Cyanotype (New Cyanotype)

New Cyanotype is the same picture made from a different iron salt, and the reason it exists is a complaint about chemistry rather than about photography. Ware’s verdict on the classic sensitiser is four words long — “All this unreliable chemistry is bad science!” — and this process is the first of his two attempts to end it. The classic process is the entry to read first; ammonium iron(III) oxalate and ammonium iron(III) citrate carry the two substances being compared.

The chemistry

The anion is the whole story. Dissolved, ammonium iron(III) oxalate provides the trisoxalatoferrate(III) ion, one iron(III) held octahedrally by three bidentate oxalates — a monomer of known structure, against the ill-characterised polymer that ferric ammonium citrate is. Ware records that this anion’s photochemistry is by far the most extensively investigated of all the iron(III) carboxylates, and that it is the basis of a well-known chemical actinometer: a light meter made of chemistry.

The measured quantum yield per mole of iron(II) formed in aqueous solution is about 1.2 between 250 and 420 nm, falling to 0.9 at 500 nm and very sharply thereafter. More than one iron(II) per photon absorbed, because the radical formed in the primary step goes on to reduce a second, unexcited complex; the theoretical ceiling is 2, and the reverse dark reaction is what keeps it below. Set that against the citrate system’s maximum of about 0.45 at 365 nm and the speed difference stops being mysterious.

⚗Why the preparation is a crystallisation and not a mixing

The sensitiser cannot simply be stirred together. Potassium iron(III) oxalate is only sparingly soluble and crystallises out, giving what Ware calls a gritty sensitiser and, in the paper, “quite pretty, but totally unwanted fern-like patterns”. The preparation therefore turns the problem into the method: a hot, concentrated potassium ferricyanide solution is added to an excess of very concentrated ammonium iron(III) oxalate, the mixture is left in the dark to cool and crystallise for one to two hours, and about 15 g of solid potassium iron(III) oxalate is filtered off and discarded.

What is left is a single-bottle sensitiser with most of the potassium removed and an image of what Ware calls an “ammonium blue” of very good colour, more resistant to peptization and alkalies than the potassium form.

The shelf life depends on which of the two versions you mixed, and the figures should not be run together. With the dichromate, Ware gives several years, and on his web article at least four to five. Without it — which is how the formulary prints it — his own three printings disagree: six months to a year at Cyanomicon 7.3.2, “a few months” three pages later at 7.3.3, and at least a year in the 2009 workshop notes without saying whether the dichromate is in or out. The mechanism is the same in every account: with no oxidant present, traces of reducing impurity slowly make Prussian blue in the bottle, seen as a sediment and as fog on the print.

Ware’s own printing of the formulation is 30 g of ammonium iron(III) oxalate trihydrate, 10 g of potassium ferricyanide and 0.1 g of ammonium dichromate, made up to 100 cc — and the dichromate is the one solid this course does not print. Under the chromium policy, chromium(VI) is never used at any level anywhere in the course.

That is an omission rather than a refusal, and the difference matters. The dichromate is a preservative rather than a working ingredient, and its originator marks it OPTIONAL in the chemicals table of his 2009 workshop notes, writes that it “may be omitted to satisfy prevailing safety regulations” and states elsewhere that it “can be entirely omitted”. So the sensitiser is published here as a working procedure at Level B, without the fourth solid, in its formulary entry — which states the omission in every section it touches rather than quietly closing the gap. What is given up is shelf life: months rather than years, recovered by mixing less at a time.

Historical workflow

There is not much history here — the process is thirty years old — but there is a lineage, and it is the reverse of the one you would expect.

Herschel had the choice in 1842 and did not take it. Ware notes that the oxalate was “shunned, or overlooked”, and that the siderotypes ran instead on the citrate, which sufficed for gold, silver and mercury but could not reduce platinum. The oxalate’s first documented print-out application came later: Ware credits Giuseppe Pizzighelli in 1887 with observing print-out using sodium iron(III) oxalate.

And the trade got there before the amateurs did. Wall’s 1912 dictionary already lists the salt as “used in platinotype and blue-printing processes”, and his 1924 formulary gives blueprint sensitisers containing sodium or ammonium ferric oxalate with a flat trade-off attached: they give “greater speed than the above, but the paper does not keep so well”. Ware’s transcription of Kwech’s commercial formulae shows the same bargain industrialised — the fastest blueprint papers, exposing in fifteen seconds of bright sunlight against four minutes for the “regular” formula, carry ammonium ferric oxalate and a very low ferricyanide concentration.

On the course’s reading, that caveat is why the citrate remained the amateur’s salt even while the trade used the oxalate; and when the drawing office stopped making blueprints, the fast papers went with it. New Cyanotype, in 1995, applied the substitution to Herschel’s process a hundred and fifty-three years after he declined to make it.

The image

Colour. The image substance is Prussian blue, as in the classic process, but Ware distinguishes the ammonium form as of very good colour and more resistant to peptization — which is a real difference to a printer, because peptization is why a cyanotype bleeds blue into its wash water long after it should have stopped.

Scale, which is the reason to change. Ware gives the exposure scale as about 2.2 against about 0.9 for the classic sensitiser. That is the difference between a process that can print a normal negative and one that needs a very flat one, and it is a bigger practical change than the speed gain.

Highlights. Because the oxalate can be mixed in chemically equivalent amounts with the ferricyanide, there is no excess iron(II) to bleed into the highlights — a fault the classic formula’s excess produces and the troubleshooting atlas carries.

Dilution as a control. Ware notes that the sensitiser may be made more dilute, up to 200 cc instead of 100, which prints faster but yields a less intense blue.

Permanence

Identical in kind to the classic cyanotype, because the image substance is the same pigment: no silver, no fixer, no residual thiosulfate; alkali destroys it irreversibly and it wants an unbuffered mount; light fading is reversible in the dark.

Two differences are Ware’s own and both are improvements rather than changes of kind. Better penetration of the paper fibres, so less image washes away — a mechanical permanence rather than a chemical one. And a pigment form he describes as more resistant to alkalies, which does not make the process alkali-tolerant and does move it in the right direction.

Hazards

Three substances set the level and one of them sets the course’s policy.

Potassium ferricyanide, Level B, carrying the incompatibility the whole cyanotype family is governed by: it must never meet a concentrated acid, which may release hydrogen cyanide.

Ammonium iron(III) oxalate, Level B. Soluble oxalates are systemic toxins, and the preparation above works with a very concentrated solution, hot.

Ammonium dichromate, at 0.1 g in 100 cc in Ware’s printing and not in this course’s. It is a chromium(VI) compound, and under the chromium policy that is not a matter of controls: chromium(VI) is not used at any level anywhere in this course. The classification is the reason — a harmonised CLP entry, signal word Danger, with H340, H350 and H372, and an HSE workplace limit whose recognised proof of control is a urine test. Ware’s own comment on the improved classic sensitiser, which also carries a dichromate, is worth having beside it — that if one is prepared to go to that trouble it would be more worthwhile to use New Cyanotype, and that the improved sensitiser should not be placed in the hands of children. His strongest statement in the chapter is against a use this entry does not otherwise cover: never use a dichromate bath to re-oxidise a cyanotype, which he calls particularly inadvisable on fabric that touches skin.

Where the course teaches it

Part XXI, in The New Cyanotype and the Other Ways to Make a Blue Print, which sits after the classic lab and the exposure-scale experiment and treats this process as one of five formulations rather than as the successor to one. It opens on the seven documented faults of the classic sensitiser and the single substance they all come from, states what Ware changed in 1995 and what he changed again afterwards, and closes on a decision procedure whose first question is not about chemistry at all but about the ultraviolet density range of the negatives you already own.

The question this entry used to flag is answered, and the answer is the second of the two branches it named. The course teaches the New Cyanotype as a practice, with the dichromate omitted, because the originator marks it optional and says in print that it may be left out — so no dichromate-free substitute had to be found and verified; the published formula already permitted one. Its formulary entry carries the procedure at Level B and states the omission wherever it bears, and Part XXI’s dichromate callout sets out the classification, what is given up, and the fact that Ware’s own published contrast curve for acid development is captioned as being for New Cyanotype without dichromate.

And the contrast control is the reason to reach for this formulation rather than another. Part XXI ranks four places a cyanotype’s contrast can be set, and the development bath is the one where only the New Cyanotype offers a usable range — about 2.6 developing in water down to about 2.1 in 1 per cent nitric acid, which brackets the 2.2 this entry’s tonal-scale field records. The classic process sets contrast in the negative and nowhere else.

There are five cyanotypes in the literature, not three. Ware devised three of them. Beside the classic sensitiser and this one sit his improved classic, the Simple Cyanotype — devised in 2019, published in the 2020 revision of Cyanomicon and again as a standalone sheet in 2022 — and Mike’s Cyanotype, which reaches the Simple Cyanotype’s solution by buying the citric acid with its ammonia already attached, for workers who cannot obtain or handle strong ammonia. The Simple Cyanotype buys no iron complex at all: a simple iron(III) nitrate goes into excess citric acid and is neutralised with ammonia, and the ammonia dose sets the contrast — which Ware believes is the first fully controllable contrast in any cyanotype sensitiser. None of the three later formulations is a separate row in this atlas’s register, and they are named here so that a reader meeting five cyanotypes in the literature knows why there are five.