Toned cyanotype

Toning a cyanotype is the only operation in this atlas that begins by deliberately destroying the image. The first bath is the standard failure mode of the process, run to completion on purpose; the second puts something else where the pigment was. The course can document the first half in detail and cannot name the product of the second, and this entry is organised around that division. The tannic acid toner carries the formula with its quantities, its two disagreeing printings, and the same gap stated at greater length.

The chemistry

The bleach is documented, and it is fast. The image substance of a cyanotype is Prussian blue, and its one real weakness is alkali. Ware reports Holtzman’s finding that a buffer at pH 9.4 completely decolourises it by irreversible hydrolysis in one to ten minutes, and that 0.25 mol/L sodium carbonate at about pH 10.7 does it in under half a minute. Household ammonia is far above pH 9.4. A bath of it does not lighten a cyanotype; it takes the image substance apart, and leaves iron in the paper.

The reason that figure matters beyond this process is that pH 9.4 is the pH of a saturated solution of calcium carbonate — the alkaline reserve routinely put into archival board. The toner and the storage failure are the same reaction.

⚗What the second bath does, and why this page will not say

The print comes out of an alkaline bath with its blue destroyed and iron in the paper, goes into a tannic acid bath, and comes out brown to black. That is the observable, and it is the whole of what the corpus establishes.

No source read for this course states what the brown-to-black substance is. The tannic acid page covers its two documented photographic roles — Russell’s tannin dry plate and its action as a gelatin hardener — and says nothing about cyanotype toning. The kit sheet gives an instruction and no chemistry.

Writing “iron tannate” or “iron gall” here would be easy, would be plausible, and would be a guess. Rule 1 forbids exactly that, so the product is left unnamed. A reader who wants to know should treat it as an open question rather than as something the literature has settled and this course failed to find.

Why the two baths are never combined. Tannic acid is an acid and the bleach is a strong alkali; mixed, they neutralise each other and neither does its job. The sheet’s mix both solutions separately is the practical form of that, and the ten-minute wash between them is the other half — it is there so that alkali does not arrive in the tannic acid.

Historical workflow

Alkali-and-tannin toning of cyanotypes is long-standing practice, and the quantities the course holds are a modern supplier’s kit sheet which credits the ideas to Jan Arnow’s handbook of alternative processes. The sequence it gives is: tone a finished, fully processed cyanotype; bleach until the colour has gone; wash ten minutes in cool water; tone until the colour you want; wash fifteen minutes in running water; dry.

Neither bath has a published time. The sheet gives two visual end points and reserves its numbers for the washes, which is a real statement about the process rather than an omission: the print can be removed at any point in the second bath, and that is where the range between brown and black comes from.

Three other toners appear on the same page and are named here for completeness rather than described: green from a 1 per cent sulphuric acid solution, and violet from either a mild borax solution or a warm 5 per cent lead acetate solution, each used until the desired colour is reached and then washed for fifteen minutes. Lead acetate is a substance this course does not hand to a reader.

⁇Two printings, one factor of ten

Two Photographers’ Formulary printings of the same toner are in the corpus and they do not agree. Both give the tannic acid bath at 20 g per litre. The ammonia bath is 10 mL of 28 per cent ammonia in 100 mL of water in one sheet and 10 mL in 1000 mL in the other — a tenfold difference in the strength of the bleach, and not a rounding.

Both are printed rather than averaged, because an average of two disagreeing printings is a third formula nobody published. What can be said is that the bleach’s end point is visual, so the weaker bath is the slower one rather than a different result — and the weaker bath is the one that gives a person time to stop.

The image

Brown through to black, per the sheet’s own heading, chosen by time in the second bath.

The blue does not come back. A cyanotype bleached in alkali and then under-toned is not a lighter cyanotype; it is a print whose image substance has been hydrolysed. The ordinary weak fading of a cyanotype is reversible — Prussian white re-oxidises in air, or in a half-minute bath of 0.3 per cent hydrogen peroxide — and this is not that.

Highlights are the fragile part, as in every cyanotype: there was least Prussian blue there to begin with, so there is least for the second bath to work on.

The paper matters and the course cannot quantify it. A cyanotype’s image sits in and on the fibres rather than in a binder, so the result depends on the sheet as well as on the baths. No source read here puts a number on that, and this page does not.

Permanence

Two statements, and the second is a caution rather than a claim.

What has been done is irreversible. Alkaline hydrolysis of Prussian blue is not the reversible fading the process is otherwise famous for. Whatever the print’s permanence now is, it is not the cyanotype’s.

Whether the toned image is more or less permanent than the blue one, this course cannot say, because it cannot name the image substance. That is an unusual position for a permanence section and it is the honest one: a permanence argument requires knowing what is being asked to last.

What still applies is the storage rule, to whatever is left in the paper. Unbuffered mounts, unbuffered enclosures, no chalk-buffered board — and, after toning, a wash that actually removes the ammonia, because a print left carrying alkali is not a finished print.

Hazards

Ammonia at 28 per cent is the hazard on this page. It is a corrosive alkali with a vapour that announces itself, worked in an open tray beside a print, and its own encyclopaedia entry carries the classification and the handling. Ventilation is the control, and the bath is the reason this process is Level B rather than the Level A the untoned cyanotype assignment reaches.

Tannic acid is a Level A material.

Lead acetate, named on the same supplier sheet for violet tones, is not offered here at any level.

And the general cyanotype rule survives into the toning tray: the print, and the water it was rinsed in, still carries ferricyanide chemistry from the original process, and must not meet a concentrated acid.

Where the course teaches it

Part XXI, in Lab: Toning the Prussian Blue Image, which follows the classic cyanotype lab and the exposure-scale experiment and is Level A — the gentlest practical page in the part, because nothing in it is a silver salt or an oxidiser.

Its design is what makes it a lab rather than a demonstration: one coating, four prints and two densitometer channels. The alkali that ruins a cyanotype is used deliberately — which is the whole conceptual move, since alkali is the pigment’s principal vulnerability everywhere else in the course — and a tannin bath puts an iron gallotannate where the pigment was. The density change is measured rather than described, in two channels, because a colour change that is only described is not a result.

The formula, with its provenance, its disagreement between printings and its unfilled mechanism, is in the formulary, which is still where a reader should go for the weights.

What this entry adds to the lab is the placement, and it is unchanged: toning is not a rescue for a cyanotype that went wrong — that is the break/fix page’s subject — and it is not reversible. It is a decision to make a different print out of a finished one.