Lab: Mixing, Coating and Printing a Classic Cyanotype

Nobody can tell you how long to expose a cyanotype. Ware gives around twenty to thirty minutes under a typical ultraviolet lamp; Bergger give about ten minutes under a cloudless Paris sky at noon in summer; Bostick and Sullivan tell you to open the frame after four minutes and look. Those three numbers describe three different light sources, two different sensitiser strengths and one variable — the sun — that is not the same twice. What they have in common is the instruction that follows every one of them: make a test strip.

So this session’s real product is not a print. It is a dose you can repeat, written on a label, for your lamp and your paper and the jar of iron salt you happen to have bought. The print comes out of it, and so does a second number nobody publishes for you either: how much density your paper gains between coming off the drying line wet-looking and being fit to judge the next morning.

Purpose

To make the two classic cyanotype stock solutions from weighed solids, coat sheets by both of the methods the process uses, establish an exposure on your own ultraviolet source by a stepped test, process to a stable blue, and record the run in the form the exposure-scale experiment will analyse.

Three things make this the right session to learn the alternative-process workflow on. The chemistry is two salts and water; a wasted sheet costs pennies rather than the most expensive solution in the course; and the image prints out, so every judgement is visible while you are making it, which is true of nothing in Parts XVIII and XIX.

This page does not teach the chemistry again — that is the previous lesson’s job — and it does not restate the formula, whose provenance, variants and ingredient functions are on the formulary page.

Learning objectives

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

• Make up a solution to a final volume rather than by adding water, and say why the distinction changes the concentration.
• Explain why the sensitiser is two bottles, in terms of the reaction already under way in a mixed one.
• Coat by rod and by brush, judge from the sheet when the volume was wrong, and say which method costs more per unit area and why.
• Establish an exposure by stepped test on your own source, and state the conditions that void it.
• Read a printing-out image and recognise a correct exposure, which looks like an over-exposure.
• Run the wet sequence and say what each bath is for, including which of them is not rinsing.
• Measure the dry-down, and write a batch record another person could reproduce from.

Prerequisites

• Iron, Light and Prussian Blue — the mechanism this session is the craft half of.
• What These Processes Need — the negative, the paper and the light. Its paper rules are assumed here, not repeated: unbuffered, 100 per cent cotton, no optical brightener.
• Build a UV exposure unit and Build a contact printing frame, or the sunlight route below.
• Concentration and dilution, for per cent w/v and making up to a volume; and Reading an SDS and GHS labels, because you will read two of them before you weigh anything.
• A negative of short density range, or a photogram subject. The classic process has an exposure scale of about 0.9, three stops, which Ware compares with a grade 3 silver-gelatin paper. A negative made for platinum will not print on it at all.

Safety classification

Level B. The classification rubric takes a formula’s level from the highest of its chemicals, and here that is potassium ferricyanide at B. Ammonium iron(III) citrate and Prussian blue are both Level A. So the letter is carried by one substance and by two things done to it — weighing it dry, and putting it in a room that also contains acid.

Where the course and its main source disagree. Ware rates both salts 1 for health on the four-point HMIS scale, writes that the chemicals of the traditional formula are not dangerous and can be handled by children under supervision, and states that from all points of view there is no safer photographic process. The course still classifies at B, because its rubric is keyed to the chemical pages and one of those carries ECHA notifications including a reproductive-toxicity statement. The disagreement is recorded rather than smoothed, and the practical consequence is small: gloves, eye protection, dust control at the balance, acid nowhere near the bench.

⚠What is not a hazard here, and why that is worth saying

There is no hydrogen cyanide. The cyanide is bound to iron in a low-spin complex, and the Environment Agency’s waste guidance makes the same distinction in regulatory language, exempting complex cyanides such as ferrocyanides and ferricyanides by name from the statements it assigns to cyanide salts. Ware’s qualification of the frequently repeated warning is worth having exactly: the heating has to be very strong and the acid very concentrated to get much hydrogen cyanide, and neither is done in cyanotype. Note the shape of that sentence. It is not “there is no risk”; it is “these two conditions create the risk, and this procedure creates neither” — which is why the acid rule below stays absolute.

There is no chromium(VI). The improved classic sensitiser adds ammonium dichromate and Ware argues against it in the same paragraph that prints it. There is none in this session, in the sensitiser or in any bath; the chromium ruling says why.

The exposure is the hazard people forget, and it is why this page declares requiresUV.

Hazards

Hazard	Where it arises	Control
Dust of either solid, to eye and airway	Weighing 10 g and 5 g of powder	Weighing SOP, no sweeping motions, splash goggles for the citrate because nearly nine in ten classifying notifiers give the serious-eye-irritation statement, and the P2 filter the international chemical safety card specifies for the ferricyanide where dust cannot be avoided
Eye and skin irritation from solutions	Mixing, coating, every tray	Nitrile gloves and eye protection. The citrate stains skin yellow-brown and the stain turns blue in the wash, which is a marker of contact rather than a hazard
Hydrogen cyanide, only if the rule is broken	Ferricyanide meeting concentrated acid, or heated	The acid rule below. The safety card states that the solid decomposes on heating and on contact with acids producing toxic gases including hydrogen cyanide, and Princeton record cases of cyanide poisoning from treating Farmer’s reducer with acid
Ultraviolet to eye and skin	Every minute the source is energised, and the whole of a sunlight exposure	Part XVI’s enclosure and interlock, or, outdoors, shade, covered skin and eye protection. ICNIRP put personal protection last in the hierarchy and so does this page
Ingestion, by hand to mouth	Any bench where food has been	Nothing that has held either salt goes near food or a food refrigerator, which both supplier sheets say
Stains to bench, clothing, sink	Coating and processing	The solutions stain wood, metal and many plastics. Blotting paper under the sheet, a tray under everything

⚠The acid rule, which is the only absolute on this page

No acid comes into this room while the ferricyanide bottle is open, and none ever goes into the waste container. Not a stop bath, not an acid fixer, not vinegar, not a descaler, not bleach in the same drain.

The single exception is the deliberate, dilute, weak acid development bath — 1 per cent w/v citric acid or less — a controlled use thousands of times below the “very concentrated” of the warning, and optional in this session.

The reason to keep the rule absolute rather than proportionate is that a waste bottle is where the two things actually meet. Nobody plans to add acid to spent ferricyanide. They top up a part-used container whose label they did not read.

Required PPE

Nitrile gloves throughout, from the first weighing to the last tray. 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, and both supplier sheets say simply to wear gloves for handling, coating and processing. A glove that has been in the citrate and goes into the ferricyanide bottle has done what a shared pipette would.

Chemical splash goggles at the balance and for making up the stocks; ordinary safety spectacles for the trays. The eye statements are the strongest thing either chemical carries, and the highest-risk moment is a powder being tipped, not a 5 per cent solution being poured.

A P2 dust filter if you cannot weigh the ferricyanide without raising dust — the specification comes from the international chemical safety card, not from a general rule.

Sun protection, if you take the sunlight route. Outdoors the personal protection is sun protection and not chemical protection: shade for the frame if you can arrange it, covered arms, a hat, eyes off the sheet. A twenty-minute exposure is a twenty-minute exposure for you as well. And an apron, because the citrate stain is permanent.

Ventilation

The control here is dust extraction at one step and separation from acids everywhere else; it is not airflow, because nothing in this session produces a vapour. Neither salt is volatile, neither solution evolves anything at room temperature, and the only gas the process makes is the carbon dioxide released by the photolysis inside the printing frame — which matters because a sealed sandwich traps it into bubbles that blur the image, and which is why the frame carries a felt blanket or a porous backing.

Work in a room with the ordinary through draught HSE ask for wet photographic work, on a lipped, wipeable surface. A strong draught at the balance is worse rather than better, because it lifts powder: shut the window for the ninety seconds you are weighing and open it afterwards.

Materials

• Five sheets of unbuffered 100 per cent cotton paper, hot-pressed, cut to 8 × 10 in — two for the coating comparison, one cut into strips for the dose test, one for the print, one spare. Bostick and Sullivan specify at least 32 lb for a small sheet and hot-pressed at 47 lb or more for larger work; the paper lesson has the rest.
• A negative the same size as the print, of short density range, or a photogram subject that lies flat.
• Two brown glass bottles of 100 mL, one small opaque bottle for the working mixture, and labels.
• Blotting paper or clean newsprint, paper or masking tape, and a soft pencil.
• Distilled or de-ionised water, about 2 L, for the stocks and for the final rinse if your tap water is hard or above pH 7; and household 3 per cent hydrogen peroxide, 50 mL, for the re-oxidation option.
• A step wedge from Part XIII if you have one. The test strip works without it and works much better with it.

Chemicals

The formula is not restated here. The two stocks below are Ware’s §7.1 at half his volume, which leaves both concentrations exactly as he publishes them; the classic cyanotype sensitiser page carries the full formula, its provenance, the function of every ingredient, and the table of every other proportion that has been published under the same name.

Stock solution A — the light-sensitive half. Half batch:

Chemical	Quantity	Form
Ammonium iron(III) citrate	10 g	Green variety, glassy scales or powder. Record the supplier and the lot: Ware gives the green form’s iron content across present-day commercial material as 14 to 18 per cent by weight, and no supplier is obliged to tell you which
Purified water	to make 50 mL	Distilled, de-ionised or pharmaceutical. A make-up volume, not an addition
Thymol	a few crystals, optional	Floated on the surface against mould. They do not dissolve. Thymol carries its own hazards and its own page

Stock solution B — the precipitant. Half batch:

Chemical	Quantity	Form
Potassium ferricyanide	5 g	Ruby-red crystals. General Purpose Reagent grade, 98 to 99 per cent, is what Ware specifies as adequate
Purified water	to make 50 mL	As above

What the session actually consumes: about 10 mL of mixed sensitiser — 5 mL of each stock, so 1 g of citrate and 0.5 g of ferricyanide. The rest stays on the shelf, which is the argument for making a stock rather than a print’s worth.

The processing baths:

Bath	Made from	Strength	Optional?
First bath, plain water	Tap water below pH 7, or distilled	—	The default
First bath, weak acid	5 g citric acid in water to 500 mL	1 per cent w/v	Optional. Read the callout under Procedure before choosing it
Wash	Running water, cool	—	Required
Re-oxidation	50 mL of 3 per cent hydrogen peroxide, made up to 500 mL	0.30 per cent w/v	Optional

🧮Why 0.3 per cent, and why three independent sheets arrive at it

Ware’s instruction for the classic process is half a minute in dilute (0.3 per cent) hydrogen peroxide. Two suppliers publish the same bath as a dilution of the 3 per cent bottle you can buy in a pharmacy, and neither states a percentage. Work them out and they land in the same place:

3 % × (100 ÷ 1000) = 0.30 % w/v

Bostick and Sullivan — 100 mL of 3 % into 900 mL of water

3 % × (50 ÷ 550) = 0.27 % w/v

Photographers' Formulary — 50 mL of 3 % added to 500 mL of water

Note the difference in the two arithmetics, because it is the distinction that governs the stock solutions as well: the first is a make-up volume and gives an exact strength, the second an addition, so the total is 550 mL and the strength is nominal. Both are within rounding of Ware’s figure — three parties who are not quoting one another.

Equipment

A balance reading to 0.01 g, checked by the balance and thermometer check. Ware’s preface to this formula says the concentrations “are not critical so they do not need to be made up with great precision”, and given that the citrate’s iron content is uncertain by a quarter between batches, weighing to a milligram would be false precision. Weigh accurately anyway, because a recorded weight is what makes next month’s comparison mean anything.

A 100 mL and a 50 mL graduate, a beaker, a stirring rod and a funnel, verified under the glassware SOP.

Three syringes or pipettes, labelled and never interchanged — one for A, one for B, one for the mixture. Ware’s instruction, and the most important sentence of bench discipline here: a pipette that has been in the ferricyanide and goes back into the citrate starts the dark reaction inside your stock, and the stock will not tell you.

A coating rod 6 to 12 mm across, with a straight section wider than the image area, and a broad soft brush with no metal ferrule. Comparing them is half the point.

A contact frame with a split back. Not a convenience: a printing-out process is judged by looking at it, and a frame you cannot open halfway is a frame that makes you guess.

A UVA source, enclosed and interlocked, from Part XVI, or the sun. Three or four trays larger than the sheet, a timer, opaque card, and — if you built one — the reflection head of your densitometer.

Nothing metal touches anything: no metal tongs, clips, chipped enamel trays or ferrules. An iron–cyanide chemistry finds a trace of iron and prints it as a blue spot.

Estimated cost

Band £, and it is the cheapest practical session in the course. The capital is a balance, a rod, a frame and a UV source, every one of which is either already yours from Parts II and XVI or is on the planner’s list. What this session adds to the shelf is two jars of solid, and one of them — the ferricyanide — you may already own from Part XX’s toning work, where it is the oxidising half of the bleach.

If none of that is available, a premixed kit is a legitimate route and the two kit pages carry the published strengths and yields: Bostick and Sullivan state about 200 prints of 8 × 10 from a 250 mL kit, and Photographers’ Formulary about twenty-four from theirs. Pre-coated cyanotype paper is the other bought route, and the planner prices it.

Estimated consumables cost

Every price with a number is the planner’s own dated UK figure and every quantity comes from the Materials and Chemicals sections. Most of this table has no number in it, and that is the finding rather than a blemish.

Consumed	This session	Sourced price	Cost this session
Nitrile gloves	2 pairs	£6.64 to £14.99 per box of 50 to 100	£0.27 to £0.60
Unbuffered cotton paper, 8 × 10 in	5 sheets	Not priced. Hot-pressed cotton paper is one of the gaps src/data/prices.json already names	—
Ammonium iron(III) citrate, green	10 g into the stock; about 1 g leaves with tonight’s trays	£17.99 per 230 g of a cyanotype-branded pack, checked 7 September 2026; the listing does not state whether the grade is the green or the brown	£0.78 weighed, about £0.08 of it gone
Potassium ferricyanide	5 g into the stock; about 0.5 g leaves with tonight’s trays	£17.99 per 230 g, checked the same day	£0.39 weighed, about £0.04 of it gone
Citric acid, if the acid bath is used	5 g	£10.00 per 250 g of the monohydrate	£0.20
Hydrogen peroxide, 3 per cent, pharmacy	50 mL	Not priced	—
Distilled water	about 2 L	Not priced	—

The priced rows come to about £1.64 to £1.97, a floor rather than a total: three of the seven consumables have no sourced price, and the paper is one of them. What the table now establishes is the ratio in money as well as in yield. Bergger publish that their 2 × 300 mL set coats about three hundred 8 × 10 sheets, so two jars of solid are a couple of hundred sheets’ worth; the £1.17 of salt weighed out tonight leaves about twelve pence of itself on the five sheets, against a paper the file still cannot price and that the part overview cites at about 96p a sheet. The paper is the expensive part of a cyanotype, and that stops being true the moment you move to platinum.

Equipment is deliberately absent, because a balance is not consumed and neither is a coating rod, and a page that quietly counts them has stopped measuring what the consumables calculator needs.

Waste streams

Three streams, and one rule that governs all of them.

• Spent sensitiser and coating residue — the cup, the blot, the paper towel from the rod. Small in volume, highest in concentration.
• The first bath and the wash water — dilute unreacted citrate and ferricyanide, plus the peptised Prussian blue clouding off the sheet, which is image substance rather than dye and is the process working as designed.
• The peroxide bath, if used: the same dilute chemistry plus a decomposing oxidiser.

The rule: nothing acidic goes into any of them, ever — the disposal face of the acid rule, and why cyanotype waste is not simply “iron salts”. Label each container with contents and date, under the labelling SOP and the general chemical waste SOP.

⁇How hazardous is this effluent, actually?

Ware argues that it is not, on specific grounds rather than general ones: the citrate is permitted food additive E381 and has been prescribed as an iron tonic; ferricyanide reduces in the environment to the more stable ferrocyanide, permitted as E536 and used as an anti-caking agent in table salt; and Prussian blue is so insoluble that it is used clinically as an antidote to thallium and caesium poisoning. He concludes that treating the effluent as chemical waste is unnecessary from an environmental viewpoint, names and rebuts a published article that reached the opposite conclusion, and is writing about septic tanks and land drainage rather than about what a sewer authority permits. Princeton’s guidance lists ferricyanide solutions among the photographic solutions to treat as hazardous waste, without qualification by dilution.

What this course does. It publishes no jurisdiction-specific disposal instruction and makes no exception here. Neither control this page insists on — the acid rule, and no dichromate — is about dilution, so neither is touched by the argument. Ware’s position is recorded because it is well founded, not because it settles what may go down a particular sink.

Alternative route

Four readers cannot follow the page as written, and one part of it has no alternative at all.

No ultraviolet unit — print in the sun, and it is not a compromise. Every cyanotype made before electric light was made this way, and Part XVI’s UV page argues the case in its own alternative route. The sun is nearly a point source, so it gives the sharpest contact print of anything on the list. What you give up is repeatability — the ultraviolet content of sunlight varies with latitude, season, hour and cloud, which is why Bergger hedge their ten-minute figure with four conditions — and what you must supply instead of a dose is a record: date, time, orientation, cloud, under the daylight exposure SOP and the outdoor exposure session SOP, with the judgement made through a split-back frame. Everything else here is unchanged, including the test strip, which matters more outdoors rather than less.

No frame. Plate glass on a board with clips will hold a negative in contact and the frame page gives the geometry; what you lose is inspection without disturbing registration, and the workaround is a second, sacrificial coated strip beside the frame that you can uncover and look at.

Cannot weigh, or cannot keep the solids at home. Take a premixed kit: the Photographers’ Formulary and Bostick and Sullivan pages carry the published strengths, drop tables and yields, and everything from stage 3 onward applies unchanged.

Cannot handle the chemistry, or cannot stand at a sink. Pre-coated cyanotype paper and fabric are sold ready to expose and the planner prices them. The exposure, the inspection, the wash and the measurement are still yours; what you cannot do is vary the formulation, so read the exposure-scale experiment for the measurement and take its formulation results as given.

One thing has no alternative, and it should be said plainly. There is no version of this session that makes a print without ultraviolet light. An enlarger will not do it and a household lamp will not do it, and no amount of time compensates, because the sensitiser barely absorbs where those sources emit. Sunlight through an open window is the minimum.

Preparation

The day before. Cut and mark the paper; choose a side; write the sheet number, the paper name and the intended coating method on the back in pencil, in a corner that will be trimmed. Check your tap water with narrow-range pH paper. If it is above 7, or hard, plan the final rinse in distilled water and read the alkali warning in the chemistry lesson first, because alkaline water does not weaken a cyanotype, it dismantles it.

Two hours before. Bring the paper into the working room; Ware allows at least an hour for a sheet to reach equilibrium, and one at a different moisture content from the room will cockle when it is coated.

The bench. Blotting paper down, one end dry and one wet, the balance where neither can reach it. Trays laid out in sequence before anything is coated, because you will be carrying a wet sheet with two gloved hands. Nothing acidic in the room.

The reading. Both safety data sheets, under the SDS recording SOP, with version and date in the notebook. This session’s two are Spectrum Chemical F1001 revision G1 of 11 December 2014 for the green citrate, which classifies it Eye Irrit. 2B with the single statement “Causes eye irritation”, and LabChem 75436 version 1.0 of 19 August 2013 for the ferricyanide, which classifies it not at all and prints “No labelling applicable”. Read that second one properly: a sheet that classifies nothing is not a substance with no hazards, and it lists strong acids among its incompatible materials on the next page.

Procedure

Nine stages. The attended bench time is about three hours; the session also contains an hour of drying that you cannot hurry and several hours of re-oxidation that will finish overnight, so plan to read the result the next morning, not at the end of the evening.

The session, end to end

1. 11. Two stocks, weighed and labelled30 min — separately, to a final volume, into brown bottles
2. 22. Mix the working sensitiser10 min — equal volumes, only what the session needs
3. 33. Coat sheet 1 by rod, sheet 2 by brush35 min — taped, marked, blotted, rod rinsed at once
4. 44. Dry in the dark60 min unattended — no hairdryer
5. 55. Test strip: find the dose45 min — a stepped series on your own source
6. 66. Expose the printthe dose from stage 5, judged by inspection
7. 77. Wet processing30 min — first bath, then wash until the yellow has gone
8. 88. Re-oxidiseair overnight, or 0.3 % peroxide in half a minute and wash again
9. 99. Dry, measure, recordnext morning — the dry-down number is the second product of the session

Stages 4 and 8 are waits rather than work. Everything before stage 5 can be done under ordinary room lighting; the sensitiser responds to ultraviolet, not to a tungsten or LED bulb.

Stage 1 — The two stocks, made separately (30 minutes)

Separately is the design of this formula, not an inconvenience. The two salts react with each other in the bottle without any light at all. The reaction is slow, because the iron(III) is locked inside a citrate complex and is not available as a free ion, but it is not stopped — and that slow dark reaction is why a mixed sensitiser has, in Ware’s phrase, “a relatively short life” while the separate stocks keep for months. Every awkwardness in this stage descends from that fact.

1. Weigh 10 g of green ammonium iron(III) citrate under the weighing SOP. It is deliquescent: open the tub briefly and close it hard. If the contents have compacted into a mass, break off what you need rather than tipping.
2. Dissolve in about 35 mL of purified water at room temperature, stir until clear, then make up to 50 mL. No heat, and no dissolving order — each stock has exactly one solute, which is unusual in this formulary and part of why the process spread so fast. Ware allows the same weight to be made up to as much as double the volume, which halves the strength; pick one and record which.
3. Weigh 5 g of potassium ferricyanide, dissolve in about 40 mL, and make up to 50 mL. Ware’s phrasing for this one is the habit worth copying: he gives the make-up volume and the resulting strength in one sentence — “make up to 100 cc of a 10% w/v solution”.
4. Both into brown bottles, in a cupboard or a box. Float a few crystals of thymol on solution A if it will stand more than a week or two; they do not dissolve and you draw the sample from below them. Thymol carries hazard statements of its own.
5. Label both, under the labelling SOP: substance, strength as per cent w/v, make-up volume, date, batch number, and the supplier and lot of the solid.

✕Two mistakes that cost a session each

Adding water to a weight instead of making up to a volume. “10 g in 50 mL of water” and “10 g made up to 50 mL” are different solutions, because the solid occupies volume — and the first has no stated concentration at all. Jacquard give their stocks as additions and Photographers’ Formulary give theirs as make-up volumes, which is why the two kits cannot be compared as printed.

Assuming that A means the iron. It usually does: Ware, Photographers’ Formulary, Bostick and Sullivan and Bergger all put the citrate in A. Jacquard reverse it. Mixed in equal volumes it makes no difference to the sensitiser and every difference to a bottle you label from memory. Label by substance, not by letter.

📓Why the lot number is not bureaucracy

Ammonium iron(III) citrate is an ill-characterised substance: no single molecular formula, made by dissolving a polymeric iron hydroxide in citric acid, and an iron content across present-day green material running from 14 to 18 per cent by weight — a spread of a quarter. Two printers with different jars are not running the same experiment, and neither are you in six months. When a batch behaves differently, the lot number is the difference between a mystery and a measurement.

Stage 2 — Mix only what the session needs (10 minutes)

Five sheets of 8 × 10 in, one cut into strips, at Ware’s guide of about 1.5 mL per rod-coated sheet and up to twice that by brush, comes to about 10 mL. Mix 5 mL of A and 5 mL of B into the small working bottle or a shot-glass-sized cup, with the third, dedicated pipette. Swirl until the colour is uniform.

What it looks like: a clear yellow-green to olive-brown liquid with no precipitate. Cloudy, or with a blue tinge, means one stock has been contaminated by the other and the session stops here.

How long you have. Ware gives no number and says only that the life is relatively short. Two suppliers give one independently and give the same one: stable for about 2 to 4 hours after mixing. Treat four hours as the limit and mix again rather than push it.

⚗What a sensitiser left overnight is telling you

Mix 2 mL, cap it, leave it in the dark until tomorrow, and coat a strip beside one from a fresh mixture. The old one will be duller in colour, slower, and will give a weaker maximum density and a greyer, foggier highlight.

Nothing has gone wrong with it. It has been doing slowly, in the dark, what light makes it do quickly. The dark reaction between iron(III) citrate and ferricyanide makes small amounts of the same Prussian blue the exposure is supposed to make — distributed evenly through the liquid instead of according to your negative. That is fog: image substance where there should be none, and Ware lists “the sensitizer is already decomposed” first among the six causes in his diagnostic algorithm for fogged highlights. It also consumes ferricyanide that is then not available to the iron(II) your exposure will make, so the shadows have less to build with. A mixed sensitiser is not a stored reagent. It is a reaction you have started.

Stage 3 — Coating, by rod and by brush (35 minutes)

Work under ordinary room lighting. Both supplier sheets state it plainly: the solutions respond to ultraviolet only, no safelight is necessary and normal incandescent lighting may be used throughout. Bostick and Sullivan add one qualifier worth keeping — limit exposure to fluorescent light, which has more short-wavelength content than a tungsten or LED bulb. Shade any window; do not coat in sunlight.

1. Tape the sheet at the upper corners over blotting paper and check that it lies flat. Ware’s requirement for rod coating is that the paper be level, and a rod will find any slope you leave it.
2. Lay the negative on the uncoated sheet and mark its four corners in pencil. Bostick and Sullivan’s instruction, and it does two jobs: it concentrates the sensitiser where the image is, and it leaves you a coated-but-masked border, which turns out to be the most useful diagnostic area on the finished print. Coat a little outside the mark; the edges will never be exact.
3. Sheet 1, by rod. Pour about 1.5 mL in a line along one edge just outside the marked area, set the rod in the pool and rock it a few degrees each way so capillary action draws the liquid the whole length of it. Draw it across with very light pressure, lift at the far edge, skip back over the bead, and draw the other way. Ware’s pattern is five passes: the first two quick at three or four seconds, the last three as slowly as you can manage at ten to fifteen.
4. Sheet 2, by brush. Wet the brush with distilled water and blot it hard first — Bostick and Sullivan’s reason is that this stops the sensitiser wicking up into the bristles instead of staying on the paper. Pour the whole measured volume into the centre of the marked area and spread it immediately, horizontally across the whole sheet, then vertically, alternating. Bergger call the same thing a criss-cross pattern.
5. Blot the residual liquid at the bottom of the coating with clean blotting paper. Ware’s reason is exact: crystals formed there can damage the negative.
6. Rinse the rod under the tap at once and wash the brush thoroughly. A brush belongs to one process and is never shared; a rod can be shared between processes precisely because it can be got genuinely clean.

=How the sheet tells you the volume was wrong

Both methods have a published self-test, and both are Bostick and Sullivan’s. By brush: if you are still spreading after 30 seconds, reduce the volume by a quarter. By rod: if you can make more than six passes, reduce the volume by a quarter; if a line of solution is still standing after ten passes, blot it off with the corner of a paper towel. And the one that decides the surface: stop brushing before the paper does — over-brushing abrades the sizing and lifts fibre, which clumps in the image area and prints as texture you did not choose. Herschel wrote the same warning in 1842: a brush passed twice over one part gives the effect of a coarse and ill-printed woodcut, and the wash “cannot be in too thin a film”.

Rod coating, one sheet

1. 1Paper tape at the two upper corners — the sheet must lie level or the rod finds the slope
2. 2Pencil marks at the negative corners — sets the coated area and leaves a masked border for diagnosis
3. 3The pool of sensitiser, about 1.5 mL for 10 × 8 in — poured just outside the marked area
4. 4The rod, straight section wider than the image — rocked a few degrees to load by capillary action
5. 5Five passes: two quick, three slow — lift at the far edge, skip the bead, return
6. 6Blot the residual bead — crystals here damage the negative

Drawn, not measured. The pass count and timings are Ware's; the volume is his figure for a 10 × 8 inch sheet.

What the two methods cost and what they buy. Ware’s judgement is that the rod is the more economic implement and the brush the more painterly, and the negatives and papers lesson puts numbers on it: about 30 cm³/m² by rod against 40 to 60 by brush, fifty per cent more sensitiser per print for the life of your practice. What the brush buys is the edge. Neither is correct. Coat both sheets tonight and decide with the prints in front of you.

Stage 4 — Drying (60 minutes, unattended)

Lay the sheet flat until the reflective sheen has gone, then hang it in the dark for about an hour. Ware’s instruction, and both supplier sheets agree: Bostick and Sullivan give an hour in a dark place and state that a hair dryer is not recommended because their testing shows natural drying gives a superior image.

Warm air is allowed; a hairdryer is not. Ware’s alternative is a uniform stream of warm air at 40 °C for ten minutes followed by half an hour to an hour of resting, and his objection to the hairdryer is that direct heating with one is uneven rather than that it is hot. An uneven dry is an uneven sensitiser distribution, which prints.

Humidity barely matters here, which is unusual in this cluster. Ware’s statement is flat: prevailing relative humidity has very little effect on cyanotype, though it can profoundly affect some of the other iron-based processes. Record it anyway, because the claim is worth testing on your own paper and because the same notebook serves processes where it matters a great deal.

The dry sheet should be greenish-yellow. Photographers’ Formulary put the whole diagnostic in one sentence, and it is the best sentence on their sheet: the dry sensitized paper should appear greenish-yellow; if it is blue, ferrous salts are present either by exposure or through chemical contamination. Do not expose a sheet that has gone blue. Find out which of the three it was.

How long a coated sheet keeps. Jacquard state that sensitised paper and fabric may be stockpiled, that six months is the figure for best results in a sealed bag in a cool dry place, and that coated material which has darkened is not necessarily expired and should be tested rather than discarded. Ware gives no keeping time of his own. Coat what you will use, and test anything older than a month against a fresh strip.

Stage 5 — The test strip, which is the point of the session (45 minutes)

!Why this page publishes no exposure time for your unit, and what it publishes instead

Three sourced figures exist for a classic cyanotype, and all three belong to somebody else’s light.

• Around 20 to 30 minutes under a typical ultraviolet lamp — Ware, for the sensitiser on this page, repeated in his list of the process’s drawbacks against about 2 to 4 minutes for palladiotype.
• About 10 minutes to maximum density — Bergger, for a premixed kit of undisclosed strength, under a cloudless sky at noon in Paris in summer, offered as a tip and followed immediately by an instruction to make a test strip.
• 10 to 20 minutes under a 275 or 300 watt sunlamp at 12 to 18 inches — Photographers’ Formulary, for their own kit at 10 per cent citrate and 4 per cent ferricyanide in the mixture.

A fourth figure is theoretical and often quoted as though it were not: Ware’s photochemical calculation puts average sunlight at 30 to 40 W/m² of ultraviolet and derives 2 to 4 minutes to fully darken an unobscured proto-photographic sensitiser — a limiting estimate for the class, not a time for this formula.

Part XVI has already ruled on this and this page follows it: the UV unit page publishes one irradiance figure, calls it a design target with an assumed coupling factor, and states that the course publishes no claim it has not measured and it has measured nothing on your box. The same applies to minutes. Two units built to one drawing, at different array heights, with emitters of different ages, are two different light sources.

So what this page publishes is the bracket, and the bracket is the course’s own derivation from Ware’s 20 to 30 minutes: a stepped series of 5, 10, 20 and 40 minutes centred on his figure. That is deliberately not Part XVI’s cyanotype bracket of 2, 4, 8 and 16, which was reasoned from the roughly 8½-minute exposure Ware publishes for his faster Simple cyanotype. Bracket the process you are printing, not the family.

1. Cut one coated sheet into six strips, numbered in pencil on the back before anything is exposed.
2. Lay a step wedge across each strip if you have one, so the series tells you about the tonal scale as well as the level — enough for tonight either way, and not enough without one for the experiment that follows.
3. Expose the series a stop apart: 5, 10, 20 and 40 minutes, each strip separately and whole rather than by sliding a card across one strip — a card gives cumulative exposures with one processing history, while separate strips give four independent results and let you open the frame and look. Use the UV unit SOP, and give a mercury or fluorescent source its five minutes to reach full output first: cold lamps are a classic reason a repeat exposure does not repeat.
4. Inspect one strip through the split back, at four minutes and then every few minutes, so that you have watched the whole colour sequence at least once.
5. Process all four together, same baths, same times, dried together — anything else compares the processing rather than the exposure.
6. Read the result the next morning, after re-oxidation, and take the shortest exposure that reaches full density. That is Bergger’s own criterion: past it you are paying in time and in highlight detail for nothing.

📓What voids the dose you just established

Write it on the unit’s label with its conditions attached, because each of these makes it wrong again: a different paper or batch of paper; a different lot of the iron salt, since a quarter’s difference in iron content is a quarter’s difference in what the light has to work on; a different mixed strength, including the same stocks made up to a different volume; a different array height; accumulated lamp hours, a figure you do not know, which is why Part XVI counts them; and a year, whichever has happened first.

Stage 6 — Exposing the print

Negative orientation, and it is the one mistake that cannot be fixed afterwards. With the frame face down on the bench, lay the negative on the glass base side down, emulsion facing up, then the coated sheet face down on top of it, so that emulsion meets emulsion. Then apply Bostick and Sullivan’s check before you close the back: viewed through the glass the image must read the right way round, and if it reads reversed the print will be reversed. Any gap between the two surfaces spreads every edge by an amount the negatives and papers lesson calculates from the size of your source.

Give it the dose from stage 5, and then look anyway. The dose only saves you from starting at zero: a printing-out process is judged by inspection, which is its whole advantage.

Stage 7 — Wet processing (30 minutes)

Into water within a minute or two of leaving the frame. The sheet is still a sensitised material until it is washed, and it will go on printing in the room light.

1. First bath, face down, about two minutes with gentle agitation — Bergger’s figure for their own two minutes in plain water. Plain water is the default here. The yellow stain of unreacted sensitiser lifts within seconds and clouds the tray — Bostick and Sullivan describe exactly this — and blue will come off too. Some blue is normal.
2. Wash. Ware’s figure for the classic process is 20 minutes face down in gently running water or several changes of static water. Bostick and Sullivan give 12 to 15 minutes with agitation and a change every two minutes; Photographers’ Formulary and Jacquard both give at least 5 minutes. Those are not really in conflict, because all four state the same end point, and it is the end point that governs.
3. The end point is the disappearance of the yellow from the highlights, not a time on a clock. Ware’s refinement is worth adopting: hold the wet print up to a bluish light, or look through a blue filter, which makes a faint yellow stain in the interior of the paper obvious when it is invisible in room light.
4. Then stop. This is the one wash in the course where longer is not safer.

⚗The wash is doing three jobs and one of them is taking your picture away

It removes the unreacted iron and ferricyanide, which must go or the print is still light-sensitive. It completes the blue, because the wet paper lets iron(II) and ferricyanide find each other. And it peptises part of the image and carries it off, which is not a fault of technique: Ware’s words are that there will be “considerable leaching-out of blue pigment”, and that the washing needed to clear the paper will significantly reduce the image and truncate the tonal scale in the high values. Thin deposits lose a larger fraction of themselves than thick ones, which is most of why the exposure scale is only about 0.9.

Photographers’ Formulary state the trade-off in one sentence and it is the best short statement of it in the sources read: a short wash leaves ferric salts in the paper and the print will fade, while prolonged washing lightens the image, particularly if the water is slightly alkaline. You are choosing between two failures, and the choice is made by looking rather than by timing.

!The acid first bath: what it actually does, and what it does not

The course’s manifest for this session originally said that a slightly acidified first bath “clears the highlights better than plain water”. No source read for this page supports that, and the main one says something close to the opposite, so the manifest has been corrected and here is what the sources say.

What is sourced. Ware records for the classic process that the tonal scale may be strengthened by development first in a bath of very dilute weak acid, such as 1 per cent w/v citric acid, rather than in plain water — an effect on the scale, not on highlight clearing. And in the same sentence, that if the acid bath is too strong it may cause blue fogging of the highlights and degrade the paper white, which is the opposite of clearing them.

What acidity genuinely buys in the highlights is a different bath. Keeping the wash water below pH 7 reduces the loss of image substance, and Ware’s remedy for alkaline tap water is a spoonful of citric acid in it. That is a pH correction to the wash, not a development bath, and it is the acid instruction that applies to everyone with hard or alkaline water.

What is not transferable. The half-to-one-minute development in 1 per cent citric acid belongs to the Simple cyanotype and the mineral-acid developer to the New Cyanotype; neither is published for this formula, so neither is given here.

What to do tonight. Process the print in plain water and one test strip in 1 per cent citric acid, and compare them. That is a cleaner answer than either instruction, and it is the first of the Further experiments.

Stage 8 — Re-oxidation (half a minute, or overnight)

The reversed shadow tones come back. Prussian white is oxidised to Prussian blue by air over several hours of drying, and the print you hang up is not the print you will have in the morning. Bostick and Sullivan note the same at a longer timescale: a faded print left in the dark for two to four weeks will re-oxidise.

If you want the density now — and for stage 9 you do — half a minute in about 0.3 per cent hydrogen peroxide before the final wash does the same chemistry in seconds:

2 Fe[Fe(CN)₆]²⁻ + H₂O₂ → 2 Fe[Fe(CN)₆]⁻ + 2 OH⁻

Peroxide re-oxidation: Prussian white back to Prussian blue

Then wash again, and look at the right of that equation for the reason: the reaction makes hydroxide. Ware makes the general point that all these oxidations tend to make the system more alkaline, and alkali is the one thing that destroys a Prussian blue image outright — which is why Bostick and Sullivan’s own instruction after their peroxide bath is a further 8 to 10 minutes of washing. Their bath is 60 to 90 seconds, Ware’s about half a minute, Photographers’ Formulary’s a few seconds: take the shortest that works, and wash after it.

⚗What the peroxide bath costs, honestly

Ware states that it makes no difference to the final densities. It buys the answer now, and nothing else.

No source read for this page quantifies a cost in highlight detail, and the course will not assert one. What is documented is the alkali the reaction generates, which is a real mechanism for damage if the bath is long and the wash after it is short. There is an ordinary second risk: the pharmacy bottle degasses and loses potency on the shelf, so a print left in a bath of unknown remaining strength is in an experiment rather than a process. Use fresh peroxide, make the bath up for the session, do not store it.

Never use a dichromate bath for this. It was once common practice, especially for fabrics. Ware strongly advises against it, records that the consequences to one artist who used a dichromate bath for quilting fabrics are described in the medical literature, and notes that victims of dichromate exposure can become particularly sensitised. Ammonium dichromate and potassium dichromate are chromium(VI) compounds under the chromium ruling. Use air, or peroxide.

Stage 9 — Drying, and the measurement (next morning)

Drain for about fifteen seconds — Bostick and Sullivan’s figure — and air-dry hanging or flat on a plastic screen, in a dust-free place: a cyanotype has no binder to protect its surface.

Then measure, twice on the same marked patch: wet and blotted, straight out of the final wash, and dry, the next morning, after the re-oxidation has finished.

The wet reading carries a caveat and the notebook has to carry it too. A reflection densitometer reads light coming back off a surface, and a wet paper surface has a specular sheen a dry one does not; blotting removes most of it and not all. So the wet figure is indicative, the dry figure is a measurement, and the difference is a number with an uncertainty rather than a constant — the densitometer geometry page explains why the geometry matters more than the electronics here.

Without a densitometer the comparison still works: cut the dried strip in half and compare the halves side by side under one light. You lose the number, not the finding.

Expected observations

During coating. A clear yellow-green liquid that soaks in and darkens the paper slightly. On a heavily sized sheet it will bead rather than wet — Ware’s remedy is two strips of sensitiser rather than one, and it is a sign the paper is fighting you.

On the dry sheet. Greenish-yellow, even, the sheen gone. Not blue.

During the exposure, and this is the sequence to learn by heart:

Stage	What you see	What it means
First minute	Yellow-green pales, then greys	Iron(III) being reduced; almost no blue yet
A few minutes	Uncovered areas turn a bright, obvious blue	Prussian blue forming. This blue is not your image — Bostick and Sullivan state it is non-archival and will wash away in plain water
Later	The blue in the most-exposed areas stops deepening and begins to reverse, going grey and then a pale blue-grey; on fabric and some heavily coated papers the deepest areas read as a metallic bronze-grey	Solarisation: the blue is being reduced to Prussian white. Jacquard use the bronze appearance as their own end point for fabric
Correct exposure	Deep shadows visibly reversed, highlights distinctly green, the whole image blocked up and slightly wrong	Ware’s three criteria together. Bostick and Sullivan describe the same moment as the darkest areas beginning to reverse and become lighter
Over-exposure	The reversal spreads out of the shadows into the mid-tones; highlight detail closes up and blues over	Too far. The reversal is not the failure; its extent is

Before washing the sheet looks olive-green — Photographers’ Formulary’s word — and the print looks faded and washed out. Bostick and Sullivan say the quiet part out loud: cyanotypes must be over-exposed to create a permanent image, and judging that moment is the most crucial part of the process.

In the first tray, a yellow cloud lifting within seconds and then a fainter blue one; the image darkens and gains contrast as the paper wets. Overnight the reversed shadows fill back in and the whole print deepens. A print judged wet is a print judged wrong.

What is happening chemically

Three steps, all of them worked through properly in the previous lesson. What matters at the bench is which step each stage belongs to.

Light reduces the iron, and the citrate pays for it. All the photochemistry belongs to the iron salt: near-ultraviolet light transfers an electron from the citrate ligand to the iron, iron(III) becomes iron(II), and the citrate is oxidised with loss of carbon dioxide. The ferricyanide takes almost no part in this — its own quantum yield is around 0.01 against the citrate’s 0.45 — but it absorbs strongly in the same region, so it acts as an internal filter and slows the exposure it does not drive. That is why a lamp peaking near 410 nm is nearly wasted on a cyanotype, and it is stage 5’s problem.

Iron(II) meets hexacyanoferrate(III), and no light is needed:

Fe²⁺ + [Fe(CN)₆]³⁻ → Fe³⁺ + [Fe(CN)₆]⁴⁻

A plain electron transfer between two iron centres

The iron(III) and hexacyanoferrate(II) precipitate together as Prussian blue. Because this step needs no light it goes on happening in the wet paper — which is why the wash completes the image as well as clearing it — and slowly in a mixed bottle, which is stage 2’s problem.

Over-exposure reduces the blue in its turn to colourless Prussian white: the reversal you watch for in stage 6 and the density you get back in stage 8. A printed-out blue masks itself, absorbing the light that would make more, so when the shadows reverse the masking falls and more light gets through, and the final density after re-oxidation can be higher than it would otherwise have been. That is why the correct exposure looks over-exposed.

And the wash is the only development this process has, because everything unexposed is soluble and there is no fixer: nothing is left to fix. Stage 7 has what that costs.

Data to record

Two records, and the second is the reason the first exists. Use the lab notebook format and the batch record SOP.

The batch record, one entry per bottle: substance and strength as per cent w/v; what was weighed and what it was made up to; supplier and lot number of the solid; date and batch number; preservative or none; and observations over time.

The session log, one line. Date, room temperature, relative humidity. Paper: maker, product, surface, weight, size, which side. Both batch numbers and the mixed strength. Volume mixed, and how long after mixing each sheet was coated. Coating method and volume per sheet. Drying method, temperature and time. Source: which unit, array height, warm-up, lamp hours — or, outdoors, date, time, orientation and cloud. Exposure per strip and per print, every bath and every time, and which waste went where.

The measurement table, one row per marked patch:

Column	What goes in it
Sheet and method	1 rod, 2 brush, S1 to S4 test strips
Patch	Numbered, marked in pencil on the back before exposure
Exposure	Minutes, and the source
D wet	Reflection density, blotted, straight out of the final wash — marked as indicative
D dry	The same patch the next morning
Δ dry-down	Dry minus wet, signed
Re-oxidation	Air overnight, or peroxide and for how long
Notes	Bleeding, fog in the masked border, coating texture, edge quality

And one thing in neither table: the masked border. Ware’s diagnostic needs a region coated but never exposed and a margin never coated, both on one sheet. Your pencil marks gave you the first and the sheet edge the second. Compare them before you throw anything away: almost every fault on this page is diagnosed by that comparison and by no other observation.

Analysis

First, decide what you may claim. You have one print per condition, so the honest form of every conclusion is “on this paper, at this strength, on this night”.

Second, read the dry-down, which has two parts worth separating: drying, which changes how the paper scatters light, and re-oxidation, which changes how much blue there is. The peroxide strip separates them, its re-oxidation having finished before it dried.

🧮The arithmetic, worked on constructed figures

Every number in this box is invented to demonstrate the method. None is a measurement, none comes from a source, and none should be copied into your notebook. The course has not run this session; it publishes the procedure and the arithmetic, and your paper supplies the numbers.

Suppose a mid-tone patch on the air-dried print reads 0.62 blotted-wet and 0.88 the next morning:

Δ = D_dry − D_wet = 0.88 − 0.62 = +0.26

Dry-down, as measured

Suppose the peroxide strip, exposed identically, reads 0.79 blotted-wet and 0.87 dry:

drying alone ≈ 0.87 − 0.79 = +0.08 · re-oxidation ≈ 0.26 − 0.08 = +0.18

Separating the two effects

Two things follow, and both are worth more than the numbers. The two dry readings agree to 0.01, which is the test of Ware’s statement that the peroxide bath makes no difference to the final densities — your data either support it or they do not, and either result is a finding. And a cyanotype gains density on drying, which is the opposite sign from the dry-down every silver-gelatin printer learns in Part XIX. Two processes, two signs, one word: say which one you mean.

Third, compare the coating methods on evidence: evenness of the mid-tones, the edge, any texture from the implement, and the maximum density if you can read it. Divide the volume used by the area coated and see whether your figures land near the published 30 cm³/m² by rod and 40 to 60 by brush.

Fourth, ask what the exposure series says about the negative rather than the light. A series that goes from blank to blocked with nothing between means the negative’s density range is wrong for a process whose exposure scale is 0.9, and that is not a fault you can expose your way out of. Then name the one variable you would change first, which in this process is almost always the paper.

Troubleshooting

The formulary page carries the fault list for the formula itself. What follows is what goes wrong at this bench, and one diagnostic worth more than any of it.

=Fog or stain? Ware's algorithm, and it settles most of this section

Ware distinguishes two faults that look alike and have nothing in common. Fog is unwanted image substance — Prussian blue where there should be none — and it is grey. Stain is unwanted residual chemistry, chiefly ferric salts, and it is yellow. Then he asks six questions in order, comparing the masked border with the uncoated margin:

1. Fog visible as soon as the sheet is coated? The sensitiser is already decomposed, or there is a very hostile chemical in the paper.
2. Fog appears while it dries? Dry a second sheet in total darkness. If it still fogs, the paper is the problem; if it does not, your working light is.
3. Fog in the masked border after exposure? The mask or the negative is not dense enough in its maximum value. Include a scrap of high-blocking material for comparison.
4. Fog only after wet processing? Decomposed sensitiser, a faulty working light, a thin negative, or the wet chemistry itself.
5. Yellow stain remaining after the wash? The clearing is inadequate. Wash longer, and check the water.
6. Stain in areas that were never coated? Your wet chemistry is contaminated.

It works because stage 3 left you a masked border, which is what the pencil marks were for.

A pale, flat print with empty highlights. Peptisation, not under-exposure. Suspect, in order: a heavily sized paper the sensitiser never got inside; a wash longer than the end point required; hard or alkaline wash water. The remedies in the same order are a different paper, a wash judged by the yellow rather than the clock, and distilled water for the final rinse.

No image, or one that appears and goes. Check the paper for an alkaline buffer, then the wash water’s pH, then the age of the citrate stock, in that order.

Blue creeping out of the shadows into the highlights. Bleeding: excess iron(II) diffusing sideways, intrinsic to a formulation in which the iron is in large molar excess over the ferricyanide. Less exposure and a shorter first bath reduce it; changing formulation eliminates it.

Discrete blue spots unrelated to the negative are iron — a chipped tray, a clip, a ferrule, a nail on the line. A gritty surface is crystals, from a stock made up warm and used cold. A tacky sheet that marks the negative is deliquescence plus humidity, the third of Ware’s shortcomings: dry properly, store sealed, interleave.

Clean-up

Rinse the rod immediately, before anything dries on it, and wash the brush thoroughly — Bostick and Sullivan’s reason is that whatever is left on it will be exposed and will contaminate your next print.

Wipe the bench and bin the blotting paper into the solid waste, not the sink. Rinse graduates and the mixing cup into the waste container rather than down the drain. Wash the outside of both stock bottles: a drip of one on the shoulder of the other is how cross-contamination happens weeks later.

Gloves off last, under the PPE removal SOP, and hands washed. Yellow stains on skin will turn blue when they meet water; that is a marker that a glove failed, and it is worth noticing rather than scrubbing away without thought.

If either solid was spilled dry, follow the solid spill SOP: pick it up dry, do not wet it and do not sweep it into the air.

Storage

The two stocks: brown bottles, in a cupboard or a box, labelled and dated. Ware’s instruction and every supplier’s.

How long they keep, and here the sources genuinely disagree.

Source	What it says about the separate stocks
Ware	Storable, but the citrate solution’s surface will carry a thick furry mould within a week or two in many environments unless thymol is added. No keeping time given
Jacquard	“The stock solutions are stable long term”, and mould in the citrate solution is noted without being treated as spoilage
Bostick and Sullivan	Approximately 1 year from purchase, at room temperature, in the original separate containers
Bergger	About one year, kept dry and away from natural light
Photographers’ Formulary	Nothing at all — no shelf life for the kit, the sachets or the made-up stocks

The two commercial figures agree at about a year and describe bought solutions of undisclosed composition; Ware describes what happens to a home-made citrate solution in an ordinary room, which is a biological problem rather than a chemical one. The course’s position: a stock older than a year, or one that has grown mould, is replaced, and a stock of any age is tested against a fresh strip before a session that matters.

The mixed sensitiser is not stored. Two to four hours is the only published figure and the two suppliers who give it give the same one.

The solid citrate is deliquescent and will set solid in a humid room: stoppered, dry, cool, opened briefly. The Spectrum sheet adds that it is sensitive to light and belongs in a light-resistant container — the instruction Photographers’ Formulary printed as a footnote in an older printing of their sheet and dropped from the current one.

The ferricyanide solid keeps well; its solution keeps less well, decomposing slowly in light, which is the second reason for a brown bottle. Separated from acids, as the safety card directs — a different shelf, not a different corner of one. See incompatibilities.

Coated paper: sealed bag, cool, dry, dark; six months for best results on Jacquard’s figure.

Finished prints: unbuffered mounts only, because the calcium carbonate reserve in archival board sits at roughly the pH that destroys Prussian blue. The one storage instruction that contradicts every other process in the course.

Disposal considerations

The chemistry first. The waste is dilute ammonium iron(III) citrate, dilute potassium ferricyanide, a little potassium ferrocyanide made by the reaction, and suspended Prussian blue. The hexacyanoferrates are complex cyanides, not cyanide salts: the cyanide is bound to iron in a complex of extreme stability, which is why the Environment Agency’s waste classification guidance exempts ferrocyanides and ferricyanides by name from the hazard statements it assigns to cyanide salts for liberating a very toxic gas on contact with acid. That exemption is about what the substance is, not about how dilute it is, and it does not make an acid addition acceptable.

The general practice. Never add acid, and never top up an unlabelled container — the unlabelled container SOP exists because that is how it goes wrong. Prussian blue settles out of standing wash water as a fine sludge, so decant rather than shake before moving a container.

The one substance with a documented aquatic figure. The supplier’s sheet for potassium ferricyanide gives an LC50 for fish of 1869 mg/L and an EC50 for Daphnia of 1549 mg/L, which are high values as aquatic toxicity goes, while the ECHA notifications aggregated on PubChem include H411 in a substantial minority of reports. The course records both and settles neither.

And the caveat that is not a formality. The course publishes no jurisdiction-specific disposal instruction; 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.

Questions

1. Why is the sensitiser kept as two bottles when it is only ever used mixed? Name the reaction that makes it necessary and say why it is slow rather than instant.
2. Your label says “10 g in 50 mL of water” and your neighbour’s says “10 g made up to 50 mL”. Whose is stronger, and what would you have to know to give a number?
3. A sheet dries blue instead of greenish-yellow. Give three distinct causes, and say what single comparison on the sheet itself begins to separate them.
4. Last month’s exposure gives a thin print today, on the same unit, paper and negative. List four things that could have changed, in order.
5. A print looks perfect through the split back at twelve minutes. Why is that a reason to keep exposing? Answer in terms of what the wash does and what solarisation does.
6. The peroxide re-oxidation produces hydroxide. Why does that matter more here than in any silver process, and what step does it make compulsory?
7. You have an unbuffered cotton print and a sheet of “archival, acid-free, buffered” mountboard. What will happen, over what sort of timescale, and what should you mount it on instead?

Further experiments

Water against 1 per cent citric acid, on one exposure. Two identical strips from one coating and one exposure; develop one in plain water and one in 1 per cent citric acid before the same wash, and read maximum density and the separation of the light values on both. This settles for your paper what the sources only assert. Then do it wrong on purpose, once, with a third strip at 2 per cent, so that you have seen the blue highlight fog Ware warns about.

Rod against brush, quantified. Weigh the mixing cup before and after each coating, so you know the mass you put on the paper rather than the volume you poured, and divide by the coated area.

Test the alkali rule instead of believing it. One strip given a minute in a very dilute sodium bicarbonate solution, one not. The fastest demonstration in the course that an “archival” material can be the wrong material — and it is the first bath of the tannic acid toner done as a lesson rather than as a step.

Two lots of the iron salt, or a green and a brown grade, coated and exposed together — which turns “ill-characterised substance” into something you have seen.

Then read the alternatives you did not print tonight, all of which use the bench technique you have just learned: Simple cyanotype and Mike’s cyanotype, made to a chosen exposure scale; Herschel’s own 1842 mixture, at the far historical end of the same two chemicals; and the Pellet process, which turns it positive.

Recap

The two bottles are a consequence, not a convention. The salts react slowly in the dark, so a mixed sensitiser is a reaction you have started, with two to four hours of useful life, while the separate stocks keep for months.

Make up to a volume, and record the lot. A concentration is a mass in a final volume, and the iron salt’s own iron content varies by a quarter between batches, so the supplier and lot number are part of the formula whether the label admits it or not.

Coat by rod for economy and by brush for the edge — five passes with two quick ones for the first, horizontal then vertical for the second, and stop before the paper does.

Nobody’s minutes are yours. Ware’s 20 to 30, Bergger’s 10 and the Formulary’s 10 to 20 belong to three different light sources. The course publishes the bracket, 5, 10, 20 and 40 minutes, and the conditions that void your result.

The correct exposure looks wrong: reversed shadows, distinctly green highlights, an image that looks blocked up. The bright blue you see first is not the image and washes away.

The wash clears, completes and steals. It is the only development this process has, its end point is the disappearance of the yellow rather than a time, and the print goes on gaining density for hours after it. Peroxide buys that density now and makes hydroxide doing it, so it is followed by another wash.

And the one absolute: no acid, and no alkali. Acid never meets the ferricyanide; alkali — buffered paper, buffered board, alkaline tap water — takes the finished image apart.

Check your understanding

Question 1. Why does the classic cyanotype sensitiser have to be kept as two separate stock solutions?

• The two salts react violently if combined dry
• Ferricyanide is light-sensitive and the citrate is not, so they need different bottles
• A slow dark reaction between them makes Prussian blue in the bottle, so the mixture has a short life while the separate stocks keep for months
• Mixing them changes the pH beyond the range where the citrate complex is photoactive

Show the answer and why

Answer: A slow dark reaction between them makes Prussian blue in the bottle, so the mixture has a short life while the separate stocks keep for months

The iron(III) is locked in a citrate complex and is not available as a free ion, which makes the reaction slow rather than instantaneous - but it does not stop it. Two suppliers independently publish 2 to 4 hours for the mixed sensitiser, and Ware says only that the life is "relatively short", which is why the formula is two bottles and three pipettes.

Question 2. A test strip has been exposed until the deepest shadows have visibly reversed to a pale blue-grey and the highlights look distinctly green. What should you do?

• Stop at once - the shadows have solarised and density has been lost
• This is the correct exposure for the classic process; process it
• Halve the exposure on the next strip, because the reversal shows over-exposure
• Continue until the reversal reaches the mid-tones

Show the answer and why

Answer: This is the correct exposure for the classic process; process it

Ware gives three simultaneous criteria for a correct classic cyanotype exposure - somewhat reversed shadows, distinctly green highlights, and an image that looks rather blocked up - and Bostick and Sullivan describe the same moment as the darkest areas beginning to reverse. The reversal is not a loss: a printed-out blue masks the light that would make more, so when it reverses more light gets in, and the density comes back on re-oxidation.

Question 3. Your tap water measures pH 8.1. Which single change matters most, and why?

• Wash for longer, because alkaline water clears the highlights more slowly
• Wash in water acidified with a little citric acid, or in distilled water, because alkali hydrolyses Prussian blue and prolonged washing in alkaline water lightens the image
• Add more ferricyanide to the sensitiser to compensate for the loss
• Nothing - pH only matters for the sensitiser, not for the wash

Show the answer and why

Answer: Wash in water acidified with a little citric acid, or in distilled water, because alkali hydrolyses Prussian blue and prolonged washing in alkaline water lightens the image

Prussian blue is decomposed by alkali; a buffer at pH 9.4 destroys it in minutes, and even slightly alkaline wash water measurably lightens a print. Ware's own remedy for alkaline wash water is a spoonful of citric acid to bring it below pH 7. The Photographers' Formulary sheet names the same failure in one sentence - prolonged washing lightens the image, particularly if the wash water is slightly alkaline.

Question 4. You mix 100 mL of 3 per cent hydrogen peroxide into water to make 1 litre. What is the strength of the bath, and why is a further wash needed afterwards?

• 0.3 per cent; the further wash removes unreacted peroxide only
• 3 per cent; no further wash is needed because the peroxide decomposes
• 0.3 per cent; the re-oxidation reaction generates hydroxide ions, and alkali attacks the image the bath just intensified
• 0.03 per cent; the further wash is a matter of habit rather than chemistry

Show the answer and why

Answer: 0.3 per cent; the re-oxidation reaction generates hydroxide ions, and alkali attacks the image the bath just intensified

Three per cent of a tenth of the final volume is 0.3 per cent w/v, which is the strength Ware publishes and within rounding of Photographers' Formulary's 0.27 per cent - three independent sheets converging. The equation for the re-oxidation puts hydroxide on the product side, and Ware notes that all these oxidations tend to make the system more alkaline. Bostick and Sullivan's own instruction is a further 8 to 10 minutes of washing after their peroxide bath.

Question 5. A dried, unexposed coated sheet is faintly blue rather than greenish-yellow. Which of these could account for it? (Select all that apply.)

• Light reached the sheet while it dried
• The brush had previously carried the other stock solution
• The sensitiser was mixed several hours before coating
• The paper carries an alkaline buffer

Show the answer and why

Answer: Light reached the sheet while it dried, The brush had previously carried the other stock solution, The sensitiser was mixed several hours before coating

Blue on an unexposed sheet means iron(II) and ferricyanide have met - by exposure, by contamination, or by the dark reaction in an old mixture. Photographers' Formulary state the diagnostic in one sentence: if the dry sensitised paper is blue, ferrous salts are present either by exposure or through chemical contamination. An alkaline buffer does the opposite - it destroys Prussian blue and hydrolyses the sensitiser, so it gives a weak or absent image rather than a blue cast.

Question 6. The wash removes unexposed sensitiser and completes the formation of Prussian blue. What is the third thing it does?

• It fixes the image by converting residual iron to an insoluble compound
• It peptises and carries away part of the image substance, costing the light tones proportionally more than the shadows
• It neutralises the acid produced during exposure
• It hardens the paper sizing so the image is more permanent

Show the answer and why

Answer: It peptises and carries away part of the image substance, costing the light tones proportionally more than the shadows

Ware writes that there will be "considerable leaching-out of blue pigment" and that the extended washing needed to clear the paper truncates the tonal scale in the high values. A thin deposit loses a larger fraction of itself than a thick one, which is most of why the classic process ends up with an exposure scale of only about 0.9 - three stops - and why the New Cyanotype, whose image substance is better retained, reaches about 2.2.

Question 7. Why does this page publish a bracket of 5, 10, 20 and 40 minutes rather than an exposure time?

• Because the sources disagree and the course averages them
• Because irradiance at the paper depends on the unit, the array height, the lamp hours and the paper, none of which the course has measured on your equipment - so it publishes the calibration and its own derivation of a sensible bracket instead
• Because cyanotype does not obey reciprocity, so a single time would be meaningless
• Because the exposure depends only on the negative and cannot be generalised

Show the answer and why

Answer: Because irradiance at the paper depends on the unit, the array height, the lamp hours and the paper, none of which the course has measured on your equipment - so it publishes the calibration and its own derivation of a sensible bracket instead

Part XVI already rules that the course publishes no claim it has not measured, and it has measured nothing on your box: its one irradiance figure is labelled a design target with an assumed coupling factor. The three published times - Ware's 20 to 30 minutes under a typical lamp, Bergger's 10 minutes in Paris sun, the Formulary's 10 to 20 under a named sunlamp - describe three different sources and two different sensitiser strengths. The bracket is the course's own derivation from Ware's figure, and it is deliberately different from Part XVI's cyanotype bracket, which was reasoned from the faster Simple cyanotype.

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