Mike's Cyanotype

Every other formula in this chapter was designed to solve a problem with the picture. This one was designed to solve a problem with a bottle. Mike Ware’s Simple Cyanotype of 2019 builds its light-sensitive compound in the beaker out of citric acid, an iron salt and concentrated ammonia, and the ammonia is the step that some people cannot take: they cannot buy the reagent, or cannot store it, or cannot be in a room with it. Ware’s answer was not to weaken the formula but to buy the ammonia already attached to the citrate, as a bland white powder. The result is the same solution, made odourlessly, in five short steps, out of three solids and some water.

Ingredient	Quantity	Form the source specifies
Triammonium citrate	15 g	Triammonium citrate, (NH4)3C6H5O7, which Ware also names ammonium citrate, tribasic; formula weight 243.22, CAS 3458-72-8. Dissolved first, in 40 cc of pure water. Ware records that the solution cools as it dissolves and may be warmed to help it. This course has no chemical encyclopaedia page for it, and its ammonium citrate entry is the dibasic salt, which is a different substance
Iron(III) nitrate nonahydrate	12 g	Fe(NO3)3.9H2O, formula weight 404.00, CAS 7782-61-8; added to the citrate solution slowly, a little at a time, with stirring, and never dissolved in water on its own. The solution goes yellow-green. A pale violet to greyish-white deliquescent solid melting at 47.2 degrees C. This course has no chemical encyclopaedia page for it either
Potassium ferricyanide	10 g	Ground before weighing, which is Ware's instruction here and not in the parent formula; added last, to the yellow-green solution, and stirred until every particle has gone. The final colour is olive-brown. Section 7.5.1 asks only for General Purpose Reagent grade, about 98 per cent purity, where the parent formula prefers Analytical Reagent grade at 99 per cent and over
Water	to make 100 mL	Purified water - distilled, de-ionised or pharmaceutical. Only 40 cc goes into the beaker at the start; the balance is added at the end, in a measuring cylinder, to bring the whole to the 100 cc mark. That make-up volume fixes all four concentrations at once and is the only step in the preparation that has to be done in graduated glass.

!Two ingredients the encyclopaedia had to be extended for

Neither triammonium citrate nor iron(III) nitrate nonahydrate had a page in this course’s chemical encyclopaedia when this formula was written. Two of the three solids were substances a reader could not look up, because the register was built from the design files and had never listed either. This page needing them is how that was found; both entries now exist (triammonium citrate, iron(III) nitrate nonahydrate) and both names in the table above are links.

Triammonium citrate is the more dangerous omission, because a near neighbour does have a page and it is the wrong substance. The encyclopaedia’s ammonium citrate entry is the dibasic salt, (NH₄)₂HC₆H₅O₇, CAS 3012-65-5, molar mass 226.18, which still carries one of citric acid’s three acidic protons and whose solution is mildly acidic. What this formula needs is the tribasic salt, (NH₄)₃C₆H₅O₇, CAS 3458-72-8, EC 222-394-5, molar mass 243.22, all three carboxyls neutralised. That page says in as many words that the two are different substances with different ECHA entries, and it is right: the missing proton is the whole point of the substitution this formula makes. If a supplier’s label says only “ammonium citrate”, the CAS number on the safety data sheet settles it and nothing else does.

The warning above is the reason this note survives the entries being written. A supplier’s label that says only “ammonium citrate” is not enough to buy from, and no encyclopaedia page can protect a reader who never checks the number on the sheet in front of them. Both substances’ identities, physical descriptions and notified hazard classifications are also set out on this page, under Function of every ingredient and under Safety, because they are the figures this formula is worked from; the encyclopaedia entries carry the storage, incompatibility and waste sections written once for the whole course, and the iron(III) nitrate entry carries the hydrolysis prohibition at greater length than there is room for here.

Purpose

To make the Simple Cyanotype sensitiser without ever opening a bottle of concentrated ammonia.

That is the whole of it, and it is worth being precise about what kind of change it is. This is not a milder version of a formula, and it is not a substitution made to save money or to work around a shortage. It is a different route to the same solution. Ware states his intention plainly: the preparation should produce, odourlessly, a sensitiser that is identical with the first of the Simple Cyanotype sensitisers. Everything the parent formula does on the paper, this does on the paper.

The chemistry behind that claim is an acid-base bookkeeping exercise, and it is checkable. In the parent formula, citric acid goes into the beaker as the free acid and ammonia is then run in to take its protons off. Three ammonias per citric acid is exactly the composition of triammonium citrate. So a jar of triammonium citrate is a jar of citric acid that has already had its ammonia added, weighed out by the manufacturer under conditions you do not have to reproduce, and stable on a shelf. The arithmetic that shows the two routes really do arrive at the same place is set out under The mechanism, and it agrees to better than half a per cent.

What is bought and what is sold. Bought: no ammonia bottle, no ammonia vapour, no titration, no hydrometer, no goggles-and-extraction step in the middle of the preparation, and a preparation that a person with asthma or a shared kitchen can carry out. Sold: the contrast control. In the parent formula the ammonia volume is not a detail of the preparation, it is the exposure-scale adjustment, and the only one any cyanotype sensitiser has ever had. Take the ammonia out and you take that out with it. Ware publishes one grade here and no others.

It remains a printing-out process. The image is made by light in the frame, not by a developer afterwards; the bath the print goes into is called development by convention, and what it actually does is release iron(II) from its ligands so the blue can finish forming and wash out everything the light did not use.

Recommended uses

• When concentrated ammonia is not available to you. Ware wrote the section because correspondents told him so, and the reasons are ordinary: a supplier who will not ship it, a jurisdiction that restricts it, a school or a shared studio that will not have it on the premises, a household with no room that can be ventilated properly.
• When concentrated ammonia is available and you still should not use it. The international chemical safety card for 10 to 35 per cent ammonia solution describes it as very volatile, gives the boiling point of the 25 per cent reagent as 38 °C, and classifies it Danger: causes severe skin burns and eye damage, harmful if swallowed, may cause respiratory irritation, very toxic to aquatic life. A bottle whose contents boil a little above room temperature has a loaded headspace before anyone tips it. Ware’s own Simple Cyanotype instruction sheet opens its preparation with “PROTECT YOUR EYES with safety goggles” and “Avoid inhaling ammonia vapour!”. Anyone with asthma or another airway condition has a good reason to want a preparation that never generates a vapour at all.
• When the low-contrast grade is the one you want anyway. The exposure scale of about 2.7 is the grade most people printing digital negatives or negatives made for the siderotypes will choose, and if that is where you were going to end up, the contrast control was not buying you anything.
• When the preparation itself is the lesson. Three solids, water, a beaker, a rod and a measuring cylinder. Nothing is heated, nothing is filtered, nothing is crystallised, no vapour is evolved and nothing needs a titration. It is the shortest honest route from a shelf of chemicals to a photographic sensitiser that this formulary contains, and it can be demonstrated in front of a class.
• When ferric ammonium citrate has been the problem. Everything the parent formula gains by refusing to buy ammonium iron(III) citrate is gained here too: a defined iron content instead of one that runs from 14 to 28 per cent by weight, no mould in the bottle, and a sensitiser whose composition is the same in two hemispheres.
• When you want a long-scale sensitiser with no dichromate and no oxalate. The two substances that give New Cyanotype and the improved Classic their hazard profiles are ammonium dichromate, a listed human carcinogen, and oxalic acid. Neither is here.

When another formula is preferable

• When you need to choose the contrast. Simple Cyanotype is the same chemistry with the neutralisation left in your hands, and it publishes three grades at exposure scales of about 2.7, 2.3 and 1.8 with any intermediate value obtainable by mixing two of them. If matching a sensitiser to a measured negative is what you are doing, that formula exists for it and this one cannot do it.
• When you want the fastest sensitiser and the deepest blue. New Cyanotype is more than twice as fast, with a maximum density Ware describes as verging on black. Ware’s own one-line summaries put the trade fairly: New is “faster; finer; higher Dmax; toxic; chemically tricky”.
• When your negatives are ordinary. A negative made for grade 2 or 3 silver-gelatin paper has a density range around 0.9 to 1.2, and printing it on a sensitiser with an exposure scale of 2.7 wastes most of the scale. The classic sensitiser is built for exactly that negative.
• When you cannot get triammonium citrate but can get citric acid. Citric acid is a supermarket chemical and the tribasic ammonium salt is not. If the ammonia is available to you, the parent formula is easier to shop for.
• When the oxidising iron salt is the obstacle. Iron(III) nitrate nonahydrate is classified Danger, an oxidiser, and it is deliquescent enough that a jar left open is ruined. If that is the substance you cannot have in the building, this formula does not help, and Herschel’s 1842 cyanotype or the classic sensitiser is the place to go: two salts, water, and nothing that intensifies a fire.
• When you want a formula that two independent sources print. This one appears in a single document. That is enough under the course’s evidence rules, because its author devised it, but it is less than the classic sensitiser has and less than the parent formula has.

Mixing

Work under dim tungsten or LED lighting, not under fluorescent tubes and not near a window. Ware’s instruction is emphatic and it covers the whole preparation. The finished solution is sensitive to the near ultraviolet and the deep blue, which an incandescent or warm LED lamp emits very little of and a fluorescent tube and daylight emit a great deal of.

The order is the formula. Section 7.5.2 sets it out in five numbered steps and each colour change is a checkpoint.

1. 15 g of triammonium citrate into a beaker of 200 cc or more, and 40 cc of pure water. Stir until the colourless crystals have gone. Ware notes that the solution cools as it dissolves and may be warmed to help it along — dissolution here is endothermic, which is the opposite of what happens in the parent formula, where neutralising an acid with ammonia gives out heat.
2. 12 g of iron(III) nitrate nonahydrate, added slowly, a little at a time, with stirring until each addition has dissolved before the next goes in. The solution turns yellow-green. This is the step that must not be hurried and must not be done the other way round: iron(III) nitrate dissolved in plain water hydrolyses and polymerises at once, and the polymers are the defect the whole family of formulas exists to avoid. Here the ligand is already in the beaker and in a two-to-one molar excess, waiting.
3. 10 g of ground potassium ferricyanide, stirred in thoroughly until no solid remains. The final colour is olive-brown. The instruction to grind it first is peculiar to this section — the parent formula does not ask for it — and the reason is mechanical rather than chemical: the crystals dissolve into a solution that is already carrying about 27 g of dissolved solid in some 55 cc, and an undissolved crystal that is later coated onto paper is a blue speck in the print.
4. Transfer to a measuring cylinder and make up to 100 cc with pure water. This is the only step that has to be done in graduated glass. The make-up volume is what fixes all four concentrations at once; estimating it puts every strength on this page out by the same unknown fraction.
5. Into a well-stoppered brown bottle, labelled, dated, stored dark. Ware notes that trace impurities in the chemicals may slowly throw a very small sediment of brown iron(III) hydroxide or Prussian blue, which is filtered off if it is in the way.

⚗One instruction the parent formula gives and this section does not

Section 7.4.2 ends with “The solution should be left to stand overnight before first use”. Section 7.5.2 does not say it, and goes straight from the make-up to the bottle.

The course’s reading is that the standing period applies here too, and the reading is marked as a reading because Ware does not state it. The reason it exists in the first place is in his own appendix on citratoferrate photochemistry: the equilibria in the iron(III)/citrate system become slow above pH 3, and the sensitiser this preparation is meant to reproduce is published at about pH 4. Nothing about this route changes that; if anything it makes the point sharper, because the iron here arrives into a solution that is already neutralised rather than into a strongly acid one, so it has further to travel to reach the same speciation. Standing a fresh batch overnight costs nothing and removes a variable.

The surfactant is not an ingredient and does not go in the bottle. Ware lists 0.25 cc of Tween 20 per 100 cc of sensitiser among the chemicals needed, but his operative instruction, in section 7.4.6, is that Tween must not be added to the stock solution: it does not last well when dilute, and the right dose depends on the paper. It is mixed into the portion about to be coated, to a final strength of about 0.25 to 0.5 per cent — one drop of a 5 per cent stock per cc of sensitiser, or one drop of a 25 per cent stock per 5 cc for larger areas. Tween 80 is specifically ruled out because it foams.

✕Two shopping-list figures for the same surfactant

Ware’s chemicals list for this formula gives 0.25 cc of Tween 20 per 100 cc of sensitiser; his chemicals list for the parent formula gives 1 cc for the same 100 cc. One per cent is above the 0.25 to 0.5 per cent his instructions ask for and 0.25 per cent is exactly the bottom of it.

This is a difference between two shopping lists, not between two formulas. The operative sentence is the same for both: mix Tween into the working portion to a final strength of about 0.25 to 0.5 per cent, found by trial on your own paper. The 0.25 cc figure is the smallest amount that can do that job if every drop of the batch is used; the 1 cc figure buys some spare. Neither is a concentration to make up to.

The two-bottle version, for keeping

Ware offers the same alternative here that he offers for the parent formula: separate the ferricyanide from the iron, because it is the two together in one bottle that eventually makes Prussian blue in the dark.

• Follow steps 1 and 2 above, then bottle that solution — about 55 cc — without further dilution. Ware says it should keep well.
• Dissolve 10 g of potassium ferricyanide in 50 cc of pure water and bottle it separately.
• Mix equal volumes of the two when the sensitiser is wanted.

🧮What 'equal volumes' does to the strength, and why the 2022 sheet is tidier

The two stocks are not the same size. The iron and citrate solution is given as about 55 cc and the ferricyanide solution as 50 cc, and mixing equal volumes therefore takes all of the ferricyanide and about fifty fifty-fifths — around 91 per cent — of the iron and the citrate. Made that way, the working sensitiser has the intended ferricyanide concentration and about nine per cent less iron and citrate than the one-bottle version.

How much this matters depends on how seriously you read the tilde. Ware writes “~55 cc”, which is an estimate of the volume the dissolved solids happen to occupy rather than a measured make-up; if the true figure is nearer 50, the two versions agree. The course’s position is that the arithmetic is worth knowing and the fix is trivial: make the iron and citrate stock up to 50 cc in a measuring cylinder before bottling it, and the two halves are then interchangeable by volume with nothing left over.

That is exactly what Ware himself does in the 2022 Simple Cyanotype instruction sheet, whose two-bottle Solution I and Solution II are each made up to 50 cc and mixed in equal volumes. He does not say anything about section 7.5 there, and the course does not claim he was correcting it — but it is his own later practice for the same arrangement, and it is the arrangement that works out exactly.

After the bottle

Section 7.5 stops at the bottle and hands the reader back to section 7.4 for everything else. In brief, and with every figure belonging to the parent formula rather than to this one:

Paper must be pure cellulose, unbuffered, internally sized with AKD — Arches Platine, Hahnemühle Platinum Rag, Magnani Revere Platinum, Ruscombe Mill’s Buxton or Herschel, Crane’s Platinotype, Weston Diploma Parchment. A buffered sheet has to be stripped of its calcium carbonate first, in 5 per cent v/v hydrochloric acid or 5 to 10 per cent w/v sulfamic acid, then washed; oxalic acid will not do it, because calcium oxalate is as insoluble as the carbonate it replaces.

Coat about 1.5 cc per 10 by 8 inch area with a glass rod, six passes or so, and blot off any excess, which would otherwise crystallise and damage a negative. Dry in the dark for an hour or two at room temperature, or with a 40 °C air stream for about five minutes after the surface has gone matte; heat-drying raises the contrast by about a stop, so the two are not interchangeable once you have calibrated. Expose five to ten minutes under an average 365 nm source, judging the print rather than the clock. Develop half a minute to a minute in 1 per cent citric acid, until Prussian blue starts to run off. Wash ten minutes face down in running water that is neither alkaline nor hard.

Behaviour

It prints out. You watch the picture appear in the frame. At the end of a correct exposure the high values are light green, the mid-tones are firm blue, and the deepest shadows have reversed to a pale blue-grey — the “solarised” look that tells you the shadow regions have gone past Prussian blue to Prussian white and will come back on oxidation.

One grade, and the reason is structural. The parent formula’s three grades are three different extents of neutralisation, selected by pouring 33.0, 38.5 or 44.0 cc of 10 per cent ammonia. This formula’s neutralisation was performed by whoever made the citrate, and it is fixed at three ammonium ions per citrate. There is no dial. What survives is the processing adjustment: developing in water instead of 1 per cent citric acid shortens the exposure scale from about 2.7 to about 1.3, with higher contrast, no fogging and a somewhat weakened maximum density. That is a large step, not a graded control, and it is the only one on offer.

Speed and scale are inherited rather than measured. Nothing has been published for this preparation. The sensitiser it is intended to reproduce is stated to be less than half the speed of New Cyanotype and about four times a typical Classic formula, with an exposure scale of about 2.7 and a maximum density of about 1.4. The course reports those as the parent formula’s figures and does not restate them as this one’s.

Sediment on the first day is expected; blue in the bottle is not. A very small brown deposit from trace impurity is normal and filterable. A solution darkening towards blue is Prussian blue forming from iron(II), which is that batch dying.

The nitrate is doing quiet work throughout. Every nitrate ion that came in with the iron is still in the bottle — about 0.089 mol against 0.030 mol of iron — and Ware credits it with the keeping quality, the return of the reversed shadows in the acid bath, some assistance to print-out through the hygroscopic ammonium nitrate in the dried coating, and the absence of the mould that grows on ferric ammonium citrate solutions. His four claims and their different levels of confidence are under The mechanism.

Image characteristics

Colour. Prussian blue, and the same Prussian blue as the parent formula if the two solutions really are identical. Ware makes no statement about which cation occupies the pigment lattice in this sensitiser and neither does this page.

Tonal scale. Long and well separated, with delicate gradation in the high values. An exposure scale of about 2.7 is nine stops, three times the Classic cyanotype’s 0.9, and it is why this family of sensitisers suits negatives prepared for platinotype, palladiotype and the other siderotypes, and digital negatives calibrated to a long range.

Maximum density. About 1.4 for the parent formula, about 1.5 in Ware’s 2022 sheet, and unmeasured here. Developing in water rather than citric acid weakens it somewhat, by design.

Grain. There is none in the silver-halide sense. The image is pigment trapped in the interfibrillar space of the surface cellulose fibres, so the visible texture is the paper’s and the coating’s. What makes the difference between a clean print and a mottled one is whether the sensitiser got into the fibres or stayed in the coarse pores between them, which is what the surfactant, the coating technique and the drying rate are all about.

Permanence. Cyanotype permanence, which is a question about alkali and about light. The pigment is destroyed by alkali, so buffered wrappings and mounts above about pH 9 must be avoided. Prints can fade somewhat in daylight or bright gallery light around 2000 lux, but the loss is regained on dark storage in air over a few days, and exhibition at 50 to 200 lux should cause no measurable fading.

The mechanism

The chemistry falls into two halves. The first happens in the beaker, in the light, and is where this formula differs from every other cyanotype. The second happens on the paper and is common to the whole family; it is treated at greater length on the Simple Cyanotype page and in compressed form here.

In the beaker: buying a neutralisation instead of performing one

What the parent formula does. Citric acid, a triprotic carboxylic acid, is dissolved first so that the solution is strongly acid before the iron arrives; the acid suppresses the hydrolysis that would otherwise begin the instant an iron(III) salt met water. Ammonia is then run in, taking the protons off the carboxyls:

C₆H₈O₇ + 3 NH₃ → 3 NH₄⁺ + C₆H₅O₇³⁻

What the ammonia does in the parent formula: it converts citric acid into triammonium citrate, in the beaker

Which is to say the parent formula manufactures triammonium citrate as an intermediate. The substitution this page describes is simply to buy that intermediate. Ware’s own preparation of the parent sensitiser has that reaction as its third step, and this preparation has it as its first ingredient.

The target of the whole exercise is the compound isolated and characterised by Matzapetakis and co-workers in 1998 — the monomeric, water-soluble dicitratoferrate(III) anion, whose ammonium salt they crystallised out with ethanol and determined by X-ray structure as (NH₄)₅Fe(C₆H₄O₇)₂·2H₂O. The citrate in it acts tetrabasic, having lost four protons including the one on its hydroxyl, and terdentate, with one carboxylate left uncoordinated:

Fe³⁺ + 2 C₆H₄O₇⁴⁻ → [Fe(C₆H₄O₇)₂]⁵⁻

The monomeric dicitratoferrate(III) anion: one iron, two tetrabasic citrate ligands

Ware’s insight, in both formulations, is that the photographer does not need the crystalline solid: the solution is the sensitiser.

🧮The arithmetic that shows the two routes arrive at the same solution

Ware states that this preparation “should be identical with the first one” of the parent formula. That is a claim about amounts, and the source publishes every number needed to check it. The molar masses are Ware’s own, from his chemicals and hazards list.

Citrate. The parent formula takes 13 g of citric acid monohydrate, formula weight 210.14, which is 0.0619 mol. This one takes 15 g of triammonium citrate, formula weight 243.22, which is 0.0617 mol. They differ by 0.3 per cent.

Ammonium. The parent formula’s low-contrast grade takes 33.0 cc of ammonia solution at 10 per cent w/w and specific gravity 0.959. That is 31.65 g of solution, 3.165 g of ammonia, formula weight 17.03, so 0.1858 mol. Three ammonium ions per citrate here gives 3 × 0.0617 = 0.1850 mol. They differ by 0.4 per cent.

Iron and ferricyanide are the same weights in both, 12 g and 10 g. So both solutions contain, in 100 cc, about 0.0297 mol of iron(III), 0.0304 mol of ferricyanide, 0.062 mol of citrate, 0.089 mol of nitrate and 0.185 mol of ammonium. They are the same solution to within half a per cent, which is finer than a 0.1 g balance reading on a 12 g weighing. Ware’s word “identical” is arithmetically well founded, and this is the course’s working rather than his.

⚗Why three ammonia per citrate is a floor, and where the other two grades went

Run the same arithmetic across all three of the parent formula’s grades and something falls out that neither source states in these terms. Dividing the moles of ammonia by the moles of citric acid gives:

Grade	Ammonia at 10% w/w	Ammonia : citrate, mol/mol	Published exposure scale	Published pH
Low contrast	33.0 cc	3.00	~2.7	~4
Medium contrast	38.5 cc	3.50	~2.3	~6
High contrast	44.0 cc	4.01	~1.8	~8

The three grades are three, three and a half, and four equivalents of base per citrate, and the two ends of that range are the two chemically meaningful points: three is the tribasic salt, four is the tetrabasic citrate the dicitratoferrate complex actually contains.

The middle row’s exposure scale is the one figure in that table its author gives twice. Cyanomicon 7.4.2 heads the medium grade “~2.4” and its step-tablet plate labels the strip 2.4; Cyanomicon 7.4.13 heads the same grade “~2.3” and the 2022 instruction sheet gives 2.3 throughout. The table above uses 2.3 because it is the figure the later and more specialised document gives consistently, and the difference — a third of a stop — changes nothing in the column that matters here.

That is why this formula has one grade and can have only one. Triammonium citrate carries exactly three ammonium ions, so it lands exactly on the low-contrast point and there is no way to move off it without adding a base — which is the substance the formula exists to avoid. To reach the medium grade from 100 cc of this sensitiser you would have to supply another 0.031 mol of base, and to reach the high grade another 0.062 mol, and any base you chose would bring a cation the formula was not designed around.

This is the course’s arithmetic and the course’s reading of it. Ware publishes the volumes, the weights, the exposure scales and the pH values; he does not publish the equivalents-per-citrate column and does not connect the high-contrast grade to the tetrabasic ligand. The table is offered because the numbers are his and the pattern in them is exact, not because he asserts it.

⁇The one chemical difference the sources do not discuss: the pH history is reversed

The two routes reach the same composition by opposite paths, and only one of them keeps the iron in acid throughout.

In the parent formula the iron dissolves into a strongly acid solution of free citric acid, which is precisely the condition that suppresses hydrolysis, and the pH is then raised to about 4 by the ammonia. In this formula the citrate is already fully neutralised before the iron goes anywhere near it, so the iron dissolves into a solution that starts on the alkaline side of neutral and is brought down towards pH 4 by the acidity of the ferric salt itself.

Why it may not matter. The ligand is present in a two-to-one molar excess and is waiting for the iron rather than being assembled around it, the solid is added in small portions with stirring so that no local excess of iron ever exists, and the colour Ware records after that step is yellow-green — the colour of the complex — and not the brown of a hydroxide. Those are three good reasons to expect the same product, and Ware evidently expects it.

Why it is recorded anyway. Ware writes “should be identical”, which is a prediction, and neither document publishes a comparison of the two sensitisers made side by side — no matched step tablets, no pH measurement of this preparation, no shelf-life figure. His appendix also notes that equilibria in this system are slow above pH 3, and a route that spends its early minutes at higher pH has more equilibrating to do. The course therefore states the expectation as an expectation and puts the experiment that would settle it at the foot of the page. It is not a warning; it is the difference between a measured claim and a reasoned one, and Rule 7 asks for the distinction to be visible.

On the paper: making the picture

One: light reduces the iron. The citratoferrate(III) absorbs in the near ultraviolet, an electron moves from a citrate ligand to the iron, and iron(III) becomes iron(II) while the ligand decarboxylates. The initial organic photoproduct has been identified as acetone dicarboxylic acid, which may lose two further molecules of carbon dioxide to end as acetone; the nature of the iron(II) photoproduct remains unknown and is likely to be a citrato complex of some kind. Because the source leaves that half open, this page gives no balanced equation for the step. What it can give is the number: the quantum yield at 365 nm is about 0.45, falling to 0.28 at 436 nm.

Two: the iron(II) meets the ferricyanide. The iron(II) is still wearing carboxylate ligands and cannot enter the Prussian blue lattice until they are stripped off, and hydrogen ions are what strips them, by competing for the carboxylate to make the undissociated acid. That is the entire reason there is an acid bath at all, and the reason developing in water instead costs maximum density.

Three: the pigment forms. Free iron(II) is competed for by both oxidation states of hexacyanoferrate present in the coating:

Fe²⁺ + [Fe(CN)₆]³⁻ → Fe[Fe(CN)₆]⁻

Prussian blue, where exposure has been light and ferricyanide predominates

Fe²⁺ + [Fe(CN)₆]⁴⁻ → Fe[Fe(CN)₆]²⁻

Prussian white, where exposure has been heavy and the ferricyanide is largely reduced

The second reaction is solarisation, first observed and named by Herschel, and on a print-out process it is useful rather than a fault: reversing the shadows to white lets more light into regions the blue would otherwise mask, so more product forms and the final maximum density is higher.

Four: the white goes back to blue. Prussian white reduces atmospheric oxygen, which is why the reversed shadows recover on their own over hours and why dilute peroxide does the same in half a minute:

4 Fe[Fe(CN)₆]²⁻ + O₂ + 2 H₂O → 4 Fe[Fe(CN)₆]⁻ + 4 OH⁻

Aerial reoxidation during processing and drying

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

The same conversion, in half a minute, with dilute hydrogen peroxide

Both liberate hydroxide, which is why these oxidations are best carried out under acid conditions: enough hydroxide will begin hydrolysing the pigment it has just made.

⚗Ware's four claims for the nitrate, with his own levels of confidence

Every nitrate ion that came in with the iron stays in the bottle — 0.089 mol against 0.030 mol of iron — and Ware offers four things it does. The first is a prediction from chemistry, the second an observation, the last two likelihoods, and the page keeps his hedging.

1. Keeping the bottle alive, stated as a prediction from chemistry. At low pH the nitrate ion is mildly oxidising, so it should stop any iron(II) appearing in the dark; iron(II) sharing a bottle with ferricyanide is Prussian blue, and Prussian blue in a bottle is a dead sensitiser. This is the duty ammonium dichromate performs in New Cyanotype, discharged here by an ion that is not a chromium(VI) compound and is not a carcinogen. It is worth stressing for this page in particular that the nitrate is the only preservative in the bottle, and that the substitution described here leaves it untouched: the ammonia went, the nitrate stayed.
2. Bringing the reversed shadows back, stated as an observation. In the 1 per cent citric acid bath at about pH 4 the nitrate takes a hand in reoxidising solarised Prussian white to Prussian blue.
3. Helping the image print out, stated as a possibility. Ammonium nitrate left in the dried coating is hygroscopic, so it should hold a little water in the sheet, which is what the Malde-Ware print-out platinum and palladium process is observed to depend on.
4. Suppressing mould, stated as a likelihood. A bottle of ferric ammonium citrate grows a mat on its surface within a fortnight; these sensitisers do not.

Ware also records that adding ammonium nitrate to improve the stability and quality of blueprint paper sensitiser was the subject of United States patent 2,113,423, as long ago as 1938. The course reports that as his citation; it has not read the patent. He publishes no equation for any of this and neither does this page. The nitrate couples are strongly pH-dependent and the kinetics in a citrate-buffered iron solution are not established by the sources; the effect and the reasoning are, the balanced chemistry is not.

Deeper: the ferricyanide is not the light-sensitive ingredient

It does respond to light; it is simply very inefficient about it. Its own photolysis opens with a photoaquation:

UV + [Fe(CN)₆]³⁻ + 2 H₂O → [Fe(CN)₅(OH₂)]²⁻ + HCN + OH⁻

Photoaquation of hexacyanoferrate(III), the first step of its own slow photolysis

and the reported quantum yields for the aquation and reduction reactions are of the order of 0.01, roughly one fortieth of the citrate system’s 0.45. That single ratio is why Herschel’s ferricyanide-only “proto-cyanotype” needs hours where this needs minutes, and it is why the ferricyanide here can be treated as photochemically inert on the timescale of a print.

It is not optically inert, and that is a different problem. Ferricyanide absorbs strongly in exactly the band the iron salt must be exposed in, so a large part of the light entering the coating is captured by a molecule that does nothing useful with it. The calculated fraction of incident light actually absorbed by the photoactive component of a typical cyanotype coating is 0.65 at 365 nm and 0.03 at 420 nm. Two consequences follow. This process wants a genuine 365 nm source rather than a violet one, and the near-equimolar ferricyanide in this formula buys pigment at some cost in speed.

Function of every ingredient

Triammonium citrate, 15 g — the ligand and the base in one white powder. Also sold as ammonium citrate, tribasic, and as the food additive E-380; (NH₄)₃C₆H₅O₇, formula weight 243.22, CAS 3458-72-8, freely soluble in water, a citrate salt in which all three carboxyl groups are deprotonated and paired with ammonium counter-ions. It is doing three jobs. It supplies the citrate ligand at two moles per mole of iron, which is what keeps the iron(III) monomeric instead of letting it condense into the hydroxy-bridged polymers that make commercial ferric ammonium citrate so variable. It supplies the ammonium ions — three per citrate — that the parent formula titrates in as ammonia, which is the entire point of the substitution. And its citrate is the ligand light destroys: the electron that reduces the iron comes off it, and it decarboxylates in the process.

More or less of it. The 0.0617 mol is a two-fold molar excess over the iron and that ratio is the design. Below two citrates per iron there is not enough ligand to keep every iron monomeric, and the hydrolysed polymers this formula exists to avoid begin to appear. Above it, you are adding both ligand and base together in a fixed 1:3 proportion, so you cannot move the pH by adding more — you only raise the total salt concentration, and with it the viscosity and the drying behaviour of the coating. The one thing you cannot do with this ingredient is choose the contrast, because the base is welded to the ligand.

What it interacts with. Everything. It is the reagent that decides the pH, the speciation and the light sensitivity, and it must not be confused with the dibasic salt on the ammonium citrate page, which retains one acidic proton and would leave the preparation under-neutralised by a third of an equivalent.

Iron(III) nitrate nonahydrate, 12 g — the iron that light reduces, and the nitrate that keeps the bottle alive. Fe(NO₃)₃·9H₂O, formula weight 404.00, CAS 7782-61-8, EC 616-509-1, IUPAC name iron(3+);trinitrate;nonahydrate. A pale violet to greyish-white deliquescent solid, density 1.68 at 21 °C, melting at 47.2 °C. It is chosen because it is a simple salt — a pure crystalline compound of known composition containing monomeric hydrated iron(III) ions — where ferric ammonium citrate is an ill-defined polymer sold at iron contents from 14 to 28 per cent. Its anion is a spectator during the dissolution and anything but a spectator afterwards: the four roles Ware assigns the nitrate are under The mechanism.

More or less of it. Less iron and the citrate excess rises, the sensitiser gets slower and the maximum density falls, because there is less of the thing light acts on. More iron and the citrate excess falls below two-to-one, which is the condition that lets hydrolysis restart; the nitrate load rises with it, and so does the hygroscopicity of the dried coating.

What it interacts with. The citrate, which must already be in the beaker before it goes in. Water on its own, which is the one thing it must never meet alone. Its own jar’s headspace, because it is deliquescent and water it takes up from the air changes what a 12 g weighing means. And, as a classified oxidiser, combustible material anywhere near the balance.

Potassium ferricyanide, 10 g — the precipitant, and an optical filter you cannot avoid. K₃[Fe(CN)₆], molecular mass 329.25, CAS 13746-66-2, a red crystalline powder of density 1.89 g/cm³ dissolving to 46 g per 100 mL of water. It takes no part in the exposure worth counting; its job begins the moment light has made iron(II), which it captures as Prussian blue. At 0.0304 mol against the iron’s 0.0297 mol it is very close to equimolar with the iron. In concentration it is 10 per cent w/v, exactly twice the 5 per cent of the classic sensitiser, whose 20 g of citrate and 10 g of ferricyanide are made up to 200 cc between them. The corresponding molar comparison cannot honestly be made, because the classic sensitiser’s iron content is the very thing that is not a fixed number: ferric ammonium citrate is sold at 14 to 28 per cent iron by weight. Either way, the ferricyanide here is generous, and generosity is paid for in the internal filtering described under The mechanism.

More or less of it. Less and the deepest tones run short of the anion they need, and unreduced iron washes away instead of becoming pigment. More and the coating absorbs still more of the exposing light without using it, so the sensitiser slows down for no gain.

What it interacts with. Acids, and this is the rule that has no exceptions — see Safety. Silver, in any form and in any shared tray, because ferricyanide is the oxidising half of Farmer’s reducer. And light, feebly, which is why the working light is dim from the moment it goes in.

Purified water, to make 100 cc — the ingredient that sets every concentration. Distilled, de-ionised or pharmaceutical. Forty cubic centimetres are measured in at the start, purely to have something to dissolve the citrate in; the rest is added at the end to bring the whole to the mark. Tap water brings calcium, which damages Prussian blue, and alkalinity, which destroys it.

Two things in Ware’s chemicals list that are not ingredients of this solution. Tween 20 — polyoxyethylene sorbitan monolaurate, a non-ionic surfactant — is listed at 0.25 cc per 100 cc but is explicitly kept out of the stock bottle and mixed into the working portion instead, to a final 0.25 to 0.5 per cent; its function is to carry the sensitiser into the interfibrillar space of the surface cellulose fibres, where the pigment is trapped, rather than leaving it in the coarse pores between them, from which it simply washes out. This course has no encyclopaedia entry for it. Citric acid at 200 g is listed as the processing chemical, not as a sensitiser ingredient: 10 g in a litre makes the 1 per cent development bath, and its function there is to supply the hydrogen ions that strip carboxylate ligands off the iron(II) photoproduct so the pigment can finish forming. At 1 per cent w/v the bath holds about 0.05 mol of citrate per litre against the sensitiser’s 0.62, so it is roughly twelve times the more dilute of the two, and it is doing an entirely different job.

Interactions

Citrate to iron, 2.08 to 1. This is the ratio the whole design turns on and it is fixed by the two weights. It has to be at least two for the dicitratoferrate complex to be the majority species, and the small excess is insurance against weighing error and against the deliquescence of the iron salt.

Ammonium to citrate, exactly 3 to 1, and not adjustable. In the parent formula this ratio is the variable that sets contrast, running from 3.00 at the widest exposure scale to 4.01 at the narrowest. Here it is a property of the substance in the jar. Everything that follows about this formula’s single grade follows from that one sentence.

Ferricyanide to iron, 1.02 to 1. Near enough equimolar, and it is worth resisting the temptation to read a stoichiometry into it. Only a small fraction of the coating’s iron is ever reduced by light, so the ferricyanide is present as a reagent waiting for whatever iron(II) appears rather than as a measured partner. The pigment’s own composition is not a fixed ratio either: Ware’s reading of the infrared evidence is that cyanotype Prussian blue may be closer to the “insoluble” form Fe₄[Fe(CN)₆]₃, a 4:3 ratio of iron(III) to iron(II), than to the 1:1 ammonium form, and he adds in the same breath that the relative proportions are not critical. He also notes that the pigment this process makes is nominally Turnbull’s blue, whose lattice is missing a third of its hexacyanoferrate groups. Neither source draws a stoichiometric conclusion from the ratio in the bottle and neither does this page.

Nitrate to iron, 3 to 1, unavoidably, because it arrives as the counter-ion. About 0.089 mol per 100 cc, which is roughly 7 g of ammonium nitrate equivalent — a figure that matters twice, once for the keeping quality Ware credits it with and once for the waste stream.

The acid bath and the sensitiser are the same chemistry at different strengths. The bath is citric acid at 1 per cent w/v, about 0.05 mol of citrate per litre; the sensitiser carries about 0.62 mol per litre of the same anion, some twelve times as much. That the developer is a dilute solution of the sensitiser’s own ligand is not a coincidence: it is why these sensitisers tolerate a citric acid bath and tolerate very little else. Ware is explicit that the Simple and Classic sensitisers accept only a very weak acid, above which they fog blue in the highlights, which is why New Cyanotype’s nitric acid developer must never be brought to this formula.

This sensitiser and New Cyanotype must not be mixed. Ware’s caveat on incompatible cyanotype processes is chemical, not stylistic: the point of New Cyanotype’s laborious preparation is to remove most of the potassium ions and replace them with ammonium, because potassium ferric oxalate is sparingly soluble and will crystallise on the paper. This formula’s ferricyanide brings potassium in by the 0.09 mol. The two are different chemistries that happen to share a colour.

Alkali is an interaction, everywhere. Buffered paper, alkaline sizing, alkaline wash water, buffered mountboard: all of them attack Prussian blue, and the same hydroxide that the reoxidation reactions liberate is why those oxidations are done under acid conditions.

Variants

The two-bottle version is the only variant Ware publishes for this formula, and it is set out under Mixing along with the arithmetic of the “equal volumes” instruction. It exists because the ferricyanide and the iron in one bottle are what eventually makes Prussian blue in the dark.

The water-developed version. Omit the citric acid bath and process in water alone. Published for the parent sensitiser at an exposure scale of about 1.3 against 2.7, with higher contrast, no fogging and a somewhat weakened maximum density. It is a change of processing rather than of formula, so it applies here; it is also the only contrast adjustment this preparation has.

Simple Cyanotype is the formula this one re-routes, and the relationship runs the other way from most pairs in this formulary. This is not a modification of the parent: it is a second preparation of the same solution, published by the same author, with the neutralisation moved from the darkroom bench to the chemical supplier. The parent formula’s three grades, its intermediate contrasts by proportional mixing of two finished sensitisers, and its own separated-storage arrangements are all on that page. Nothing here can reach the medium or high contrast grades, for the reason set out under The mechanism.

Ware’s own five-way summary, which is the shortest guide to where this entry sits among its neighbours. The adjectives are his.

Formulation	Ware’s own one-line summary
Classic cyanotype	easy to prepare; uncertain; limited tonal range
Classic improved	more toxic; better range and density
New cyanotype	faster; finer; higher Dmax; toxic; chemically tricky
Simple cyanotype	easy to make; cheap; contrast control; non-toxic — Ware’s own summary, which the course does not adopt as a hazard classification
Mike’s cyanotype (this page)	Simple cyanotype without using ammonia

Toning. A finished print is Prussian blue on paper like any other, so the tannic acid toner and the rest of the toned cyanotype repertoire apply. Nothing in the sources this page rests on tests toning against this sensitiser specifically.

Safety

Level B, and the honest statement of the level is that this formula removes the largest single reason the parent formula has one and keeps the rest. Under the course rubric, Level B assumes splash goggles, stronger ventilation, an eyewash within reach and some experience with concentrated reagents or with ultraviolet sources. What survives here is the ultraviolet exposure unit, which every cyanotype needs, and an oxidising, deliquescent iron salt on the balance. What disappears is the bottle of concentrated ammonia, and with it the vapour, the goggles-and-extraction step in the middle of the preparation, and the one substance in the parent formula that carries a corrosive classification. The rubric’s own principle is that the operation sets the level and not the substance; here one operation has been deleted outright.

The classification does not go below B, because it cannot sensibly sit under the classic sensitiser, which contains no oxidiser at all and is also Level B for its ultraviolet exposure.

The substances, with the classification and the source for each.

• Iron(III) nitrate nonahydrate is now the most hazardous thing in the preparation. Signal word Danger in the aggregated ECHA notifications — 97 reports across 13 notifications — with H272 (may intensify fire; oxidiser) in 60.8 per cent of the reports carrying codes, H315 and H319 (skin and serious eye irritation) in 70.1 per cent each, H335 in 61.9 per cent, and the more severe H314 and H318 in about 29 per cent. Hazard classes Ox. Liq. 2, Skin Corr. 1B, Skin Irrit. 2, Eye Dam. 1, Eye Irrit. 2, STOT SE 3. PubChem’s hazards summary adds that it may cause liver damage and may induce methaemoglobinaemia. Ware’s own condensed rating is health 1, reactivity 2, contact 2. Treat it as an oxidising solid: keep it off paper, cloth, sawdust and a wooden bench, weigh it on clean glass or plastic following the weighing SOP, and keep the jar closed against the deliquescence.
• Triammonium citrate is an irritant and nothing worse. Signal word Warning, aggregated from 278 reports across 16 notifications: H319, serious eye irritation, in 97.5 per cent of the reports carrying codes; H315 and H335 in 45 per cent each; and 7 of the 278 reports state that it meets no GHS criterion at all. Hazard classes Eye Irrit. 2, Skin Irrit. 2, STOT SE 3. Ware’s condensed rating is health 2, contact 2. It is a permitted food additive, E-380, which is a statement about a purified substance in food quantities and not a licence to be careless with a jar of it.
• Potassium ferricyanide is classified as a low toxic hazard and is mildly irritating to eyes, skin and respiratory tract; the card notes that its health effects have not been adequately investigated and that it may be hazardous to aquatic organisms. Avoid inhaling the dust — and note that this formula asks you to grind it, which is exactly the operation that generates dust. Grind it in a covered mortar or a closed bag, not in an open dish under your face.
• Citric acid appears only as the 1 per cent processing bath. Its solution in water is a medium strong acid; it reacts with oxidants and bases and attacks metals. Handled as a solid it is a nuisance dust and an eye irritant.
• Ammonia does not appear at all, and that sentence is the formula. The ammonium hydroxide page is where its hazards are set out, and the reason to read it here is to know what has been avoided.

⚠Cyanide complexes, oxidisers and ultraviolet, in one place

Ferricyanide and acid. The cyanide in potassium ferricyanide is bound to iron in a complex; it is not the hazard that potassium cyanide is, and the two must never be spoken of as the same substance. The international chemical safety card records that it decomposes on heating to give toxic gases including hydrogen cyanide, and that it reacts with acids, producing cyanides; its storage instruction is “separated from acids, dry”. Ware’s qualification is that the heating has to be very strong and the acid very concentrated to liberate much hydrogen cyanide, and nothing on this page approaches either condition — the strongest acid used anywhere is 1 per cent citric. The rule that follows is nonetheless absolute: no mineral acid ever goes near the ferricyanide jar, the sensitiser or the trays. The course’s fuller treatment, including one genuine disagreement between a university safety guidance document and established darkroom practice, is on the ferricyanide page, and it is stricter than Ware.

An oxidiser and an organic acid, and why this route engages that pairing less than the parent does. The safety card for citric acid gives its storage rule as “separated from strong oxidants, strong bases, metal nitrates and metals”. The parent formula deliberately puts a metal nitrate into concentrated free citric acid, under controlled conditions, and its page says so. This preparation never brings the two together: the citrate is already fully neutralised when the nitrate is added to it, so what is in the beaker is a salt solution and not a solution of the free acid. That is a real difference and the course states it as its own reading, because no source it has read comments on the point. It does not license improvisation. The nitrate is still added as a solid, in small portions, to an aqueous solution, stirred, at room temperature, never heated, never mixed dry, and never at a scale beyond the published quantity.

Ultraviolet is on the same list, and this process uses a great deal of it. Ware’s answer on hydrogen cyanide under ultraviolet is specific: the traditional process, ferricyanide together with an iron(III) salt, will not emit any significant, perceptible or harmful amount during a normal exposure and processing, whereas paper coated with ferricyanide alone — Herschel’s proto-cyanotype, which needs twenty to fifty times the exposure — can give a perceptible odour when the frame is opened. The practical rule is the one this formula follows anyway: the iron salt is always there, the exposure is minutes rather than hours, and the frame is opened in a ventilated room.

Ultraviolet, as an exposure hazard. Five to ten minutes at close range under a source that does not feel warm and gives no warning. Enclose the lamp, protect the eyes, cover the skin, and switch it off before reaching into the box. A facial tanning unit is a domestic appliance being used as laboratory equipment.

Working light. Dim tungsten or LED throughout the preparation and the coating. This is a photographic control rather than a hazard control, but it belongs with the rest of the setup: lay the glassware out where you can find it before the lights go down.

Staining is a nuisance rather than a hazard. The sensitiser marks skin, clothing, wood and anything absorbent, and Prussian blue is not easily removed once it has formed.

Storage

The iron salt is the one that will not wait. Tightly closed, dry, and bought in a size you will use. It is deliquescent and it melts at 47.2 °C, so a warm room and a loose lid between them will turn a jar into a slurry. A jar that has gone damp or begun to slump is no longer the substance the formula asks for, and a 12 g weighing from it is not 12 g of Fe(NO₃)₃·9H₂O.

The citrate keeps well and is the easiest thing in the formula to store, which is one of the quiet advantages of this route: the reagent that has replaced a corrosive volatile liquid is a stable food- grade powder with an indefinite shelf life in a closed jar. Keep it dry all the same, because a citrate that has taken up water weighs more per mole than the formula assumes and the error is invisible.

The made-up sensitiser: brown glass, stoppered, labelled, dated, dark, cool. Ware publishes no keeping time for this version, which is recorded as an absence rather than filled in from the parent formula. The best available figure is the parent’s several weeks at room temperature and much longer refrigerated at about 5 °C, and it is a figure for the other preparation. Look at the bottle before you coat: a clear olive-brown liquid is sound, and darkening towards blue, or a blue deposit, is Prussian blue forming in the bottle and the end of that batch.

Expect a trace of sediment on the first day — brown iron(III) hydroxide or a little Prussian blue from impurities in the chemicals — and filter it off if it is in the way. Sediment that keeps returning is a different problem and is under Troubleshooting.

Separate the two halves if weeks are not enough. The two-bottle arrangement is under Mixing. The iron and citrate stock is said to keep well on its own; nothing is published about how long the mixed working solution lasts, and the parent formula’s answer for the equivalent mixture is an unpredictable few weeks or even days.

Nothing else goes into the stock bottle, and that includes the Tween.

Coated paper is a short-term store and the paper decides how short. Within a few hours if possible, longer in a cool, dark, desiccated enclosure. Light yellow is sound; green or blue is chemical fog and a verdict on the sheet rather than on the coating.

Finished prints keep away from alkali. Buffered wrappings and mounts above about pH 9 must be avoided. Fading under bright light is largely recoverable on dark storage in air over a few days; exhibition at 50 to 200 lux should cause no measurable fading.

Incompatibilities

Alkali, at every stage and in every form — buffered paper, alkaline sizing, alkaline tap water, household cleaners, buffered mountboard, and any sheet sold as archival or permanent on the strength of an alkaline reserve, which is exactly the wrong paper for this process. Where a buffered sheet cannot be avoided, the pre-treatment is 5 per cent v/v hydrochloric acid or 5 to 10 per cent w/v sulfamic acid to destroy the calcium carbonate, followed by a wash. Oxalic acid is ruled out on chemical grounds: calcium oxalate is as insoluble as calcium carbonate, so it removes nothing.

Hard water, at every stage, for the calcium it carries.

Mineral acids, anywhere near the ferricyanide. This includes nitric acid, the standard New Cyanotype developer, which must not be brought to this formula for the chemical reason under Interactions as well as the hazard reason under Safety.

New Cyanotype sensitiser, mixed with this one, for the potassium reason under Interactions.

Combustible material in contact with the dry nitrate. It is a classified oxidiser: paper, cloth, sawdust and a wooden bench are not where it is weighed or spilled.

Silver in any form, and any vessel that has held it. A cyanotype bench and a silver bench should not share glassware, because the ferricyanide that makes the blue here is the oxidising half of Farmer’s reducer and will quietly bleach a gelatin-silver print given the chance. Label the cylinders and the trays and keep them apart.

Bare metal trays, tongs, clips and sinks. A chemistry built on iron and hexacyanoferrate will find whatever iron a wet fitting gives it and record the contact as a blue mark.

Gelatin-sized papers with Tween 20, which Ware notes may interact unfavourably, and Tween 80, which foams.

Waste

Three streams and none of them is silver. The spent development bath — 1 per cent citric acid carrying peptized Prussian blue and dissolved sensitiser, replaced after two or three prints per litre; the wash water, which carries the rest of the unexposed sensitiser, the ammonium nitrate and the potassium salts out of the paper; and the occasional failed batch of sensitiser, which is the only concentrated item on the list.

Test the pH before anything else. Ware’s general instruction for cyanotype waste is that if it seems very acidic — test it with pH paper — it can be neutralised with a cheap alkali such as lime or chalk. Do that in a labelled container rather than in the sink, and expect any excess alkali to destroy the Prussian blue in the bath, which is chemically expected and visually startling.

Never acidify a ferricyanide-bearing stream, and never send it to a drain that is also taking a chlorine-releasing household cleaner.

The nitrate is a nutrient, not an inert. A discarded 100 cc batch carries roughly 7 g of ammonium nitrate equivalent, and nitrate is what eutrophies watercourses. Nothing from this process goes onto a garden, into a soakaway or into a watercourse. Bottle it, label it, and follow the general chemical waste SOP and the course’s disposal ruling. Check your local regulations, which are what actually govern and are the only document that can answer the question.

⁇Whether this effluent needs treating as chemical waste at all

Ware says no, and gives reasons. His section on environmental issues holds that cyanotype processing solutions are too dilute to warrant it, and that handling them as chemical waste is an expensive and needless consumption of resources. He argues it substance by substance: ferricyanide reduces in the environment to the more stable ferrocyanide, which is a permitted food additive; Prussian blue is insoluble enough to be used clinically as an antidote to thallium and caesium poisoning; nitrate is of low toxicity, and ammonium nitrate goes onto farmland by the tonne. His readers are practitioners working away from a mains drain, on septic tanks and land drainage.

What kind of statement that is. A chartered chemist’s assessment of the toxicology of his own process, and a sound one on its own terms. It is not an account of what any sewer undertaking permits, and it fixes no quantity or concentration at which a discharge stops being negligible — which is the question a person standing at a sink is actually asking.

Where this course stands. It gives no jurisdiction-specific disposal instruction on any page and will not start on this one: bottle it, label it, follow the SOP, and do what your local regulations require. Ware’s argument is reproduced because a reader deserves to know that this is among the more benign chemistries in the building, not because it decides what may go down a drain. On one point the course is deliberately more cautious than he is, and it is the point this formula shares with its parent: a discarded batch of sensitiser is a concentrated nitrate solution, not rinse water, and it is the item on the bench most worth keeping out of the ground.

Troubleshooting

The sensitiser has darkened towards blue, or thrown a blue deposit. Iron(II) has formed in the bottle and made Prussian blue with the ferricyanide. Filtering will clear the liquid but will not restore what has been consumed. Make a fresh batch, keep it colder and darker, use it sooner, and consider the two-bottle arrangement. If a fresh batch does this within days, suspect the iron salt: a deliquesced jar is the commonest cause.

A brown sediment on the first day. Iron(III) hydroxide from trace impurity, which Ware treats as normal and says may be filtered off. If it keeps returning, look at the order of addition rather than at the chemicals: iron added too fast, or in lumps, meets a local excess of water and not enough ligand.

Blue specks in the print, and a clean-looking bottle. Undissolved potassium ferricyanide carried onto the paper. This is why section 7.5 asks for it to be ground before it goes in, and it is the one instruction in this preparation that has no counterpart in the parent formula.

The print is flatter than expected on a good negative. This sensitiser has one grade, and its exposure scale of about 2.7 wants a negative with an ultraviolet density range of 1.8 to 2.8. A negative made for grade 2 or 3 silver-gelatin paper carries about a third of that. Develop in water instead of citric acid, which shortens the scale to about 1.3, or use a formula built for that negative.

The print is more contrasty than expected. Check what the development bath was, and check the drying: heat-drying the coated sheet raises the contrast by about a stop compared with air-drying.

Blue highlights. The first remedy is to make the citric acid development bath more dilute than 1 per cent, not to shorten the time; the second is a bath of 1 per cent ammonium oxalate for a few minutes, for which this course has no encyclopaedia entry. If the highlights were already blue before development, this is fog and not a processing fault — see the next entry.

The coating goes green or blue before it is exposed. Chemical fog. Ware’s diagnostic algorithm, Table 7.1 in Cyanomicon 7.6, separates six causes by asking when the fog appeared, and it needs two reference areas on every sheet: a margin of uncoated paper, and a border that was coated but masked during exposure. Fog visible on the coating itself means the sensitiser is already decomposed, or the paper is very hostile. Fog that appears during drying means a hostile paper chemical if it happens in total darkness and a faulty safelight if it does not. Fog that appears only after exposure means the negative’s maximum density is too low. Distinguish fog from stain: fog is unwanted image substance and reads grey to blue, stain is residual iron salts and reads yellow.

A yellow cast left in the paper after washing. That is stain rather than fog — unreduced iron salts still in the sheet, which the wash has not carried out. Wash for longer, in water that is neither alkaline nor hard, and check the wash volume rather than only the clock. Left in, it is the fault that turns into a print quietly going darker in its mount over the following months.

Weak blacks. The development bath was water rather than 1 per cent citric acid, which lowers the maximum density by design; or the sheet was underexposed, which on a print-out process you can see, since the deep shadows should be reversed to a pale blue-grey before processing; or the sensitiser has aged and part of its iron is already committed to blue in the bottle.

The shadows stay reversed. They are Prussian white and they need oxygen. Air will do it over hours; 50 cc of 6 per cent hydrogen peroxide per litre of the first wash bath does it at once, and changes only when the density arrives.

The image weakens during washing. The causes are alkaline or hard water, over-rapid drying of the coating before exposure, or a paper whose sizing holds the sensitiser in the surface pores instead of inside the fibres. A wetting agent addresses the third.

Detail that is soft or faintly doubled, worst across broad even tones. The exposure itself makes carbon dioxide as the citrate decarboxylates, and if it cannot escape it lifts the negative off the paper in patches. Back the sheet in the printing frame with papermakers’ felt, or any porous material that gives the gas somewhere to go.

A batch that behaves differently from the last one. Weigh the citrate from a jar that has been kept closed; check that the label says tribasic and the CAS number reads 3458-72-8, because the dibasic salt sold under a nearly identical name would leave the preparation short of a third of an equivalent of base.

Experiments

1. The claim, tested — this formula against the parent, side by side. This is the measurement neither source publishes and the whole page turns on it. Make 100 cc of this sensitiser and 100 cc of the parent formula’s low-contrast grade. Read the pH of each. Coat one sheet from each on the same paper with the same Tween strength, expose both step tablets in the same frame for the same time, develop both in the same 1 per cent citric acid, and read the curves. Ware’s prediction is that they are identical. If they are, the substitution is proved; if they are not, the difference will show first as a difference in speed rather than in scale, and the reversed pH history described under The mechanism is the place to look.

2. Does standing overnight matter here? Split a fresh batch in two. Coat and print a step tablet from one half within an hour of making it, and from the other after twenty-four hours in the dark, at the same exposure. The parent formula’s instruction to stand overnight comes from the slow equilibria above pH 3 in this system; section 7.5 omits it. One afternoon and one night settle whether the omission is deliberate or an oversight.

3. Order of addition, demonstrated rather than asserted. Make two 20 cc batches at one-fifth scale. In the first, follow Ware’s order. In the second, dissolve the iron(III) nitrate nonahydrate in the water first and add the triammonium citrate afterwards, then proceed identically. Compare the two solutions against a white card for colour and clarity, leave both overnight, and coat and print both. The prediction from the chemistry is that the second is cloudier, browner, slower and lower in maximum density, because part of its iron is locked into hydrolysed polymers. This is the cheapest possible demonstration of why a mixing order is part of a formula.

4. The missing grades, and whether they can be reached from here. Anyone who has both this formula and the parent formula’s high-contrast sensitiser can test the proportional-mixing rule across the boundary: mix this sensitiser with that one at 80:20, 60:40, 40:60 and 20:80, print a step tablet through each, and read the scales. If the mixing rule holds between the two preparations as it holds within the parent formula, then a worker who can borrow a small bottle of the high-contrast sensitiser once has the whole contrast range without ever handling ammonia again. Nobody has published this.

5. The two-bottle arithmetic. Make the iron and citrate stock twice: once bottled as Ware writes it, undiluted at about 55 cc, and once made up to exactly 50 cc. Mix equal volumes of each with the ferricyanide stock, coat, expose and read the maximum densities. The predicted difference is about nine per cent less iron in the first, which should show as slightly less density and slightly less speed. This is an arithmetic prediction of the course’s, and it is worth knowing whether it is visible.

6. The nitrate as preservative, measured. Split a fresh batch, keep half in the dark at room temperature and half at about 5 °C, and read a coated step tablet from each at weekly intervals against a strip coated on day one. Ware publishes no keeping figure for this preparation at all, so this experiment does not check a number — it produces the first one. A stricter version prints alongside a classic sensitiser, which contains no nitrate, kept and read the same way.

7. Acid against water, on the same sheet. Expose two identical step tablets, develop one in 1 per cent citric acid for half a minute and the other in water alone, and read both. The published figures for the parent sensitiser are exposure scales of about 2.7 against about 1.3, with a somewhat weakened maximum density for the water-developed print. This is the experiment that shows what the development bath actually does — stripping ligands off the iron(II) photoproduct so it can build pigment — and since it is the only contrast adjustment this formula has, it is worth calibrating properly.

8. The internal filter, if you have two ultraviolet sources. Print the same step tablet under a 365 nm source and under one weighted towards 400 to 420 nm, and record the time to a fixed maximum density. The calculated absorbed fractions for a typical cyanotype coating are 0.65 at 365 nm and 0.03 at 420 nm, which predicts a very large difference. It is also the explanation for why two printers with “a UV box” can report exposures for the same formula that differ by a factor of ten.