Lab: Toning the Prussian Blue Image

Every toner in Part XX had a metal to work on. Sulfide, selenium and gold each begin with a silver image and end with a different compound of the same silver atoms, sitting where those atoms sat. A cyanotype offers nothing of the kind: no silver, no noble metal to plate over anything, no halide to rehalogenate. The image is a pigment, and the only reaction that reaches it quickly is the one this part has spent five pages telling you to avoid.

So the toner takes the picture apart first. A weak alkali hydrolyses the Prussian blue, leaving iron in the paper and washing the cyanide half away as ferrocyanide; a bath of tannic acid then builds an iron gallotannate on what is left — the same class of substance as iron gall ink. Two moves rather than one, and both yours to stop wherever you like.

The interesting part is what it costs. Everyone assumes a bleach-and-rebuild loses density; Mike Ware writes that the treatment “generally intensifies the image”. This session’s product is not a brown print but a table of numbers on either side of a conversion, taken on a step tablet, against a control that went through the same trays and the same water and no chemistry at all.

Purpose

To convert a finished cyanotype from blue to purplish-brown by Ware’s published two-bath route, to stop the first bath early on a second print and skip it entirely on a third, and to measure what each of those did to a step wedge print — patch by patch, before and after, on more than one densitometer channel, against a control that received only water.

Three reasons this is a lab and not a demonstration.

It is the only indirect toning in the course whose first bath is irreversible and whose second cannot reach past it. A sepia print bleached too far can still be redeveloped; here the ferrocyanide has gone down the drain and the highlights with it. It is the cheapest way to watch a spectral-condition problem wreck a conclusion, since a blue image and a brown one do not absorb in the same place. And it is where the course has to say what it does not know: no measured permanence figure for a tannin-toned cyanotype exists in anything read here.

This page does not restate the formula: the tannic acid toner entry carries the supplier’s version with its provenance and its two disagreeing printings, and the toned cyanotype entry carries the identification question.

Learning objectives

By the end of the session you will be able to:

• Say what a toner can and cannot do to a pigment image, and why a cyanotype has to be destroyed before it can be toned at all.
• Run an alkaline hydrolysis to a chosen end point rather than to completion, and predict from the chemistry what stopping early leaves behind.
• Name the product of the second bath and say which parts of that naming are established and which are not.
• Choose between three published strengths of the same two-bath toner and justify the choice.
• Characterise a tannin source you did not buy, or decline to use one, on stated grounds.
• Measure a density change across a colour change and explain why the channel has to be reported with the number.
• State what is known about the permanence of a toned cyanotype without overstating it in either direction.

Prerequisites

• Lab: mixing, coating and printing a classic cyanotype — you need four finished prints from it, made in one coating and one exposure. Read the Preparation section below before you make them, because two of the four are step tablets.
• What a toner actually does to a silver image — the vocabulary of direct and indirect toning, and the argument that colour moves because the substance moved. Everything in it applies here except the substance.
• Iron, light and Prussian blue — the hydrolysis is derived there and is used here. Its alkali warning is this page’s first bath.
• Experiment: the exposure scale, maximum density and contrast if you have run it: its channel ruling and its error budget are reused here without re-argument.
• Concentration and dilution, for per cent w/v against per cent v/v, which this page uses in the same sentence more than once.

Safety classification

Level A, and the letter is a consequence of one decision made deliberately.

The classification rubric assigns a level to an operation rather than to a substance. The operations here are four dilute baths at room temperature on a washed sheet of paper: 5 per cent w/v sodium carbonate, 1 per cent v/v acetic acid and 1 per cent w/v tannic acid, all of them Ware’s own figures, the first and third being Level A materials on their own pages. Nothing is heated and nothing is volatile at these strengths; there is no ultraviolet, no mains equipment and no darkroom, Bostick and Sullivan stating plainly that cyanotypes are toned under normal room lighting with no safelight.

The decision that keeps it at A is the choice of alkali. Ware’s first-named bleach is 1 per cent v/v ammonia, and ammonia solution is a Level B material whose hazard is its vapour over an open tray. In the same sentence he names the substitutes, one of which is ordinary washing soda at about 5 per cent w/v; Bostick and Sullivan reached the same family independently, their Solution A being a 14 per cent potassium carbonate solution. A chartered chemist and a supplier selling a kit both bleach cyanotypes with a carbonate, so the course takes the carbonate route as the default and does not publish the ammonia bath as this session’s procedure. Choose ammonia anyway and the session is Level B, with its controls on the ammonium hydroxide page rather than here.

⚠One raised step, declared: making the acid stock

The 1 per cent v/v acetic acid bath is diluted from concentrated acid, and the dilution is a Level B operation inside a Level A session, which the rubric permits provided the page names the step and its controls rather than leaving them to be inferred.

Make it once, ahead of the session, not at the bench with prints in the trays. Concentrated acetic acid is corrosive to eyes, skin and the respiratory tract, has a flash point of 39 °C, and its international chemical safety card notes that a harmful contamination of the air can be reached rather quickly on evaporation at 20 °C. Splash goggles and gloves, a ventilated space, acid into water and never the reverse, and the stop bath preparation SOP for the sequence. Five millilitres of the concentrated acid makes the 500 mL this session needs.

Where that step is not available to you, the Alternative route below says what to do instead and what it costs.

⚠What is not a hazard here, and why saying so is part of the assessment

There is no hexacyanoferrate in these trays, if the print was washed properly. The whole acid rule of the coating lab exists because unreacted potassium ferricyanide sits in the sensitiser and in the first wash. By the time a print reaches this page it has been through a wash judged by the disappearance of the yellow, and what remains is pigment in paper. That is why an acid bath can appear in this session at all when it is forbidden in the previous one.

Two qualifications, which are why that sentence is conditional. The alkaline bath releases ferrocyanide from the image itself — that is the hydrolysis, not a contaminant — so the bleach tray and its rinse do carry a hexacyanoferrate by the end of the session, and the acid tray must never be poured into the bleach waste. And an under-washed print carries the original chemistry forward into your alkali. Both are handled under Waste streams.

There is no free cyanide and no hydrogen cyanide. Prussian blue is stable to acids of moderate strength down to about pH 1 or 2, below which Ware records some risk of release; the strongest acid in the room is a 1 per cent solution of a weak one. The Environment Agency’s waste guidance draws the same distinction in regulatory language, excluding complex cyanides such as ferrocyanides and ferricyanides by name from the statement it assigns to cyanide salts.

There is no heavy metal. Four published cyanotype toners work by walking a heavy-metal cation into the lattice; none is in this session, and the exclusion is argued below.

Hazards

Hazard	Where it arises	Control
Serious eye irritation from carbonate solution	Mixing and the bleach tray	Splash goggles. ECHA’s aggregation gives sodium carbonate H319 in 99.8 per cent of classifying reports; Japan’s NITE-CMC and Safe Work Australia go further to H318, serious eye damage, and the course takes the union of those views
Dust of either solid	Weighing 25 g of carbonate and 5 g of tannic acid	Weighing SOP, no sweeping motions, window shut for the minute you are weighing
Eye irritation from tannic acid, on a disputed classification	Mixing and the toner tray	Goggles. PubChem’s ECHA aggregation gives H319 from 82.4 per cent of classifying notifiers; the Spectrum sheet republished by Bostick and Sullivan states the substance is not dangerous under GHS and carries no H-number. Protect to the stricter
Corrosive concentrated acetic acid	Only when making the 1 per cent stock, outside the session	The declared raised step above, and the stop bath preparation SOP
An acid and an alkali in adjacent trays	The whole procedure, by design	The two rinses, which are not optional, and never combining the spent baths
Permanent brown staining of skin, clothing, bench and tray	The toner tray and every wet print leaving it	Gloves, apron, a tray under everything, and a dedicated toning tray
Ammonia vapour	Only on the ammonia route this page declines	Then the session is Level B: read ammonium hydroxide and ventilate for the vapour

Required PPE

Chemical splash goggles for mixing and for the bleach tray. The strongest hazard statement here is an eye one and it belongs to the alkali; the highest-risk moment is a 25 g weighing being tipped or a 5 per cent solution being poured. Ordinary safety spectacles are enough once four trays stand full and you are moving a print between them.

Nitrile gloves throughout. HSE’s COSHH essentials sheet takes single-use nitrile at 0.2 mm as splash protection where the safety data sheet gives no more specific advice, which is the position for all four solutions. Change them if one goes from the tannin tray into the carbonate — the point is the carry-over rather than your skin.

An apron, and not optional in the way an apron usually is. Tannic acid stains textiles brown and the stain does not wash out; the same is true of the bench, so work on a lipped wipeable surface.

No respiratory protection is specified, and the reason is worth stating rather than implying: none of the four working solutions is volatile at room temperature, and the only respiratory statements in the reading belong to the concentrated acetic acid and the ammonia bath, neither used at the bench here. Dust control at the balance is housekeeping under Princeton’s guidance on powders rather than compliance with a classification.

Ventilation

The control here is separation and rinsing, not airflow. Work in a room with the ordinary through draught HSE ask for wet photographic work. Nothing in the four baths produces a vapour at these strengths: sodium carbonate solution is a medium-strong base with no vapour hazard on its safety card, a 1 per cent acetic acid bath is far below the concentration its card is written about, and tannic acid is a solid dissolved in water.

What ventilation would be for, on the other route. The ammonia bath Ware names first is an open tray of a solution that releases ammonia gas as soon as it is poured, and there the control genuinely is air movement under the ventilation check SOP. Choosing the carbonate is choosing not to need it — a design decision rather than an absence.

Materials

• Four finished, fully washed, dry cyanotypes from one coating and one exposure, two carrying a step wedge and two pictorial. Sheets coated on different evenings are different papers as far as this measurement is concerned; Preparation says how to make them comparable.
• One further print, sacrificial, cut into six strips for the end-point trial.
• Five trays larger than the print, none of them metal or chipped enamel.
• Distilled or de-ionised water, about 4 L, and it matters more here than in most sessions: Ware reports Ian and Angela Moor’s densitometry of a 15-minute wash losing an average 18 per cent of image density in tap water at pH 7.5 to 8.5, 4 per cent in distilled and nothing measurable in de-ionised water at pH 6.3. Alkaline tap water bleaches the print you are trying not to bleach yet.
• Blotting paper, drying screens, a soft pencil, labels, a timer and narrow-range pH papers.

Chemicals

The formulation is Ware’s, not the course’s, and it is not re-derived here. The concentrations below are exactly those he publishes in his own workshop notes and in Cyanomicon §8.2 — a formula published by the person who published the process, which is the strongest evidence the course recognises for a formula of this kind. The volumes are halved to 500 mL because a print of 8 × 10 inches does not need a litre, and halving a volume leaves a concentration alone.

Bath	Chemical	Quantity	Form
1. Bleach	Sodium carbonate, anhydrous	25 g	White powder, to make 500 mL. Ware allows the hydrated form; if you use washing soda decahydrate you are weighing mostly water and the bath is weaker, so record which you used
2. Neutraliser	Acetic acid	5 mL of the concentrated acid	Made up to 500 mL, giving 1 per cent v/v. Made ahead of the session — see the declared raised step above
3. Toner	Tannic acid	5 g	Light tan powder, to make 500 mL. Ware’s own note names it properly as gallotannic acid
4. Rinse and wash	Water	about 4 L	Distilled or de-ionised for the rinses, running tap water acceptable for the final wash if it is at or below pH 7

Named and not used in this session: ammonium hydroxide at 1 per cent v/v, which is Ware’s first-listed bleach and would raise the level; sodium hydroxide or potassium hydroxide at 1 per cent w/v, which he also names; calcium carbonate as lime water and borax, which he names as further alternatives; and gallic acid, which he gives as an equal alternative to tannic acid in the second bath and which carries a heavier classification — H315, H318 and H335 across the ECHA aggregation, against tannic acid’s H319 alone.

🧮Three published strengths of the same toner, in the same units

The three sources do not use the same units, so here they are converted to one. This arithmetic is the course’s own, from figures each party published.

50 g ÷ 1 L = 5 % w/v carbonate · 10 g ÷ 1 L = 1 % w/v tannic acid

Ware, both baths

14 % × (50 ÷ 1000) = 0.70 % w/v potassium carbonate

Bostick and Sullivan, bleach: 14 % stock, 50 mL into 950 mL

40 % × (50 ÷ 950) = 2.1 % w/v · 40 % × (100 ÷ 1000) = 4.0 % w/v tannic acid

Bostick and Sullivan, toner: 40 % stock, 50 to 100 mL into 900 mL

10 g ÷ 0.5 L = 2.0 % w/v tannic acid

Photographers' Formulary, toner: 10 g in 500 mL

Read the pattern rather than the numbers. The tannin runs from 1 to 4 per cent across three independent publishers and Ware is the mildest. The alkali runs the other way: his carbonate is about seven times the strength of the supplier’s working bleach — consistent with a supplier writing a three-minute timed bath and a chemist writing a five-minute one judged by eye, though the two salts are not the same salt, so this compares strengths and not strictness and settles nothing about which bath is better.

And one coincidence worth recording. Photographers’ Formulary print their ammonia bleach two ways, 10 mL of 28 per cent ammonia in 100 mL of water and the same 10 mL in 1000 mL, a tenfold disagreement the formulary page sets out and declines to average. The weaker printing works out at 0.99 per cent v/v — Ware’s 1 per cent to within the width of the figure. That is not proof that the weaker one was intended, but it is evidence, and it is the kind a course should say out loud when it finds it.

Equipment

Nothing metal. No tongs, no steel-sprung clips, no chipped enamel. An iron-and-tannin chemistry is exactly the one that finds a trace of iron and prints it: the blue-black of iron gall ink is what a tannin bath does to a rust spot, and it will do it on your print.

A timer you can read wet, because five of the seven steps are timed and two are thirty seconds.

A reflection densitometer with two channels available. Part XV’s head as built carries matched green LEDs and a blue one for the superposition test, and the exposure-scale experiment rules that a red pair be fitted for cyanotype work because Prussian blue absorbs in one broad band centred near 700 nm. That ruling stands for the untoned print and fails for the toned one, which is the subject of the Analysis section. Whatever two channels you have, use both, on every patch, on every print, and record each one’s dominant wavelength and half-width from its datasheet.

Drying screens rather than a line, because four prints have to dry together in the same air if their next-morning readings are to mean anything.

Estimated cost

Band £, and the least expensive session in Part XXI. The capital — trays, a timer, a densitometer — is already yours by this point in the course. What this session adds to the shelf is one jar of tannic acid: the carbonate came with Part VIII’s developers and the acetic acid with Part X. The planner carries the numbers.

Estimated consumables cost

Every figure with a number is the planner’s dated UK price and every quantity comes from the Materials and Chemicals sections above.

Consumed	This session	Sourced price	Cost this session
Nitrile gloves	3 pairs	£6.64 to £14.99 per box of 50 to 100	£0.40 to £0.90
Sodium carbonate, anhydrous	25 g	£7.20 per 500 g	£0.36
Tannic acid	5 g	Not priced, and not among the gaps src/data/prices.json names: this page names a new one	—
Acetic acid, concentrated	5 mL	Not priced at this strength. The file prices acetic acid at 80 per cent, £11.89 a litre, and names the glacial reagent this bath is diluted from as a gap of its own	—
Distilled or de-ionised water	about 4 L	Not priced	—
Cyanotype prints, 8 × 10 in	5 sheets	Made in the previous lab, whose own table records the paper as unpriced under the file’s hot-press cotton watercolour paper gap	—
Blotting paper	4 sheets	Not priced	—

The priced rows come to about £0.76 to £1.26, a floor rather than a total: five of the seven consumables have no sourced price, including the one substance the session exists to use. What the table does establish is a ratio — at Ware’s 1 per cent a 500 mL bath holds 5 g of tannic acid, so a 100 g jar is twenty baths, and the paper under it costs more than the chemistry every time. Equipment is deliberately absent: a tray is not consumed and neither is a densitometer.

Waste streams

Four streams, kept apart for a chemical reason rather than a tidy one.

• The spent bleach and its rinse: a dilute alkali carrying hydrous iron(III) oxide as a fine yellow-brown suspension and ferrocyanide in solution, the cyanide half of the image substance just taken out of the print. This is the stream that matters.
• The spent acetic acid bath, a dilute weak acid carrying whatever alkali it neutralised.
• The spent toner, a dark dilute plant polyphenol with iron in it.
• The wash water, dilute everything, and the largest volume by far.

The bleach waste and the acid waste do not meet — not in a tray, a bottle or a drain trap. A carbonate and an acid react violently, generate heat and evolve carbon dioxide, which is a nuisance in a sealed container and worse in a full one, and the bleach stream is the one carrying hexacyanoferrate. Label each container under the labelling SOP and follow the general chemical waste SOP.

Alternative route

Four readers cannot follow this page as written, and one thing here has no alternative at all.

You cannot handle concentrated acetic acid. Buy a ready-diluted acetic acid solution at a stated strength if you can, which removes the raised step and changes nothing else. Otherwise omit the bath and lengthen the rinse, and know the price: an alkaline tannin solution darkens rapidly in air, the alkali carried in by the print is what starts that, and Ware’s note is that the toned colour is very sensitive to alkali. Record the omission, because it makes your prints incomparable with anyone else’s. Do not substitute citric acid, though this part uses it everywhere else. Citrate is a strong chelator of iron(III) — it is the ligand of the sensitiser you coated in the first place — and at that moment the image is iron(III) hydroxide with nothing holding it but the paper.

You cannot get tannic acid. Gallic acid is Ware’s own equal alternative at the same 1 per cent, and being a single defined compound rather than a botanical extract it is the easier substance to reason about. It carries a heavier GHS classification, so the session becomes Level B by the chemical rather than by the operation. A plant infusion is the third option and a different kind of experiment; the tannin-sources section below says what you would have to do to make it one.

You cannot read reflection densities. Compare visually and say what it is: prints dried together, viewed together under one light, ranked. That establishes an ordering and not a difference of 0.08, so every quantitative claim here is unavailable to you and what goes in the notebook is the ranking and the word “visual”.

You cannot make a cyanotype at all, for want of ultraviolet or a place to coat. Pre-coated cyanotype paper is sold ready to expose, the planner prices it, and every word here applies to a print made on it — the toning has no idea how the blue got there. Take all four sheets from one pack, because bought paper varies between them.

And the one thing with no alternative. There is no way to tone a cyanotype without destroying the cyanotype: not a shorter time, not a milder bath, not a different tannin. Every route in the literature to a substantially different colour begins by hydrolysing the pigment, and the reversal window closes within days. If the print in front of you is the only one and you would regret losing it, the answer is not a gentler procedure — it is to print it again and tone the second one.

Preparation

The four prints, which are made in the previous session

This is the part that goes wrong if it is read late. Prints that are to be compared must differ in nothing except what you do to them today, so all five come from one coating batch, one mixed sensitiser, one drying, one exposure and one processing run in the coating lab.

Two carry a step tablet and two carry a picture — the tablets are the measurement, the pictures tell you whether it means anything to your eye. Mark all four on the back in pencil before exposure and leave the coated-but-masked border the coating lab asks for. The fifth sheet is sacrificial, cut into six strips for the end-point trial.

Read every print, dry, before it is toned — not on the day it was washed, because a cyanotype goes on gaining density by aerial re-oxidation for hours, but the next morning, on both channels, patch by patch. Those readings are half your data and cannot be recovered afterwards.

The end-point trial, which is the number nobody can give you

Ware’s step 2 says five minutes “until the Prussian blue image is bleached to pale yellow”. Bostick and Sullivan time their much weaker bath at three minutes and describe the colour going bright blue, then red, then fading quickly, with the deepest shadows sometimes never fading entirely. Neither is a time for your print, whose density, paper, sizing and formulation are all yours.

So before any print you care about goes in, put the six sacrificial strips through the bleach and pull them at 30 s, 1, 2, 3, 5 and 8 minutes. Rinse, dry, look. You want two numbers: t_full, at which the blue has gone to a pale yellow-brown everywhere, and t_part, roughly a third of it.

The bench, and the written plan

Five trays in a row in the order of use, filled before the first print is wet, blotting paper down, the two acid trays at the far end from the alkali with a rinse between them, so that a mistake requires walking rather than reaching. Write the plan out first, one line per print:

Print	Bleach	Neutralise	Tone	What it is for
T — step tablet	Full, to t_full	Yes	5 to 10 min	The full conversion, and the main measurement
P — picture	Partial, to t_part	Yes	Same as T	Split toning: the second bath can only reach where the first went
N — picture	None	No	Same as T	The one-bath route: tannin on intact Prussian blue
C — step tablet	Water, for t_full	Water, 1 min	Water, same time	The control. Same trays, same times, no chemistry

The control is why this session produces a number rather than an impression. A cyanotype loses image substance to plain water by peptisation — Ware’s figures from the Moors are an average 18 per cent lost in fifteen minutes of tap water. Print C measures what today’s trays and water cost a print that received no toner, and every difference you claim for T is a difference from C rather than from T’s own starting density.

Procedure

Seven steps, from Ware §8.2, and four prints put through them differently. About two hours at the bench for the set if the washes overlap; the two densitometry passes are the previous morning and the following one and are timetabled separately.

The session, end to end

1. 10. Read all four prints dry, both channelsthe previous morning — half the data, and unrecoverable later
2. 21. Mix four baths at 500 mL20 min — carbonate, acid (made earlier), tannin, and water
3. 32. End-point trial on six strips15 min — find t_full and t_part on your own prints
4. 43. Print T, the full sequencepresoak, bleach to t_full, rinse, acid, rinse, tone, wash
5. 54. Print P, bleached to t_part onlythe same sequence with step 2 curtailed
6. 65. Print N, tannin onlyno bleach, no acid: straight from presoak to the toner
7. 76. Print C, the controlwater in every tray, for the same times as T
8. 87. Wash all four together, dry together20 min, then screens overnight in the same air
9. 98. Read all four again, both channelsthe next morning, same instrument, same anchoring

Stage 1 — Mix the four baths (20 minutes)

Weigh 25 g of sodium carbonate, dissolve in about 400 mL of water and make up to 500 mL under the mixing to a final volume SOP; the same for 5 g of tannic acid. The acid bath was made earlier. Fill the rinse and wash trays at room temperature and label everything, including the water. Note the colour of the tannin solution as it goes in, because you will want to know later whether it darkened.

Stage 2 — The end-point trial (15 minutes)

The six strips, as set out in Preparation. Do this even if you toned a cyanotype last month: a different paper, exposure or jar of citrate moves the end point.

Stage 3 — Print T, the full sequence

Ware's seven steps, with his times

Print T

Presoak 1 minBleach ca. 5 minRinse ½Acid 1 minRinse ½Tone 5 to 10 minWash 20 min

015 min30 min

Ware's own times. The bleach is the one you replace with your measured t_full; every other figure is his.

1. Presoak the dry print in water for 1 minute. A dry sheet takes up the bleach unevenly, and this is why prints come out evenly toned rather than mottled.
2. Bleach in the 5 per cent carbonate for t_full, agitating gently and continuously. The blue goes through a curious reddish stage on its way out and the image then collapses to a pale yellow-brown ghost. Stop when the blue has gone and not later.
3. Rinse in water for 30 seconds.
4. Neutralise in the 1 per cent acetic acid for 1 minute, to take out alkali the rinse left behind.
5. Rinse again for 30 seconds.
6. Tone in the 1 per cent tannic acid for 5 to 10 minutes. The image comes back darker and browner over the first minute or two and keeps going. You may lift the print out at any point, and that is where the whole colour range lives.
7. Wash for 20 minutes.

Stage 4 — Print P, curtailed at step 2

Identical, except that the bleach stops at t_part. Ware’s note is that curtailing that bath can yield interesting split-tone effects, and the reason is mechanical: the second bath can only build where the first one destroyed, so the shadows still holding Prussian blue stay blue while the mid-tones and highlights go brown.

Stage 5 — Print N, the one-bath route

Presoak, then straight into the tannin for the same time as T, then wash. No bleach, no acid.

!What this arm is and is not

No source read for this course publishes a tannin-only toning procedure for cyanotype with quantities and times. What the corpus supports is narrower. Ware records that the beverage toners of §8.3 — teas, coffees, red wine — “have all been used both with, and without preliminary bleaching and hydrolysis”; that Mercer, working with madder rather than tannin, found leaving the Prussian blue unhydrolysed gave a very rich purple; and that the best purplish-browns are said to result from the reverse order, tannin first for a few minutes, rinse, then the alkali, and optionally a brief return to the tannin.

So this arm is the course’s own arrangement of Ware’s published bath, at his strength, with the steps he publishes around it omitted. It is a legitimate experiment and not a published formula: label its result as your own observation, and do not call it “the one-bath tannin toner” as though it had a provenance.

The reasoning is the course’s, not a source’s: nothing has been destroyed, so nothing can have been lost. The Prussian blue is still there and still blue, and whatever the tannin does to iron the paper holds outside the lattice is added to that rather than substituted for it. The prediction is a smaller colour shift and a smaller density change than T; your numbers will say whether it survives.

Stage 6 — Print C, the control

Through all five trays, in order, for exactly the times print T received, with water in every one of them. This print measures the session rather than the toner.

Stage 7 — Wash together, dry together, read tomorrow

All four into the final wash together for the last twenty minutes, then onto screens in the same room. A print read in different air from its own control is not a control.

Expected observations

In the bleach: blue, then red, then a ghost. The reddish intermediate is a partly hydrolysed Prussian blue as the lattice comes apart, and it is fast. Bostick and Sullivan’s much weaker bath takes two to three minutes to reach the ghost and their sheet warns that the darkest areas may not fade entirely; Ware’s stronger carbonate will be quicker, and the shadows are still the last to go.

The ghost is yellow-brown and it is iron. Ware names it ferric hydroxide, more accurately a hydrous iron(III) oxide gel. It is the image, in that it is in the same places in the same relative amounts. It is not the picture, because it has almost no density.

In the toner: the image returns within seconds and keeps deepening. Bostick and Sullivan describe their kit’s sequence as salmon, then eggplant, then a deep burnt magenta after several minutes; Ware describes a rich purplish-brown. Different colours from the same class of chemistry, and the sources differ in more than one variable at once — a different alkali salt at a seventh of the concentration, a tannin two to four times stronger, unknown papers. Which of those moves the hue is not established by anything read here, and it is the first of the further experiments.

Staining of the paper base. Ware names it flatly as a problem in both places he publishes the procedure. Expect the white to warm; purified tannic acid stains less than an infusion.

On print P, two colours, with the boundary wherever the bleach stopped. On print N, a small shift or nearly none — and nearly none is a result, not a failure.

Overnight, less change than you are used to. An untoned cyanotype gains density for hours after the wash as Prussian white re-oxidises in air; a print whose Prussian blue has been destroyed has nothing left to re-oxidise. Print T should be stable by the evening while print C goes on moving, and that difference is the clearest bench evidence that the conversion happened.

What is happening chemically

Step one: the failure mode, run on purpose

The chemistry lesson derives this and the lab uses it. Hydroxide breaks the lattice, takes the iron(III) out as an insoluble hydrous oxide, and leaves the hexacyanoferrate(II) in solution to be washed away:

Fe[Fe(CN)₆]⁻ + 3 OH⁻ → Fe(OH)₃ + [Fe(CN)₆]⁴⁻

Ware's Appendix II.8: half the image substance stays in the paper as iron, and half leaves as ferrocyanide

Three consequences, all operational.

The alkali does not have to be strong. Ware’s §8.1 gives 1 to 5 per cent of almost any of them — aqueous ammonia, sodium carbonate, sodium or potassium hydroxide, calcium hydroxide as lime water, borax. Holtzman’s figure, the one the course quotes whenever it discusses mounting board, is that a buffer at pH 9.4 decolourises Prussian blue irreversibly in one to ten minutes, and that 0.25 molar sodium carbonate at about pH 10.7 does it in under half a minute. Your bleach tray is the archival-board failure with the clock running fast.

The reversal window is short and then it shuts. Prompt acidification can partly restore the breakdown, because freshly precipitated hydrous iron(III) oxide dissolves in dilute acid and can meet ferrocyanide again. Over a few days the gel transforms to crystalline forms dilute acid will not touch:

Fe(OH)₃ → FeO(OH) + H₂O

Ageing to goethite, and with it the end of any chance of getting the blue back

Ware calls restoration an area needing further research and notes that whether the print has been washed — which removes the ferrocyanide — is likely to decide it. Treat the bleach as irreversible at the bench, because washing is the one thing you certainly do.

And the yellow-brown left behind is the substrate for the second bath. The first step is not there to remove the picture but to convert its iron into a form a polyphenol can react with.

Step two: an iron gallotannate, which is iron gall ink

The two conversions, and where each element goes

1. 1Prussian blue in the paper fibres — mixed-valence lattice, intensely coloured by intervalence charge transfer
2. 2Alkaline hydrolysis — 5 % w/v sodium carbonate, minutes; irreversible once washed
3. 3Hydrous iron(III) oxide — the yellow-brown ghost — the image in position, with almost no density
4. 4Hexacyanoferrate(II) leaves in solution — half the image substance, into the bleach waste
5. 5Tannic acid, 1 % w/v — ten gallate groups on a glucose core
6. 6Iron gallotannate — the same class of compound as iron gall ink; composition not established here

Drawn, not measured, and schematic in the strict sense: the lattice fragment is not to scale and the product on the right has no established stoichiometry.

The method is old and its originator has a name. Ware credits John Mercer — whose “chromatic photographs” of the 1850s also produced the iron-mordanted dye toners of §8.4, and whose prints on cloth survive in the Mercer Archive at Lancashire County Record Office — with originating the alkali-then-tannin route. The procedure this page runs is Ware’s own printing of it.

Ware also names the product, and that naming answers a question this course had left open. Cyanomicon §8.2 states it in one sentence: the image is converted to ferric hydroxide as above, “and this is reacted with gallic or tannic acid to form ferric gallate or ferric tannate ‘ink’”. He then gives the molecules. Gallic acid is 3,4,5-trihydroxybenzoic acid, a simple polyphenolic acid; tannic acid — properly gallotannic acid, because there are other tannic acids in the family — is a much larger molecule of ten gallate groups, in pairs, on a central glucose. Tannins generally are plant polyphenols from gallnuts, sumac, witch hazel, tea leaves, oak bark, seeds and fruit skins.

His quotation marks around “ink” do real work. This is the chemistry of iron gall ink — the blue-black a polyphenol makes with iron, which the tannic acid page already gives as the reason a tannin bath must never meet a steel tray. You are not making an analogue of it on your print. You are making it, in the shape of a photograph.

⚗What is settled, and what this course still cannot write down

Settled, from Ware: the substrate is iron(III) as a hydrous oxide; the reagent is a gallate-bearing polyphenol; the product is a ferric gallate or ferric tannate of the iron gall ink class; the colour is a rich purplish-brown; the reaction intensifies the image; the colour is very sensitive to alkali.

Not settled by anything read for this course: the stoichiometry; whether this complex is the one the ink literature describes; whether any iron(II) is involved, as it is in making an ink, when the starting material here is entirely iron(III); how much of the colour is complex and how much is tannin staining cellulose. The course therefore publishes no balanced equation for this step, and the scheme above is drawn as shapes for that reason.

One note on the course’s own record. The formulary entry for the supplier’s version of this toner records the product as unestablished; that entry was written from Ware’s chapter 9, on the destruction of Prussian blue, and the naming here comes from his §8.2, which is where he gives it.

Where the tannin comes from, and why an infusion is an unknown

Ware’s §8.3 is the honest account of the fashion for toning cyanotypes with tea, coffee and wine. The chemistry is real and it is the same chemistry: natural products containing tannins and similar phenolic substances that react chromogenically with iron salts, “closely related chemically to the previous well-defined toning agents”. Four findings from it, in his order.

The results are muddy. Black and green teas, several varieties of black coffee and red wine, used with and without preliminary bleaching, “yield rather muddy brown and black images”.

Most of the colour difference is stain, not reaction. The source of the tannin affects the colour, but “more from staining than from the specific reaction with iron(III)”. Grape tannins tend toward purple-brown and green tea toward green-brown, and both are coloured extracts dyeing the paper base as well as reacting with the image. Oak-derived tannin is gallic and sold as a white powder: in itself it gives no stain, and it still reacts. That is the argument for the jar over the teapot, from the source.

Nobody knows how long they last. Ware states that the life expectancy of such images has not been established, quotes Mrhar’s admission that some of the procedures are “unreliable and capricious”, and cites Lukasz Brzezinski’s observation of significant colour shifts within a year or two.

And an infusion is a solution of unknown concentration — the course’s objection rather than Ware’s. A bath whose strength you cannot state cannot be repeated, compared or varied deliberately. Use one anyway if you like, but characterise it: record the mass of leaf or grounds, the volume and temperature of water, the steeping time, whether it was filtered, and the pH; make it the same way every time; and run it against a strip of the 1 per cent tannic acid bath in the same session, because a reference you can buy is what turns an infusion into data.

The other way to change the colour, which this course does not use

A second family works by a completely different mechanism: instead of converting the pigment, it walks a foreign cation into the holes in its lattice. Prussian blue is microporous and an ion exchanger — that is why it is given clinically as an antidote for thallium and radiocaesium — and a cation that enters it can interfere with the intervalence charge transfer that makes the blue.

Ware’s conjecture is that the metal needs two accessible oxidation states to have any effect, and the four that work are the four that have them: lead(II) for a violet, thallium(I) for a cornflower blue, nickel(II) for a slight greenish shift, and copper(II) for a purplish-brown “Bartolozzi red”. A fifth approach, from Humphrey Desmond Murray’s 1933 and 1935 patents, colours only the high values by precipitating an insoluble ferricyanide of manganese, cobalt, nickel, zinc or cadmium in the first wet bath.

The course uses none of them. Lead acetate is a cumulative poison in an open tray; thallium(I) sulfate is the substance Prussian blue is used as an antidote for, and Ware himself declines to recommend it to the general public; nickel(II) salts are listed carcinogens, which his own text says in two words and an instruction to read the safety data sheet; cadmium needs no argument. The chemistry and the provenance are collected on the lead, thallium and nickel toners page at Level D, and the course’s general position on additives of this kind is Part XXV’s.

Know they exist for one reason: one of them is the only toning treatment in the literature that improves a cyanotype’s permanence, and that fact is load-bearing below.

Data to record

Use the lab notebook format and the batch record SOP.

The session log. Date and room temperature. For the prints: coating batch, sensitiser lot numbers, paper, exposure and processing from the previous session, and confirmation that all five came from one coating. For the baths: substance, supplier, lot, per cent w/v or v/v, volume and water used. t_full and t_part, and the six strip times they came from. Every bath time and temperature for every print. The wash water’s pH. Drying method and the air the prints dried in.

The instrument. Head, both channels’ dominant wavelength and half-width, tile serial, date of last anchoring, and the reading repeatability figure from Part XV. Anchor once and do not re-anchor between the before and after passes; if you must, say so, because that is a systematic difference sitting inside your differences.

The measurement table, one row per patch per print per channel:

Column	What goes in it
Print	T, P, N or C
Step	1 to 21, or the alternate steps if you are reading half of them
Channel	Red or green, by name — never “the densitometer”
D before	Read dry, the morning after the print was made, before any bath
D after	The same patch, same channel, the morning after toning
Δ	After minus before, signed
Δ corrected	Δ for this print minus Δ for the same patch on print C
Visual note	Hue by eye, and whether the patch still reads as separate from its neighbours

Three things that go in no column. The colour of the tannin bath at the start and the end; the paper base at an unimaged, uncoated corner before and after, because base stain is a number too; and what happened between the evening and the next morning, which on the control should be a gain and on print T nothing.

Analysis

First, what your control did

Look at print C first. It went through five trays of water and almost certainly lost density: Ware’s figures from the Moors put fifteen minutes of tap water at an average 18 per cent of image density, distilled at 4 per cent and de-ionised at nothing measurable. That loss is the session’s, not the toner’s, and every number you report for T, P and N is Δ corrected. If print C lost a great deal, your wash water is the finding, and it applies to everything else you have printed in this part.

Second, the channel problem, which is the real teaching of this page

The exposure-scale experiment rules that a cyanotype is read on a red channel, because Prussian blue absorbs in one broad band centred near 700 nm and a green channel reads the shoulder of it. That ruling is correct for the blue print. It is not correct for the brown one, and a reader who applies it without thinking will publish a fictitious collapse in density.

The argument is the definition of a density. Under ISO 5-3 a density is defined by its geometric and its spectral conditions together, the spectral response being the product of the detector’s sensitivity and every filter in the path. Change the substance’s absorption spectrum and the same instrument is answering a different question. A purplish-brown iron gallotannate does not absorb where Prussian blue absorbs, so a red-channel reading of the toned print measures how much red light a brown substance fails to absorb, which is not comparable with a red-channel reading of a blue one however carefully both were taken.

🧮The arithmetic, on invented numbers, and what each of them may be used for

Every figure in this box was made up to demonstrate the method. None is a measurement and none should be copied into a notebook. The course has not run this session.

Suppose one mid-tone patch reads, before and after:

D 1.02 → 0.41 · Δ = −0.61

Print T, red channel

D 0.55 → 0.78 · Δ = +0.23

Print T, green channel

D 1.02 → 0.94 · Δ = −0.08

Print C, red channel

Δ corrected = −0.61 − (−0.08) = −0.53

Print T, red, corrected against the control

Three claims are available from those four lines and a fourth is not. Legitimate: the toned print absorbs 0.53 less red light than it did, allowing for what the trays cost; it absorbs 0.23 more green light; and the two signs are opposite, so this is a change of spectrum rather than simply a loss of substance, which is what a conversion should look like. Not legitimate: “toning cost half the maximum density.” No channel makes that sentence true, and no single-channel figure becomes a visual density without a spectral product this course does not have.

What to do about it. Report every figure with its channel attached, report both channels always, and treat the separation between adjacent steps rather than the absolute density as the thing that survives the conversion best. Tonal separation is what a print is made of, and a step that has stopped being distinguishable from its neighbour has been lost in any spectral condition you like.

Third, what you may claim

Ware writes, in both places he publishes the procedure, that the treatment generally intensifies the image — a statement about appearance by a chemist who measured a great deal else, not a densitometric claim in a channel. Your data can support it, contradict it, or most likely do both depending on the channel, and that mixed answer is the honest one.

Beyond that: one print per condition, one paper, one coating, one evening. Every conclusion takes the form “on this paper, at this strength, on this night”, and gives you a direction and a rough size for your own materials rather than anything about another paper, formulation, tannin strength or year.

The permanence question, and why the course will not close it

Four things have to be said and they do not point the same way.

There is almost no direct evidence. In a 1992 survey of several major collections by the photograph conservator Megan Gent, no curator interviewed could report having definitely identified a toned cyanotype. Gent did find prints of about 1906 in informal Royal Family snapshot albums in the Royal Archives at Windsor — single-layer purplish-brown prints that Ware describes as indicative of tannic acid toning, sharing album pages with conventional blue cyanotypes. That is the population: a probable identification in one album.

Ware’s judgement is negative and specific. In §9.1.7, having tested many treatments, he writes that of the many toning procedures suggested for cyanotypes, most de-stabilise the image and cannot be recommended. His chapter 8 opening is blunter: most prove “more curious than valuable”, with little reproducibility and less permanence, and he quotes the process historian W. Russell Young III to the effect that a photographer who wants another colour should use another printing method.

The one exception is a treatment this course does not use. Lead(II) acetate toning improved resistance to light fading by a factor of about four in Ware’s tests, with X-ray spectrometry indicating lead in the lattice, and he debars it as a conservation treatment on the colour shift and the toxicity of the bath in the next sentence. Nickel(II) does something comparable for alkaline hydrolysis rather than light, by about an order of magnitude, and is a listed carcinogen. The two treatments that demonstrably help are the two the course refuses.

And the comparison is genuinely difficult, for reasons of chemistry rather than diligence. The untoned print is not a stable reference: Prussian blue fades in light by photoreduction to Prussian white and regains in the dark, and Ware measures that regain at over 95 per cent within a day and 99 per cent within five, with permanent loss over repeated cycles at most of the order of 3 or 4 in his units, while McElhone’s monitoring found no detectable fading at all at 50 lux over 2000 hours. A cyanotype that has “faded” on your wall may be one that has not yet been in the dark. The toned print has no such mechanism, the pigment that did it being gone, so a display test compares a material that recovers with one that does not and will flatter the toned print early.

The course’s position. No measured permanence figure for a tannin-toned cyanotype appears in anything read here. The evidence is a specialist’s general negative judgement, one probable historical identification, and two practitioners reporting colour shifts within a year or two. That is enough to say do not tone a print for its permanence, and not enough to say how much life you have cost it. If you want an archival brown photograph, the processes that are brown to begin with are three parts away, and Ware says the same thing.

Troubleshooting

The print bleached and then would not tone. Nothing was left for the tannin to reach, and there is no repair: the ferrocyanide went down the drain and the iron that stayed is all the second bath has. Print for the toner you intend to use, which means printing darker than you would for a blue print you meant to keep.

The highlights are gone and the shadows are fine. Same cause, differently distributed: highlights held least pigment and so had least to lose, which is the general rule for every fault in this part.

The bleach is taking far longer than five minutes. Check what you weighed: 25 g of washing soda decahydrate carries only about a third as much carbonate as 25 g of the anhydrous salt this page specifies, and Ware’s own table permits either. Then check the print, since a heavily printed New Cyanotype holds more image substance than a classic one.

The tone is patchy or mottled. Agitation in the first bath, or a print that went in dry. Ware’s one-minute presoak exists for this.

The paper base has stained. Expected, and Ware names it as a problem rather than a possibility. Purified tannic acid stains least and an infusion most; a longer tannin bath makes it worse.

The colour keeps drifting after the print is dry. Record it and treat it as data. Ware gives no drying-side warning, and Brzezinski’s report of significant shifts within a year or two is the nearest thing in the corpus, so your own dated observations are worth more than anything the course can tell you.

The toned image is lightening on the mount. Check what it is mounted on. The toned colour is, in Ware’s words, very sensitive to alkali, so the unbuffered-mount rule applies to a toned print as well and possibly harder. See Storage.

A blue-black speck that is not in the negative. Iron from a tray, a clip or a nail, meeting the tannin. That is iron gall ink and it will not come out.

The tannin bath went dark and cloudy. Expected with age; a polyphenol solution oxidises. Bostick and Sullivan replenish theirs at 5 to 10 mL of concentrate per 8 × 10 print, which is their figure for their own 2 to 4 per cent bath and not a rule for a 1 per cent one. Nothing in the corpus publishes a capacity for Ware’s.

Clean-up

Empty the three chemical trays into three separate labelled containers, and the rinse and wash water into a fourth. Rinse the trays in that order — tannin, acid, then bleach — so that no rinse water carries alkali into an acid tray.

Wash the toning tray immediately and expect it to stay brown anyway. Blotting paper, gloves and the sacrificial strips go into the solid waste, not the sink.

Wipe the bench before the stains dry, because a tannin stain on a wooden bench is permanent and will find your next print’s back.

Gloves off last, under the PPE removal SOP, and hands washed.

Storage

The toned print goes into an unbuffered mount and an unbuffered enclosure, exactly like the untoned one, for a reason that survives the conversion: Ware’s note on this toner is that the colour is very sensitive to alkali. The alkaline reserve in board sold as archival is the same chemistry as your first bath, and the Prussian blue having already gone does not make the print alkali-tolerant. This is the process in the course with the strongest case for unbuffered storage and the weakest evidence about ignoring it.

The four baths. Nothing read here publishes a shelf life or capacity for Ware’s formulation; Bostick and Sullivan give ten years at room temperature for their concentrates and nothing for a working bath. Mix these fresh — the cost of doing so is thirty grams of solid. The tannic acid solid keeps dry, cool, dark and tightly closed in a labelled container that has never held food; expect solutions to darken and grow mould.

Records with the prints. A toned cyanotype is hard to identify later, which is the substance of Gent’s survey finding. Write the process, the bath strengths, the two bleach times and the date on the back in pencil while you still know them.

Disposal considerations

The chemistry first. The bleach waste is a dilute alkali carrying suspended hydrous iron(III) oxide and dissolved hexacyanoferrate(II) — the ferrocyanide that used to be half your photograph. The toner waste is a dilute plant polyphenol carrying iron and a good deal of colour; the acid waste is a dilute weak acid; the wash water is all three, greatly diluted.

The general practice. Keep them separate, label them with contents and date, never top up an unlabelled container, and never let the acid stream meet the alkaline one — a carbonate and an acid react, generate heat and evolve carbon dioxide, and doing that in a sealed bottle is how a container fails. The hexacyanoferrates are complex cyanides rather than cyanide salts, which is why the Environment Agency’s WM3 excludes ferrocyanides and ferricyanides by name from the hazard statement it assigns to cyanide salts for liberating a very toxic gas on contact with acid. That is a statement about what the substance is rather than how dilute it is, and it is not permission to add acid.

!A supplier's disposal claim that this course does not adopt

Bostick and Sullivan’s toning kit sheet ends with the claim that depleted solutions may be poured down the drain with plenty of water, and that the bleaching and toning solutions are safe for septic systems and the environment. The course does not repeat that claim and does not act on it. It is an assertion by a manufacturer, in the United States, about an unstated volume entering an unknown sewer, and it is not a hazard assessment: nothing in the sheet says what was assessed or against whose limits. WM3’s own first step says that nearly all wastes need classifying, including waste from domestic households, which is the opposite of the assumption behind a sentence like that one. The disposal ruling is what this course follows.

And the caveat that is not a formality. This course publishes no jurisdiction-specific disposal instruction, because it cannot: local regulation governs, it differs, and it is the only document that can answer the question for your drain. In England and Wales a domestic worker’s route is the council’s household waste and recycling centre, which is ILFORD’s own advice to domestic users; the disposal ruling names the government service that finds one. Check your local regulations; they govern.

Questions

1. Why can a cyanotype not be toned the way a silver print is toned? Answer in terms of what the image substance is and what a toner has to react with.
2. Your bleach reaches its end point in forty seconds rather than five minutes. Give two distinct causes, and say which one you can test with the material already on the bench.
3. Print T’s red-channel density fell by 0.61 and its green-channel density rose by 0.23. A friend concludes that toning destroyed most of the image. What is wrong with that, and what would you have to own to answer the question they were really asking?
4. Ware writes that the treatment “generally intensifies the image”. Name two ways your densitometer could disagree with him while both of you are right.
5. You want a print that is brown in the highlights and blue in the shadows. Which step do you change, in which direction, and why does that produce that result rather than the reverse?
6. A reader proposes replacing the acetic acid bath with the 1 per cent citric acid the coating lab already uses. Give the chemical objection, in terms of what citrate does to iron(III) and what the image is made of at that moment.
7. Two treatments are known to improve a cyanotype’s resistance to something. Name them, name what each protects against, and say why neither appears in this session.

Further experiments

The variable nobody has isolated: alkali strength against final colour. Bostick and Sullivan’s bleach is a seventh the strength of Ware’s and their reported colours are magentas where his are purplish-browns — but their tannin is two to four times stronger as well. Four prints, two bleach strengths crossed with two tannin strengths, one coating: the smallest design that separates the two. This is the most useful experiment on the page because the sources genuinely do not answer it.

Ware’s reversed order. He reports that the best purplish-browns are said to result from tannin first, rinse, then alkali, then optionally a brief return to the tannin. Two prints, his order and the reverse, side by side; the word “said” in his sentence is an invitation.

Gallic acid against tannic acid, at the same 1 per cent, on two halves of one print. Ware gives them as alternatives; one is a single defined molecule and the other ten of it on a sugar. Whether the print can tell is a short evening.

Characterise one infusion properly and put it beside the jar. Strong black tea, measured and timed, on one half of a print and 1 per cent tannic acid on the other, with each one’s base stain read at an uncoated corner — then repeat it three weeks later from the same tin and see whether you can hit your own result twice.

Test the alkali sensitivity of the toned image, which nobody has published. A toned strip and an untoned strip, both given a minute in very dilute sodium bicarbonate. The untoned one will be destroyed; what happens to the toned one turns Ware’s “very sensitive to alkali” into an observation, and it is the evidence the Storage section most wants and does not have.

A dated display pair. One toned print and one untoned, from the same coating, half of each masked, in one frame in one light, read every three months for a year — giving the untoned one a fortnight in the dark before each reading, or you will measure its recovery rather than its life.

Recap

A cyanotype has no metal for a toner to convert, so the toner destroys the image and builds another one — not an aggressive method chosen over a gentle one, but the only route in the literature to a substantially different colour.

The first bath is the archival failure of the whole process, run deliberately. Dilute alkali hydrolyses Prussian blue to a hydrous iron(III) oxide and washes the ferrocyanide away, and Ware’s carbonate keeps the session at Level A where his own first-named ammonia would not.

The second bath makes iron gall ink: ferric gallate or ferric tannate in Ware’s naming, the same reaction that puts a black spot on a print that met a steel clip. What the course cannot write is a formula for it, and it says so rather than guessing.

Stopping the first bath early gives two colours, because the second can only build where the first destroyed.

Measure against a control that got only water, because five trays cost a cyanotype density on their own — 18 per cent in Ware’s cited figures if your tap water is alkaline.

Report the channel with every density, because a single-channel comparison across a conversion is a number about the instrument as much as about the print.

And do not tone for permanence. Tone because you want the print to be brown.

Check your understanding

Question 1. Why must a cyanotype be bleached before it can be toned to a substantially different colour, when a silver print does not have to be?

• Because Prussian blue is too dense for a toner to penetrate the paper fibres
• Because the image substance is a pigment with no metal to convert or plate over, so the iron in it has to be released as iron(III) hydroxide before a polyphenol can react with it
• Because the sensitiser leaves ferricyanide behind that would poison the toning bath
• Because a cyanotype is a printing-out image and printing-out images cannot be toned directly

Show the answer and why

Answer: Because the image substance is a pigment with no metal to convert or plate over, so the iron in it has to be released as iron(III) hydroxide before a polyphenol can react with it

A silver toner works on metallic silver: it converts it, plates it, or replaces it. A cyanotype contains no silver and no halide. Ware describes the route the literature actually takes, which is to hydrolyse the Prussian blue with dilute alkali so the image becomes a hydrous iron(III) oxide in the same places, and then to react that iron with gallic or tannic acid to form a ferric gallate or tannate ink. Half the original image substance, the hexacyanoferrate, is washed away in the process.

Question 2. You read a toned print and its untoned control on the red channel your densitometer carries for cyanotype work, and the toned print reads 0.5 lower everywhere. What have you established?

• That toning cost 0.5 of maximum density
• That the toner is a reducer and the loss is image substance
• That the toned print absorbs less red light, which is expected of a brown substance and is not by itself a loss of image
• That the bleach went too far and the print is spoiled

Show the answer and why

Answer: That the toned print absorbs less red light, which is expected of a brown substance and is not by itself a loss of image

A density is defined by its geometric and spectral conditions together, which is the only thing this course quotes ISO 5-3 for. Prussian blue absorbs in one broad band near 700 nm, which is why the red channel is the right one for a blue print; a purplish-brown iron gallotannate does not absorb there. The same instrument is answering a different question about a different substance, so the red-channel fall has to be read beside the other channel, where the sign is likely to be opposite, and beside a control that went through the same water.

Question 3. Ware gives five minutes in the alkali bath. Why does this page make you find your own time first, on sacrificial strips?

• Because carbonate solutions vary in strength from batch to batch
• Because the end point is visual and depends on how much image substance your print holds, which depends on your paper, exposure and formulation
• Because the bath is unstable and weakens within minutes of mixing
• Because the published figure is for ammonia and not for carbonate

Show the answer and why

Answer: Because the end point is visual and depends on how much image substance your print holds, which depends on your paper, exposure and formulation

Ware writes "ca. 5 minutes until the Prussian blue image is bleached to pale yellow", and the second half of that sentence is the instruction. Bostick and Sullivan time a much weaker bath at three minutes and warn that the deepest shadows may not clear at all. A heavily printed sheet, a differently sized paper or a New Cyanotype rather than a classic one all move the number, and the bleach is irreversible once the print has been washed, so the time is found on material you are willing to lose.

Question 4. Which of these is a fair summary of what is known about the permanence of a tannin-toned cyanotype?

• Toning stabilises the image, because the tannate is more stable than Prussian blue
• Toning is neutral: the print behaves like an untoned cyanotype afterwards
• No measured figure exists in the sources this course holds; the specialist judgement is that most toning treatments de-stabilise the image, and the two treatments shown to help are heavy-metal ones the course excludes
• Toned cyanotypes are known to fade within one to two years

Show the answer and why

Answer: No measured figure exists in the sources this course holds; the specialist judgement is that most toning treatments de-stabilise the image, and the two treatments shown to help are heavy-metal ones the course excludes

Gent's 1992 survey found no curator who could definitely identify a toned cyanotype, so the historical population is nearly empty; Ware states that most suggested toning procedures de-stabilise the image and cannot be recommended; lead(II) improved light-fading resistance by about four times and nickel(II) improved alkali resistance by about ten, and both are excluded here on toxicity. The one to two year figure belongs to two practitioners reporting colour shifts in beverage-toned prints, which is an observation rather than a measurement and is not a general result.

Question 5. Why does this page refuse to substitute 1 per cent citric acid for the 1 per cent acetic acid bath, when the coating lab uses citric acid throughout?

• Citric acid is a stronger acid and would fog the highlights blue
• Citrate is a strong chelator of iron(III), and at that moment the image is iron(III) hydroxide held only by the paper
• Citric acid would react with the tannic acid in the next bath
• Citric acid leaves a residue that stains the paper base brown

Show the answer and why

Answer: Citrate is a strong chelator of iron(III), and at that moment the image is iron(III) hydroxide held only by the paper

The bath's only job is to neutralise alkali carried out of the bleach, and acetic acid does that without complexing anything. Citrate is the ligand of the sensitiser you coated in the first place: it holds iron(III) in solution, which is exactly what it would do to the hydrous iron(III) oxide that is currently the entire image. The general lesson is that a substitution has to be judged against every property of the substance, not just the one you were substituting for.

Check my answersClear