it runs the cyanotype’s chemistry the other way round. There the light makes iron(II) and the hexacyanoferrate is a precipitant; here the silver image reduces the ferricyanide and an added iron(III) salt supplies the other half. Potassium ferrocyanide and Prussian blue carry the chemistry.

The chemistry

The general pattern is one reaction with the metal left blank, and Kodak’s 1928 primer sets it out. Metallic silver reduces ferricyanide to ferrocyanide; the ferrocyanide so produced then forms an insoluble coloured compound with whatever metal ion is in the bath, deposited exactly where the silver was. The primer names three: iron citrate for blue, uranium nitrate for reddish brown, and copper citrate for red. Eder worked the scheme out in 1876 by way of lead, and it applies to the whole ferricyanide family.

So a blue toner is a two-part operation in one bath: the silver image is converted to silver ferrocyanide by the ferricyanide, and the iron(III) salt then takes the silver’s place as insoluble iron blue.

⚗Prussian blue is a family, and 'soluble' is a misnomer

Ware’s account: the whole family of iron blues shares one face-centred cubic lattice that accommodates variable amounts of water and metal ions, so compositions run from KFe[Fe(CN)₆]·5H₂O — the so-called soluble Prussian blue — to Fe₄[Fe(CN)₆]₃·15H₂O, the insoluble form. Iron appears twice and in two oxidation states, bridged by cyanide, and the deep blue comes from intervalence charge transfer between them: absorption is a strong broad band near 700 nm, in the red, so what is left of white light is blue.

Ware is emphatic that soluble is a misnomer: all forms are highly insoluble, and the apparent solution is a colloidal dispersion of very fine particles. That is not pedantry. It is why a cyanotype bleeds blue into its wash water, and why a blue-toned print keeps releasing colour long after you expect it to have stopped.

What more or less of the reagent does is not what a printer expects. More ferrocyanide does not make a deeper blue: the depth follows how much silver was bleached, and a partial bleach gives partial toning. What the concentration changes is how much loose pigment ends up in the paper base rather than on the image — and loose pigment is the enemy of a clean highlight.

Historical workflow

Fix, wash, tone, wash — and the wash is where blue toning differs from everything else in this cluster.

An alkaline wash aid must not follow a blue toner. Ware’s finding for the cyanotype governs here too: alkali hydrolyses Prussian blue irreversibly, and a buffer at pH 9.4 — no more alkaline than saturated calcium carbonate — decolourises it in one to ten minutes. A print that has just been toned blue and is then put through a hypo clearing agent or an alkaline bath is a print being bleached.

The same rule follows it out of the darkroom. A blue-toned print wants unbuffered mounts and enclosures, which is the opposite of the standard conservation precaution for paper and the same exception the cyanotype carries.

The image

Prussian blue, and a conservator can find it. The Getty atlas confirms the result on real prints: a blue-toned silver gelatin photograph shows Prussian blue, identified by a higher iron concentration in the maximum-density areas than in the highlights. That is a satisfying identification because it tests the mechanism rather than just the element — the pigment is where the silver was, so the iron tracks the density.

Highlights are the risk. Loose pigment in the paper base is what a blue toner leaves when the bath is too strong, and it lands where there is least image to cover it.

Depth is set by the bleach, so control of a blue-toned print happens in the first half of the operation rather than the second.

Permanence

This is the toner the manufacturer will not make a permanence claim for, and the atlas records that rather than softening it. ILFORD’s own paper sheet warns that metal replacement toners such as blue (iron) and red (copper) may give no extra protection and the image might fade.

That is worth stating plainly beside the other three entries in this cluster. Sulfide converts the image to a compound forty orders of magnitude less soluble than silver chloride and carries Kodak’s strongest permanence wording. Selenium has an observed increase in stability and a detectable signature. Gold plates the particle with a metal that does not tarnish. Blue toning is a colour decision, not an archival one, and it adds two new vulnerabilities of its own.

Alkali. The pigment is destroyed by it, irreversibly, and archival board is buffered with alkali.

Peptization. The pigment goes on dispersing into wash water and, over time, moving where it is not wanted.

Hazards

Level B, set by the two hexacyanoferrates. Potassium ferricyanide carries the signal word Warning with serious eye irritation and aquatic toxicity with long-lasting effects, and potassium ferrocyanide is classified alongside it.

⚠The incompatibility that governs the whole family

A hexacyanoferrate holds its cyanide bound to iron rather than free — which is why the salts are Level B rather than Level D. Ware’s sentence is the one this course repeats: potassium ferricyanide “is incompatible with concentrated acids, which may release deadly hydrogen cyanide gas.”

A darkroom is a room with an acid stop bath in it. So a ferricyanide bath, a bleached print and the water either was rinsed in never meet an acid, a bleach or a heat source, and the ferricyanide waste stream is kept separate from the acid one. That rule is the same in the cyanotype, in a sepia bleach and here.

The spent bath carries iron, hexacyanoferrate and silver from the bleached image. The aquatic classification is the reason it does not go to a drain; the silver is the reason it is worth collecting anyway.

Where the course teaches it

Part XX, Toning Chemistry, in two lessons, and the second of them is where the ILFORD warning above finally has somewhere to sit.

Gold, Iron-Blue and the Other Metal Toners teaches the mechanism as deposition rather than conversion — a pigment built where the silver was — and then does two things this entry cannot. It insists on asking which “blue toner” you mean, because the name covers more than one chemistry; and it treats the alkali problem as what it actually is, a permanence consequence rather than a hazard, which is the correct place for it given that alkali destroys Prussian blue irreversibly. Copper and the red toners are handled in the same lesson, for the same reason: they are the other members of the deposition family.

Toning, Permanence and What the Evidence Actually Supports is where ILFORD’s statement belongs and where it is examined rather than repeated. That lesson’s comparison table ranks the three routes that carry a protection claim — sulfide, selenium and gold — naming the kind of evidence behind each, and then sets iron-blue and copper against them as the counter-example: the demonstration that toning does not protect because it is toning. That is a stronger placement than the ranking of four this entry originally anticipated, and it is the right one.

What a Toner Actually Does to a Silver Image supplies the frame: three routes, of which this is the third, and the observation that a toned print whose colour is a new substance behaves differently from one whose silver was converted.

And the same pigment made the other way round is a whole part. Part XXI builds Prussian blue photochemically from iron and light with no silver anywhere, and its toning lab uses the alkali sensitivity deliberately. Reading the two together is the cheapest way to see that the image substance and the route to it are separate questions.

What was already written is unchanged: the potassium ferricyanide, potassium ferrocyanide and Prussian blue encyclopaedia entries carry the mechanism, the pigment’s structure and both of its vulnerabilities.

One historical route is named here and not described. Pellet’s positive cyanotype of 1877 uses potassium ferrocyanide as a developing bath rather than a sensitiser — paper coated with gum arabic, ammonium iron(III) citrate and iron(III) chloride, developed blue only where unreduced iron(III) remains, which inverts the tonal relationship and gives a direct positive. It is a different process from this one and from the classic cyanotype, it is not a row in this atlas’s register, and the Getty atlas calls it much more difficult and delicate than the ordinary cyanotype. Part XXI’s variants lesson now treats it as well, under “Pellet’s process, and what makes a positive possible at all”.