Ware's improved Classic cyanotype

Two hundred milligrams of an orange salt is the entire difference between a cyanotype sensitiser this course publishes as something to coat and one it publishes only as chemistry. Everything else in the formula below is either in the classic sensitiser already or is a plant acid you could buy in a hardware shop. The dichromate is not, and the rule it meets is not a judgement made on this page.

There is a second reason to read the entry, and it has nothing to do with hazard. This is the best worked example in the whole formulary of a formula whose provenance is the interesting part: an improvement claimed by nobody in particular, printed by a chemist who says he does not know where it came from and would rather you used something else, traceable to a supplier’s web page where it appears twice at two different sets of weights. Learning to read a formula’s paperwork is a skill, and this one rewards the practice.

☠Level D: this page is not a procedure, and the reason is a course-wide rule

Ammonium dichromate is a chromium(VI) compound. The chromium policy is the single place that rule lives, and it is unambiguous: chromium(VI) is never used, at any level, anywhere in this course. That is not decided here and it is not restated here in other words — read it there, together with the reason the policy has to name the oxidation state every time, since chromium(III) as chrome alum is permitted at Level B.

So this entry does what a Level D page does: the chemistry, the workflow in outline, the image characteristics, the hazards, and no procedure. No mixing sequence, no coating routine, no exposure time and no processing time appears below as an instruction. The composition is published because a formula that cannot be read cannot be identified, and identification — of a bottle on a shelf, of a print in a collection, of a recipe circulating on a forum — is the use a reader has for this entry.

The cyanotype sensitiser this course can publish is the classic sensitiser, which is this formula without the two additions and which contains no chromium at all. The one Ware himself recommends instead is discussed under When another formula is preferable, with the complication that it too contains dichromate.

Stock solution A — the light-sensitive half, acidified

Ingredient	Quantity	Form the source specifies
Ammonium iron(III) citrate	25 g	the green variety; dissolved in about 70 cc of distilled water at room temperature and made up to 100 cc after the acid has gone in
Oxalic acid	0.5 g	the dihydrate, as Ware's chemicals list writes it; half of the 1 g the list gives for the pair of bottles
Water	to make 100 mL	Distilled water, which is what section 7.2.2 specifies for this formula rather than the "purified water" of the classic sensitiser in 7.1.

Stock solution B — the precipitant, acidified and stabilised

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	10 g	dissolved in about 80 cc of distilled water before the two solids below are added
Oxalic acid	0.5 g	the dihydrate; the second half of the 1 g total
Ammonium dichromate	0.2 g	(NH4)2Cr2O7; the whole of the dichromate, in the bottle that contains no organic reductant
Water	to make 100 mL	Distilled water, to make 100 cc after all three solids have dissolved.

Mixed in the ratio — the sensitiser, as the source directs it be made

1 part Stock solution A + 1 part Stock solution B

These solutions must be stored in brown bottles in a dark box. Mix equal volumes of solutions A and B just before use.

Recorded because how the solutions meet is part of the formula, not because the course asks anybody to do it. Halving both stocks, the mixed sensitiser works out at 12.5 per cent w/v ammonium iron(III) citrate, 5 per cent w/v potassium ferricyanide, 0.5 per cent w/v oxalic acid dihydrate and 0.1 per cent w/v ammonium dichromate. Ware gives no working time, no temperature and no keeping figure for the mixture, and neither does the supplier's article.

Purpose

To make the classic cyanotype’s highlights clear. That is the whole of the published claim. Ware introduces the formula in one sentence — “A widely-circulated ‘improved’ formula follows, but it is not known where it originated” — and gives its claim in the next: “It is claimed that the highlight areas remain whiter.” The version on the supplier’s page his endnote points at is offered by its contributor in the same terms: a better formula that “gives whiter whites and no bleeding”.

Both halves of that claim answer real, documented defects of the classic sensitiser, which is why the formula circulates. Ware’s own list of the seven shortcomings of the classic process includes, as number five, that a significant proportion of the image substance is peptized and washes out, so that the high values lose gradation and the scale is truncated; and, as number six, that in heavily exposed areas the classic sensitiser produces excess iron(II) which diffuses into adjacent highlights and blues them, the defect known as bleeding. A formula that promised to fix the highlight end of a cyanotype would be worth having.

What it does not do is answer the shortcoming that matters most. The classic process has an exposure scale of only about 0.9 — three stops — and Ware’s remedy for that is not an additive at all but a different iron salt. Nothing in the two additions below turns a short-scale sensitiser into a long-scale one.

Why the page exists at all. It is in the register because the classic sensitiser’s own entry sends a reader here, because the formula is genuinely widely circulated and a reader will meet it, and because the reasoning that ends in “no” is more use to a student than the absence of a page. Ware himself publishes it and then argues against it in the same paragraph, which is the model this entry follows.

Recommended uses

These are the uses the sources record. They are set down as description, not as recommendation; this course recommends none of them.

Ordinary pictorial cyanotype on paper, which is the only application either source describes. The supplier’s article uses its version for contact prints from full-size negatives on hand-coated paper, exposed to sunlight or a UV unit, washed in running water, and optionally brought up in a dilute hydrogen peroxide bath.

Where the classic sensitiser’s highlights will not clear, which is the fault the formula is addressed to and is worth naming precisely: not a low maximum density, not a slow exposure, but tone where the paper should be white.

Nothing in this course. No lesson, lab or assignment in Part XXI uses this sensitiser, and none will. Cyanotype is taught with the classic sensitiser and with the chromium-free formulations Ware published afterwards.

⁇A formula whose author is unknown, and what the paperwork actually shows

The disagreement is about origin, not about chemistry. Ware prints the formula as anonymous: widely circulated, origin unknown. His endnote 614 gives a single URL, the technical-articles index of the alternative-process supplier Bostick & Sullivan.

What is on that page. The Internet Archive’s copy of the article behind that index, unchanged in content from its 2006 capture, prints the additives twice. At the top, as the supplier’s house formula signed by Dick Sullivan: ferric ammonium citrate 27.2 g with 0.5 g of oxalic acid to 100 mL, and potassium ferricyanide 9.2 g with 0.5 g of oxalic acid and 0.2 g of ammonium dichromate to 100 mL. Lower down, under “Other variations”, as a reader’s contribution from Bob Schramm at a 250 cc scale — 68 g, 1.3 g, 23 g, 1.3 g, 0.5 g — which divides by 2.5 to exactly the house figures, and which carries the claim of “whiter whites and no bleeding” that Ware reports.

What that establishes and what it does not. It establishes that the two additions, at 0.5 g of acid per bottle and 0.2 g of dichromate, were in print on that page by 2006 — which is where Ware says he found them. It does not establish who devised them, because the house version and the contributed version are the same numbers and the page gives no dates. And it does not corroborate the formula on this page, because Ware’s citrate and ferricyanide weights are 25 g and 10 g rather than 27.2 g and 9.2 g. Two of the five weights differ, and they are the two that are not additions.

What this course uses and why. Ware’s section 7.2, because it is the printing that names its source, states its own uncertainty, and comes from a chartered chemist who has measured the alternatives. The supplier’s article is cited beside it as the origin trail and as a variant, never as independent agreement.

And a caution about the source it leads to. The same web page carries reader contributions including a “rapid sensitiser” built on uranium nitrate and a toner containing lead acetate. That is what a supplier’s community page is: a mixture of house technical writing and correspondence, of very uneven authority, and no part of it is evidence for a hazard, a mechanism or a dose.

When another formula is preferable

• The classic cyanotype sensitiser, for everything. It is this formula minus the two additions, it contains no chromium, it is published in this course at Level B, and it is the sensitiser Ware describes as safe enough that he has no misgivings putting it in the hands of five-year-olds.
• Herschel’s 1842 cyanotype, for the historical article, where the point is what Herschel actually mixed rather than what prints well.
• The New cyanotype of 1995, which is what Ware himself recommends here — with a complication the reader must be told about. His sentence is that if you are prepared to go to this degree of trouble to introduce an admixture of oxalate and dichromate, which are poisonous, it would probably be more worthwhile to use the New cyanotype, “which uses oxalate entirely”. That answers the oxalate half. It does not answer the dichromate half: the New cyanotype’s own chemicals list carries 0.1 g of ammonium dichromate per 100 cc of finished sensitiser, which is the same 0.1 per cent w/v the improved Classic arrives at after mixing. Ware states elsewhere that the New cyanotype’s dichromate may be omitted to satisfy prevailing safety regulations, at the cost of a shelf-life reduced from several years to a few months. Under this course’s chromium policy the New cyanotype as published is Level D for the same reason this page is, and only its dichromate-free version could be anything else.
• The Simple cyanotype of 2019 and Mike’s cyanotype, which are the honest answer. Ware’s own summary of the Simple sensitiser is that it “contains neither dichromate nor oxalate”, so it may safely be used by children, is not harmful to the environment, and shows none of the mould growth that afflicts a citrate stock. It also has the contrast control the improved Classic does not: sensitisers can be formulated to an exposure scale of about 2.7, 2.3 or 1.8, and mixed to land anywhere between. These are the modern substitutes, and they are substitutes for the whole formula rather than for one ingredient of it.
• The Photographers’ Formulary kit, if the attraction was not weighing anything. Read its sheet first: it supplies an optional dichromate contrast solution, and that solution is the part this course does not use.

Mixing

This section gives no mixing procedure, and the omission is deliberate.

Section 7.2.2 of the source prints one: a dissolving order for each bottle, the water to start with, the point at which the additions go in, and the make-up. It is not reproduced here. The Level D policy forbids a sequence of steps, and it forbids it for a reason that applies exactly to this formula: a partial procedure reads as permission and omits the controls. The controls that would belong to weighing 0.2 g of a carcinogenic, sensitising, oxidising solid are not controls a domestic darkroom has.

What can be said without giving instructions is the shape of the thing, because the shape is what makes the chemistry legible.

It is a two-bottle sensitiser, mixed one to one at the moment of coating, exactly like the classic formula it modifies, and for the same reason: a citrate and a ferricyanide in one bottle make Prussian blue in the dark, slowly, and the mixture has a short life. Nothing in the two additions changes that.

The acid is split between the two bottles and the dichromate is not. Half a gram of oxalic acid goes into each stock; the whole 0.2 g of ammonium dichromate goes into the ferricyanide bottle, and none of it into the iron bottle. Both of those placements are chemically deliberate and both are discussed under The mechanism.

The chemicals list and the preparation do not say the same thing, and a reader could be caught by it. Section 7.2.1 heads the formula with a shopping list — citrate 25 g, ferricyanide 10 g, oxalic acid 1 g, ammonium dichromate 0.2 g, distilled water 200 cc — in which the acid and the water are totals for the pair of bottles, while the citrate and the ferricyanide are each the whole content of one bottle. Section 7.2.2 then divides the acid and the water and not the other two. Anyone who read the list as a single solution’s worth would double the acid. The quantities in the table above are 7.2.2’s, bottle by bottle, which is the only unambiguous reading.

There is no published mixing temperature, no target pH and no order of addition recorded here. The source specifies room temperature for the dissolving and prints no pH figure anywhere for this sensitiser or for the classic one. A pH would be worth having — the mechanism below turns on it — and nobody has published one.

Behaviour

Recorded from the sources so that the formula can be understood, not so that it can be run.

No time, no temperature and no exposure figure is published for these weights, by anybody. Ware gives none in section 7.2; the supplier’s article gives an end point rather than a clock, and gives it for its own different weights. The classic sensitiser’s 20 to 30 minutes under a UV lamp is a figure for a different formula and is not transferable, because at least one of the two additions is reported to change the sensitivity.

No shelf life is published either, for the stocks or for the mixture. This is the most conspicuous gap in the record, because shelf life is precisely what the dichromate is for. Ware states of the same reagent in the New cyanotype that its presence usually gives a shelf-life of several years and that omitting it may shorten the life to a few months; nobody has said what it does in a citrate stock at 0.1 per cent, and no keeping figure appears in either printing of this formula.

No capacity is published. The supplier’s article says an 8 by 10 inch print takes “about 1 dropper shoot of each solution” and calls the measurement surprisingly accurate, which it may be in the hands of the person who wrote it, but a dropper-full is not a volume and the sentence describes a different formulation.

No characteristic curve exists. This matters more than it sounds. Ware’s general complaint about this whole class of proposals is exactly that: additives to improve the traditional sensitiser “are rarely accompanied by direct experimental comparisons to demonstrate their putative benefits”. For the New cyanotype he publishes D/logH curves with and without dichromate; for the improved Classic there is nothing to plot.

What the source does commit to. In the list of five cyanotype formulations that opens his formulary chapter, Ware summarises this one in six words: “more toxic; better range and density”. That is a judgement rather than a measurement, but it is the author’s own and it is the closest thing to a performance statement anyone has published.

Image characteristics

Whiter highlights, claimed twice and measured never. It is the only property either source attributes to the formula, and both attributions are second-hand: Ware reports the claim without endorsing it, and the contributed version states it as personal experience.

No bleeding, claimed once. The contributor’s phrase is “whiter whites and no bleeding”, and bleeding has a precise meaning here — Ware’s sixth shortcoming, excess iron(II) migrating out of the shadows into the neighbouring highlights and turning them blue. It is worth noticing that the formula makes the arithmetic of that defect worse, not better, which is set out under the mechanism.

Probably more contrast, which the classic process cannot afford. Ware records that dichromate “will also tend to increase the contrast of the sensitizer significantly” and shows it in his New cyanotype curves; the Photographers’ Formulary sheet, dosing a citrate sensitiser with potassium dichromate at a strength within about a third of this one, expects a loss of two steps on a Kodak No. 2 step tablet. Against an exposure scale already only about 0.9, a further shortening is not obviously an improvement, and no source claims it as one.

Possibly a longer printing range, from the acid, pulling the other way. Hnatek’s 1955 investigation, as Ware reports it, found that oxalic or tartaric acid added to brown ammonium ferric citrate increased the sensitivity and the printing range considerably. Two additions pushing contrast in opposite directions, neither measured in combination, is a good description of what is not known about this formula.

The image substance is unchanged. It is Prussian blue, made by the same reaction, and neither source reports a difference of colour, of maximum density or of permanence. A print made with this sensitiser and one made with the classic are not distinguishable as objects by anything either source describes.

The mechanism

The cyanotype reaction itself is not this page’s to explain: the photoreduction of iron(III) in a citrate complex, the precipitation of Prussian blue by ferricyanide, the solarisation of the shadows to Prussian white and their re-oxidation on drying are all set out on the classic sensitiser’s page with their sources. What this formula adds is two reagents, and each has a documented job.

What the acid is for

Ware’s survey of modifications to the cyanotype sensitiser gives three connected statements, and together they are the case for the oxalic acid.

First, an experiment. In 1955 Hnatek published a detailed investigation of the enhancement of the sensitivity of brown ammonium ferric citrate by additions of carboxylic acids. He found that oxalic or tartaric acid increased the sensitivity and the printing range considerably, and that citric or acetic acid had no significant effect. That last clause is the interesting one: citric acid is an acid too, so whatever oxalic acid is doing is not only lowering the pH.

Second, a general mechanism for clearing. Most acids will assist clearing, Ware writes, because a low pH prevents hydrolysis of the iron(III) to its insoluble yellow hydrated oxide. This is the sentence that connects the additive to the claim. A sensitiser whose iron has partly hydrolysed leaves hydrated ferric oxide in the paper; that residue is yellow rather than blue, but it is tone where there should be none, and it does not wash out with the rest. Ware’s own appendix takes the argument further for the brown salt, whose extensively hydrolysed polymeric forms he expects to leave exactly such residues, with consequences for the colour and the stability of the print.

Third, an optimum. The light sensitivity of ammonium ferric citrate has been found to pass through a maximum at pH 3 to 4. The photochemical appendix says why: in the iron(III)–citrate system a photo-inactive monomer predominates between pH 0.5 and 1.5, while the photo-active dimer forms above pH 2, and the quantum yield at 365 nm reaches 0.45 at pH 4 and then falls as the pH rises, probably because the iron(III) hydrolyses and oligomerises. There is a window, and the additive’s job is to put the sensitiser in it.

⚗The chemistry

No balanced equation for this photolysis appears here, because none can honestly be written. Ware gives the reaction in dissected form: light on the dimeric citratoferrate(III) anion [Fe₂(C₆H₄O₇)₂(OH₂)₂]²⁻ yields two iron(II), acetone dicarboxylic acid CO(CH₂COOH)₂ and carbon dioxide, with a second decarboxylation that can take the acetone dicarboxylic acid on to acetone and two more carbon dioxide. Two electrons move and one of the two citrate ligands is consumed, which is what makes it a two-electron event on a two-iron anion. As printed, that statement does not balance — the second citrate has to go somewhere and Ware says plainly that “the nature of the iron(II) photoproduct remains unknown”, probably a citrato complex of some kind. Balancing the equation would mean choosing that product, and choosing it would be inventing chemistry, so this page states the transformation and stops where the evidence does. The quantum yield of the reaction is 0.45 at 365 nm and 0.28 at 436 nm, against about 0.01 for the photolysis of ferricyanide alone, which is why the classic sensitiser is a great deal faster than a sheet coated with ferricyanide by itself.

That the reacting species is the dimer is the whole reason pH matters. Below about pH 2 the solution holds the photo-inactive monomer, above about pH 4 the iron begins to hydrolyse and oligomerise towards the polymeric hydroxy-bridged particles the appendix describes. An acid that lands the sensitiser between those two is buying speed, and an acid that overshoots is losing it. Nobody has published the pH of this formula, so nobody can say from the sources which of those it does.

What the dichromate is for

A preservative, and a specific one. Ware states it twice for two different formulas and in the same terms. In his survey of sensitiser modifications: “The presence of a small amount of dichromate assists sensitizer stability and shelf-life by preventing premature formation of Prussian blue.” In the New cyanotype, at the same 0.1 per cent w/v of the finished sensitiser: ammonium dichromate is a strong oxidising agent, “intended here as a preservative”, whose addition prevents impurities in the chemicals from forming Prussian blue in the sensitiser over time, and usually gives a shelf-life of several years.

The failure it prevents is easy to picture. A sensitiser bottle contains iron(III) and ferricyanide. Any process that makes a trace of iron(II) in the dark — a reducing impurity in the citrate, a reductant leached from a stopper, stray light — precipitates Prussian blue on the spot, and a blue sensitiser fogs every highlight it is coated on. A strong oxidant present in small amount re-oxidises those traces as fast as they appear. Ware’s Simple cyanotype does the same job with excess nitrate instead, and says so in as many words: the nitrate acts as the preservative “just as the dichromate does in New Cyanotype”. The advantage he claims for the substitution is that nitrate does not bring a chromium(VI) compound into the process at all; his environmental section adds that nitrate itself is of very low toxicity, ammonium nitrate being spread on farmland in massive quantities as a fertiliser.

Why it is in the ferricyanide bottle. Neither source explains the placement, and the course’s reading is offered as an inference rather than as the source’s claim: solution A is a concentrated solution of an organic reductant. The photographic use of dichromate everywhere else in the subject is its reduction from chromium(VI) to chromium(III) at the expense of an organic colloid, and CAMEO’s reactivity profile for the salt records it reacting readily with reducing materials of every kind. A dichromate stored for months in a citrate solution would be consumed by the citrate; stored in a ferricyanide solution, where everything is already oxidised, it has nothing to attack until the two bottles meet. That is a coherent reason for the placement, and it is not one either source states.

What it is not. It is not a contrast agent here, whatever it does to the contrast, because neither source offers it as one. Where a dichromate is sold as a contrast agent for a citrate cyanotype — the Photographers’ Formulary kit’s optional 1 per cent potassium dichromate solution, six drops per 2 mL of working sensitiser — it is added to the working bath rather than to the stock, and the sheet publishes the effect it expects: a loss of two steps on a Kodak No. 2 step tablet.

Deeper: the arithmetic of the three additions

The numbers below are this course’s, worked from published weights. They are here because three separate questions about this formula turn out to be answerable with a calculator, and because the answers are not what the word “improved” leads you to expect.

🧮Concentrations in the mixed sensitiser

Equal volumes of A and B halve everything, so per litre of mixed sensitiser: 125 g of ammonium iron(III) citrate, 50 g of potassium ferricyanide, 5 g of oxalic acid dihydrate and 1 g of ammonium dichromate.

How much iron is in excess. Ferricyanide is a defined compound and Ware gives its formula weight as 329.26:

50 g/L ÷ 329.26 g/mol = 0.152 mol/L

Ferricyanide

The iron salt is not a defined compound, so it has to be reckoned through the iron content Ware gives for present-day green material, 14 to 18 per cent by weight:

125 g/L × 0.14 ÷ 55.85 g/mol = 0.313 mol/L

Iron, at 14 % Fe

125 g/L × 0.18 ÷ 55.85 g/mol = 0.403 mol/L

Iron, at 18 % Fe

So 2.1 to 2.7 moles of iron(III) per mole of ferricyanide, against 1.7 to 2.1 for the classic sensitiser at Ware’s own weights. The image-forming step is one iron(II) to one ferricyanide. Raising the citrate from 20 g to 25 g while leaving the ferricyanide at 10 g raises the surplus iron by a quarter — and surplus iron(II) with no ferricyanide left to meet is precisely Ware’s account of bleeding. The formula whose contributor claims it eliminates bleeding moves the stoichiometry in the direction Ware associates with causing it. That is not a refutation of the claim, because a claim about a print is settled by a print; it is a reason to want the measurement that nobody has made.

Whether the oxalic acid can be acting as a ligand. Oxalic acid dihydrate has a formula weight of 126.07:

5 g/L ÷ 126.07 g/mol = 0.0397 mol/L

Oxalic acid

Against 0.313 to 0.403 mol/L of iron, that is one oxalate for every eight to ten iron atoms. The trisoxalatoferrate(III) anion needs three oxalates per iron, so even if every oxalate went to that complex it could account for at most about 3 to 4 per cent of the iron. Whatever the acid is doing, it is not converting a citrate sensitiser into an oxalate one — for that Ware’s New cyanotype uses 30 g of ammonium iron(III) oxalate and no citrate at all. Hnatek’s finding that citric acid did nothing and oxalic acid did something leaves a real question here that the arithmetic sharpens rather than settles: either a few per cent of a much more photoactive complex is worth having, or the difference lies in strength alone. Oxalic acid’s first proton comes off at pK_a1 1.25 against citric acid’s 2.87, some forty times stronger, so an equal weight of oxalic acid moves the pH very much further.

How much reducing power the dichromate can mop up. The dichromate ion takes six electrons when its two chromium(VI) atoms fall to chromium(III):

1 g/L ÷ 252.07 g/mol × 6 = 0.0238 mol of electrons per litre

Dichromate

That is enough to re-oxidise about 6 to 8 per cent of the iron in the bottle, which is exactly the right size for the job Ware describes: a scavenger for traces made by impurities over months, not a bulk oxidant. It is also a useful check on the reasoning, because a dose big enough to hold all the iron in the iron(III) state would prevent the picture as well as the fog.

Deeper: what the sources leave unresolved

Three things, and each is a real gap rather than a rhetorical one.

The claim itself has never been tested against a control. Whiter highlights is a statement about a print, and settling it needs two prints: the same negative, the same paper, the same light, one sensitiser with the additions and one without, read off a step tablet rather than admired. Ware’s objection to this entire class of proposals is that they arrive without that comparison, and his own practice is to publish the curves.

The acid may cut against the claim rather than for it. Ware’s one measured statement about acid and highlights in a citrate sensitiser concerns the developing bath, not the coating: the Simple and Classic sensitisers “can only tolerate a very weak acid (under 1 per cent citric), otherwise stronger acids will cause blue chemical fog in the highlights”. A bath is not a coating and the two cases need not behave alike, but a formula that puts 0.5 per cent of an acid forty times stronger than citric into the sensitiser itself, and claims cleaner highlights for it, is at least owed an explanation nobody has written.

Solution B holds an oxidiser and a reducing acid in the same bottle. Oxalic acid is a reductant — it is one of the classic reagents for destroying chromium(VI) in the laboratory — and CAMEO records ammonium dichromate reacting readily with reducing materials of every kind. What that pairing does to the keeping of solution B over months in a brown bottle is not addressed by either source, and no shelf life is published for it. It is the one experiment on this page that could be done without a print, and it cannot be done in this course, because doing it means having the dichromate.

Function of every ingredient

Ammonium iron(III) citrate, 25 g in solution A — the only ingredient light acts on. A glassy, ill-characterised, water-soluble iron(III) salt of citric acid, supplied as green or brown “scale” flakes whose iron content runs from 14 to 28 per cent depending on the variety and the maker. In light it does the whole photographic work: the citrate ligand gives up electrons, the iron(III) becomes iron(II), and the amount of iron(II) at any point on the sheet is proportional to the exposure there. Ware’s formula specifies the green variety, which carries more citrate per iron atom than the brown and which Valenta first prepared in 1897. More of it than the classic 20 g is the one change this formula makes to the classic pair, and it buys density at the price of a larger surplus of iron over ferricyanide — the arithmetic is above. Less of it gives a thinner, cleaner, longer-scale print, which is the direction Christina Anderson’s 10/10 mixture takes the classic formula. Its interaction with everything else on this page is the reason the sensitiser is in two bottles: mixed with ferricyanide it makes Prussian blue slowly in the dark, and — on the course’s reading of why the dichromate is kept out of this bottle — a chromium(VI) salt stored in a citrate solution would be reduced by it.

Oxalic acid, 0.5 g in each solution — the acid, and possibly a little more than an acid. The strongest of the plant acids, a colourless crystalline dihydrate, and a systemic poison in its own right. Its documented job here is pH: a low pH prevents the hydrolysis of iron(III) to insoluble yellow hydrated oxide, which is what leaves a residue in the highlights that will not wash out, and the sensitivity of ammonium ferric citrate passes through a maximum at pH 3 to 4 which an acid addition can reach for. Hnatek’s 1955 comparison found oxalic and tartaric acid raised sensitivity and printing range considerably while citric and acetic did not, which suggests something beyond acidity — but the arithmetic above shows there is only enough oxalate to touch a few per cent of the iron, so if it is ligand chemistry it is ligand chemistry at the margin. More of it would push the sensitiser towards the pH range where the photo-inactive monomer takes over, and Ware’s warning about stronger acids fogging highlights in the developing bath is at least a caution. Less of it, or none, returns the formula to the classic sensitiser’s problem of a slowly hydrolysing iron stock. It is split between the two bottles rather than concentrated in one, which puts acid into the mixture from the first second of contact. Its interaction to watch is with the dichromate sharing solution B.

Potassium ferricyanide, 10 g in solution B — the precipitant and the filter. Deep red crystals of potassium hexacyanoferrate(III), the anion Herschel paired with the iron salt in 1842 and the one every negative-working cyanotype has used since, in one salt or another. It has two jobs at once. Where light has made iron(II), it precipitates Prussian blue immediately; everywhere else it absorbs blue and ultraviolet strongly and acts as an internal filter between the light and the iron that could use it, which is part of why the classic process is slow. It has a weak photochemistry of its own, at a quantum yield around 0.01 against the citrate’s 0.45. More of it would bring the stoichiometry closer to equivalence and, on Ware’s reasoning about bleeding, should reduce the surplus iron(II) — the improved formula goes the other way and raises the citrate instead. Less of it starves the shadows of maximum density. Its one incompatibility is with strong acid, discussed below, and the 0.5 g of oxalic acid in its own bottle is the place where this formula puts an acid closest to it.

Ammonium dichromate, 0.2 g in solution B — the preservative, and the reason for the whole classification. Bright orange-red crystals of (NH₄)₂Cr₂O₇, freely soluble — about 36 g per 100 g of water at 20 °C, which is roughly three times the potassium salt’s solubility and the historical reason printers preferred it. Its job here is to be a strong oxidant present in a small amount: it re-oxidises the traces of iron(II) that impurities generate in storage, before they can find the ferricyanide and precipitate Prussian blue in the bottle. More of it raises the contrast, on Ware’s account of the same reagent in the New cyanotype and on the Photographers’ Formulary sheet’s measured two steps, and raises the hazard proportionately. Less of it, or none, shortens the keeping — a few months rather than years, by the only figures published, which are for a different formula. Potassium dichromate can be substituted for it, which Ware states directly. It is the single ingredient that puts this entry at Level D, and the argument for that is not its quantity: it is that the operation the formula requires is weighing two tenths of a gram of a solid classified as causing cancer and as a respiratory sensitiser, on a domestic balance — which is the operation most likely to raise dust.

Interactions

Citrate and ferricyanide: the reason for two bottles. The pair reacts slowly in the dark without any light at all, because any iron(II) present precipitates Prussian blue and there is always some. That is why the classic sensitiser is stored as two solutions and why Ware records the mixture as having a relatively short life. Both additions in this formula are addressed to that reaction from opposite ends: the acid keeps the iron(III) from hydrolysing, the dichromate keeps iron(II) from persisting.

Dichromate and citrate: kept apart on purpose. A dichromate in a citrate solution is the gum-and-carbon reaction waiting for light, and CAMEO’s profile has the salt reacting readily with reducing materials generally. Putting the whole 0.2 g in the bottle without organic matter is, on the course’s reading, why the placement is what it is. It also means the dichromate only begins to work on the sensitiser at the moment of mixing.

Dichromate and oxalic acid: together in solution B, unexamined. The two are an oxidant and a reductant in the same bottle, and neither source says what months of storage do to that pair. This is recorded as an open question, not as a defect.

Ferricyanide and acid: the question every cyanotype page has to answer. A hexacyanoferrate in strong acid can release hydrogen cyanide. The concentrations here are not that case and two independent sources say so plainly. The Photographers’ Formulary’s kit sheet states that the cyanide groups are chemically bound to the iron and not free to act as a poison, that potassium ferricyanide “is not the insidious poison that sodium cyanide is”, and that hydrogen cyanide is released only in strong acid, which the cyanotype process does not use. Ware’s own assessment is that the heating has to be very strong and the acid very concentrated to get much hydrogen cyanide, and that neither is done in cyanotype. What is different about this formula is only that the acid is inside the bottle rather than in a separate bath — 0.5 g of oxalic acid dihydrate per 100 mL, about 0.04 mol/L, which is a dilute solution of a comparatively strong acid rather than a strong acid solution.

The two additions pull contrast in opposite directions. The dichromate is reported to raise contrast significantly; the acid, on Hnatek’s evidence for the same kind of addition, lengthens the printing range. Nobody has measured the two together, in this formula, on a step tablet. That is the sentence to remember about the whole entry.

Everything against the paper. Every cyanotype sensitiser is destroyed by an alkaline buffer. Ware’s instruction for his own formulations is to avoid papers buffered with chalk, because alkalies are hostile to cyanotype chemistry, and to treat buffered paper with dilute hydrochloric or sulphamic acid if it cannot be avoided — never with oxalic acid, because calcium oxalate is as insoluble as calcium carbonate and leaves a gritty texture. That last point is worth carrying away from this page even if nothing else is.

Variants

The Bostick & Sullivan house formula, the version behind Ware’s endnote, differs in two of its five weights — the citrate and the ferricyanide, which are the two that are not additions — and in nothing else:

Per 100 mL	Ware, Cyanomicon 7.2	Bostick & Sullivan article
Ammonium iron(III) citrate, solution A	25 g	27.2 g
Oxalic acid, solution A	0.5 g	0.5 g
Potassium ferricyanide, solution B	10 g	9.2 g
Oxalic acid, solution B	0.5 g	0.5 g
Ammonium dichromate, solution B	0.2 g	0.2 g

Both are mixed in equal volumes immediately before coating. The supplier’s figures look like a conversion from some other unit and Ware’s look rounded; neither source explains its own numbers.

Bob Schramm’s contributed version, on the same page under “Other variations”, is the same formula written at 250 cc: 68 g of the citrate with 1.3 g of oxalic acid, and 23 g of ferricyanide with 1.3 g of oxalic acid and 0.5 g of ammonium dichromate. Divide by 2.5 and it is the house formula exactly. It is the printing that carries the claim, “whiter whites and no bleeding”.

Oxalic acid in a cyanotype sensitiser is not new and was once commercial. Kwech’s 1924 account of American blueprint manufacture, which Ware tabulates, gives four grades of sensitiser, and the two fastest carry oxalic acid at 0.5 and 1.3 per cent w/v of the mixed sensitiser — the first of those the same concentration as this formula. Those papers also used sodium and ammonium ferric oxalate and ferric chloride alongside or instead of the citrate, and reached 15 seconds of bright sunlight at the fastest grade against four minutes at the slowest.

Dichromate in cyanotype is also not new, and its worst use was as a bath. Commercial blueprint papers were commonly “developed” in a dichromate solution of about 7 per cent to re-oxidise solarised Prussian white quickly, and the same practice was recommended for cyanotype on fabric, reputedly to improve light-fastness. Ware is emphatic on both counts: under no circumstances should a dichromate bath be used for re-oxidation, and there is a documented case-history of an artist who suffered considerable ill-health as a direct consequence of using dichromate baths on quilting fabrics, with the observation that victims of dichromate exposure can become particularly sensitised to it. A 7 per cent bath is seventy times the concentration this formula puts in a sensitiser, held in an open tray, in contact with the hands, on a material destined to touch skin.

The dichromate-free direction, which is where the subject actually went. David Knierim’s review of the type AB blueprint sensitiser, which Ware reproduces with permission, set out to get speed and good storage “without any toxic dichromate added, so making it suitable for popular use by children”. Ware’s own Simple cyanotype and Mike’s cyanotype go further and contain neither dichromate nor oxalate, using excess nitrate as the preservative that dichromate provides elsewhere.

No course variant of this formula is offered, and the omission is a decision. It would be easy to publish these weights with the dichromate struck out and call it safer. This course will not, for two reasons. The claimed benefit is unmeasured, so a modified version would be a modification of nothing demonstrable; and the preservative is the ingredient being removed, so what remains is a classic sensitiser with an acid in it and a shorter keeping life than the version that has the dichromate — which the classic sensitiser’s own page already covers, at Level B, with its variants and its evidence. A reader who wants what this formula promises should read the Simple cyanotype instead.

Safety

Level D: historical study only, no procedure. The criteria that applied are the rubric’s for chromium(VI) compounds “in any quantity”, which the classification rubric places at Level C, and the chromium policy’s ruling that this course, which teaches in a domestic darkroom and can supply neither a fume cupboard nor a specialist waste route, does not use chromium(VI) at any level. The consequence is the Level D treatment: chemistry, outline, characteristics and hazards, and nothing to follow.

What the classification of the dichromate says. The aggregated ECHA notifications for ammonium dichromate — 194 reports across 10 notifications — give signal word Danger under six pictograms and a hazard list that runs from oxidiser to long-term aquatic toxicity. In 100 per cent of the reports: toxic if swallowed, harmful in contact with skin, causes severe skin burns and eye damage, may cause an allergic skin reaction, fatal if inhaled, may cause allergy or asthma symptoms if inhaled, may cause genetic defects, may cause cancer, causes damage to organs through prolonged or repeated exposure, and very toxic to aquatic life with long lasting effects. HSE’s EH40 sets chromium(VI) compounds at 0.01 mg/m³ as chromium over eight hours, with the Carc and Sen notations and a biological monitoring guidance value — which is to say the recognised way of knowing whether the control worked is to test the worker’s urine.

Ware’s own position, which is not this course’s and is worth reading beside it. He judges that the very small quantity of dichromate used in these sensitisers “does not represent a serious risk if the chemical is handled sensibly and proper precautions taken”, while rating its health hazard 4 out of 4 on the scale he uses, noting that it is a known human carcinogen, that external contact can cause ulcers and dermatitis, and that this sensitiser “should not be placed in the hands of children”. He is describing a chemist’s laboratory and a chemist’s habits. This course is written for a kitchen table and a domestic balance, and the operation it has to assess is not the use of a dilute solution but the weighing out of 0.2 g of a friable carcinogenic solid, which is where dust is raised and where the sensitisation risk lives. That difference in setting, not a disagreement about the chemistry, is why the two conclusions differ.

The reaction hazard is separate from the health hazard. CAMEO describes the solid as readily ignited, burning to a voluminous green residue of chromium(III) oxide, capable of rupturing a closed container as it decomposes, and acting as a strong oxidiser if mixed with or contaminated by combustible material. The ammonium dichromate page carries the decomposition and its equation.

No protective equipment is listed on this page, and the omission is the same one the encyclopaedia entry makes: a PPE list implies a procedure, and there is no procedure here to protect.

What is not a hazard here, and why that has to be said. Two of the four substances in this formula are conspicuously mild, and it would be dishonest to let the Level D banner tar them. Ammonium iron(III) citrate is rated 1 out of 4 for health in Ware’s own condensation of the safety sheets, has been prescribed as an iron tonic, and is a permitted food additive. Potassium ferricyanide is a low-toxicity salt whose cyanide is locked to the iron and unavailable as a poison; of the plain two-chemical cyanotype Ware writes that from all points of view there is no safer photographic process, and this course still puts that formula at Level B, because a printer handles ferricyanide solutions repeatedly and has to keep every acid away from them. Oxalic acid is a genuine systemic poison, and it is the ingredient that would set the level of a dichromate-free version of this formula — at Level B, not worse. The entire distance between Level B and Level D on this page is 0.2 g of one salt, and saying so precisely is more useful than a general warning.

Storage

No storage procedure is given, because no formula is given to store. What the source directs for its own readers is recorded as description: the two stocks are kept in brown bottles in a dark box, and the mixture is made only immediately before use.

No shelf life has been published for either bottle or for the mixture, which is a striking gap in a formula whose second additive exists to extend shelf life. The nearest published figures belong to a different sensitiser: several years for the New cyanotype with dichromate, a few months without it.

If you already have such a bottle — bought, inherited, or mixed before finding this page — the chromium content governs everything about how it is kept. It is a solution of a notified carcinogen and respiratory sensitiser: it belongs labelled with its contents, out of reach of children and animals, away from anything combustible or reducing, and it does not go down a drain when you are finished with it. The waste section says what the chemistry of dealing with it is; local regulation says what you may actually do.

Incompatibilities

Strong acids, with the ferricyanide. Not the acid inside the formula, which is dilute, but a concentrated acid brought to the bench: a hexacyanoferrate and strong acid can release hydrogen cyanide, and heat makes it worse. No acid stop bath, no concentrated descaler and no hydrochloric acid paper pre-treatment belongs anywhere near a ferricyanide solution or a tray that held one.

Reducing materials and combustibles, with the dichromate. Paper, wood, sulfur, aluminium and plastics are on the standard incompatibility list for chromates, and they are the contents of an ordinary workroom. The solid is heat- and shock-sensitive and its decomposition is self-sustaining. See the incompatibility matrix and the chromium policy.

Alkali, with everything. Alkalies are hostile to cyanotype chemistry at every stage: an alkaline-buffered paper decomposes the iron(III) sensitiser before it is exposed, and alkali destroys Prussian blue in a finished print. Chalk-buffered “acid free” paper is the commonest cause, and calcium carbonate is the substance to look for on the specification.

The New cyanotype sensitiser, specifically. Mixing a New cyanotype sensitiser with a classic or Simple one is, in Ware’s words, a chemical nonsense: the whole point of the New cyanotype’s preparation is to remove potassium ions, and adding a potassium-rich citrate sensitiser puts them back, where sparingly soluble potassium ferric oxalate can crystallise in the coating and wreck the print. The two families also want different developing baths — a mineral acid for the oxalate sensitiser, an acid under 1 per cent citric for the citrate ones, since stronger acid fogs citrate highlights blue.

Anything that shares a tray, a rod or a syringe. Cross-contamination between the two stocks makes Prussian blue in the bottle, which is the failure the dichromate is there to prevent; the classic formula’s own instruction is a dedicated pipette for each solution and a third for the mixture.

Waste

The cyanotype chemistry is benign and the dichromate is not, and Ware says so in exactly those terms. His environmental section states that apart from dichromates there is nothing in a cyanotype sensitiser or processing bath, when dilute, that can be described as hazardous: ferric ammonium citrate is not poisonous and is a permitted food additive, ferricyanide reduces in the environment to the still-less-toxic ferrocyanide which is itself a food additive, oxalates are poisonous concentrated but occur naturally in rhubarb and spinach at the 1 per cent level, and Prussian blue is so insoluble and inert that it is used as an antidote to thallium and caesium poisoning. “The only chemical in this work that is recognised as hazardous is potassium or ammonium dichromate.”

The chemistry of dealing with chromium(VI) is reduction, and the choice of reductant matters. Ware reviews the reported methods — sodium sulfite, sodium thiosulfate, iron filings, sugar — and notes that all of them require rather acidic conditions, which is itself a hazard to arrange. His preferred reductant is ascorbic acid, which is vitamin C, is inexpensive, and has the property the others lack: it works at neutral or even slightly alkaline pH, so the chromium can be reduced without first acidifying it. Reduced to chromium(III), the metal can be precipitated and the solid handled as a solid.

What this course does not repeat. The Photographers’ Formulary kit sheet instructs the reader to dispose of excess solid dichromate by washing it down a drain with copious water. That was written for a particular jurisdiction and a particular era and it is not a universal instruction; this course gives none. Read the disposal page, and then read what your own authority requires, because local regulation governs and nothing here overrides it.

The cleanest waste decision available is not to create the stream. That is not a slogan: it is the practical argument for the chromium-free cyanotype formulations, and it is why the course teaches those.

Troubleshooting

Written for reading a print or a bottle, not for running a process. The corresponding section on the classic sensitiser’s page is the one that belongs to a procedure this course publishes.

A blue or green sensitiser in the bottle is decomposition, and it is the specific failure the dichromate exists to prevent. Prussian blue formed in storage fogs every highlight the sensitiser is coated on, and no amount of washing recovers it. Where the improved formula’s dichromate has been left out, or has itself been consumed, this is the fault to expect first.

Fogged highlights have six candidate causes and they can be separated. Ware’s diagnostic table distinguishes a sensitiser already decomposed, a hostile chemical in the paper reducing the sensitiser, inadequate safelighting, a negative or mask not dense enough in its maximum value, faulty wet chemistry, and inadequate clearing — and separates them by when the tone appears: on coating, during drying, after exposure, after wet processing, or as a stain after the wash. The comparison that makes the table work is a coated but masked border against an uncoated margin of the same sheet. It is also worth distinguishing fog from stain by colour: unwanted image substance is grey-blue, unwanted residual ferric salt is yellow.

A gritty coating with small white spots is crystallisation, not dust. Ware describes exactly this symptom for potassium ferric oxalate coming out of an oxalate sensitiser; the general lesson is that a sparingly soluble salt in a concentrated sensitiser prints as white specks.

Yellow staining that will not wash out is hydrolysed iron. Hydrated ferric oxide left in the paper is the residue a low pH is meant to prevent, which is the acid’s whole case in this formula.

Identifying a print that was made this way is not possible from the print. The image substance is Prussian blue either way, and neither source reports any characteristic that distinguishes an improved Classic print from a classic one. Nor is the bottle identifiable by eye: two tenths of a gram of an orange salt in 100 mL of a 10 per cent ferricyanide solution changes nothing a person could reliably see. Only the label can tell you, which is the argument for treating an unlabelled two-part cyanotype sensitiser of unknown origin as possibly dichromate-bearing rather than assuming it is the classic pair.

Experiments

Every experiment below is done without chromium(VI). The first two are the comparisons this formula needed and never got, rebuilt on the classic sensitiser so that a reader can actually run them; the third is the one that cannot be run here at all, described so that its absence is visible; the fourth needs no darkroom.

1. Does the acid alone do what the formula claims? Coat two sheets from the classic sensitiser, one as published and one with citric acid added to the mixed sensitiser, print both under the same step tablet on the same paper in the same session, and read the highlight steps against a border that was coated and masked. Citric acid is Hnatek’s null result, which makes it the right control: if the highlights clear better with it, the effect is acidity; if they do not, that is a point in favour of the oxalate being more than an acid. Record the pH of both mixtures, because nobody has published one.

2. What does the excess iron actually do? The arithmetic above says the improved formula raises the iron-to-ferricyanide ratio by a quarter. Make two classic sensitisers at 20 g and 25 g of citrate against the same 10 g of ferricyanide, print a subject with a hard edge between a dense shadow and a clear highlight, and look along that edge for bleeding at both. This is the claim that the formula’s own stoichiometry appears to contradict, and it needs no chromium to test.

3. The experiment this page cannot run. To settle the published claim you would coat the improved sensitiser and the classic sensitiser side by side, expose both under a calibrated step tablet, read densities, and plot two characteristic curves — the comparison Ware says this class of proposal never arrives with, and the one he publishes for his own formulas. It requires ammonium dichromate, so it is not offered here. If you find such a comparison published anywhere, it is worth more than this entire page, and it should be cited into the formulary.

4. Read the paperwork instead. Take any formula circulating online without an author, follow its citations back as far as they go, and write down at each step what the source actually says, who said it, and whether the numbers changed on the way. This entry is a worked example of that exercise: a claim, an endnote, an archived page, two sets of weights, and a conclusion that is mostly about what nobody measured.