Weak cyanotype maximum density

Owned by Break/Fix — the cyanotype that went wrong, whose fourth case ranks the causes by how often the sources make them the answer — the water first, because it is measured, cheap and usually wrong — and prints Ware’s washing comparison behind that ranking: four-hour density losses of 0.14 to 0.31 in tap water against 0.02 to 0.03 in purified water. It names this entry as carrying the rest. The exposure-scale experiment owns the measurement of maximum density itself. This entry was written earlier from the encyclopaedia pages for the pigment and the sensitiser.

What you see

A blue that will not go deep. The shadows are a mid blue rather than something approaching black, the highlights are blown out rather than delicately separated, and the print looks contrasty rather than rich — which is the tell, because losing the high values while keeping the deepest ones raises apparent contrast while lowering density.

Ware describes the result of the commonest cause precisely: a serious loss of gradation in the high values, truncating the tonal scale and conferring an artificially high contrast.

Likely causes

1. The image washed out. Ware’s fifth shortcoming of the classic process: a significant proportion of the image substance — “soluble” potassium Prussian blue — is peptised and washes away during wet processing, because of poor absorption and lack of retention by the paper fibres.
2. Poor absorption in the first place. If the sensitiser stays in the coarse pores between fibres rather than in the interfibrillar space within them, the nanoparticles of pigment are not trapped and are easily washed out.
3. Too little ferricyanide. Ware records why the commercial blueprint papers could never be pictorial: to avoid potassium ferric oxalate crystallising out they had to stay at 5 per cent or less in potassium ferricyanide, and so yielded a maximum density too low for pictorial purposes.
4. Under-exposure, which on this process is easy to commit because the print-out image is not the final image.

What is happening: the chemistry and physics

Prussian blue is only as insoluble as its lattice makes it. The Prussian blue page records the disagreement between sources on its solubility, and records Ware’s resolution of it: what looks like a solution is a peptised colloid — a dispersion of very fine particles that will pass a filter. That is exactly what a wash bath does to an image made of very fine particles that are not held.

Where the particles form decides whether they stay. Ware’s paper-problems section is the mechanism: the image substance is often in nanoparticle form and must be securely trapped within the surface cellulose fibres; if the sensitiser merely sat in the coarse pores, the tiny particles are easily washed out, leaving “empty, blown-out highlights and murky unresolved shadows”.

The cation matters too. The “soluble” potassium form peptises most readily; eliminate the potassium and the ammonium Prussian blue that results is more resistant, loses very little on wet processing, and reaches — in Ware’s words — a maximum density verging on black.

And the losses are measurable. Ware’s washing table gives density losses after four hours in tap water of around 0.14 to 0.31 in density units across five historic formulations, against 0.02 to 0.03 in purified water for most of them. The water is a variable in the maximum density, which is not obvious until somebody measures it.

!The confirming test: watch the wash water, and wash a control in pure water

Look at the wash bath. Blue coming off the print, and blue that keeps coming, is peptisation. That is the diagnosis, and it takes no equipment.

Then split the print. Cut an exposed sheet in two and wash one half in tap water and the other in distilled or deionised water, for the same time. Ware’s own figures and both earlier studies he cites point the same way — the Moor experiments gave an average loss of 18 per cent in tap water, 4 per cent in distilled and effectively none in deionised, at fifteen minutes.

• The pure-water half is markedly denser: the image was washing out, and the fix is the water, the absorption, or the formulation.
• Both halves are equally weak: the pigment was never made in quantity, and the causes are exposure or the sensitiser.

Distinguish it from over-exposure, which looks similar in the highlights. Ware’s classic process reverses at heavy exposure — the shadows pale to a grey-blue and the print looks solarised — so a weak print with separated shadows is under-exposed, and a weak print with reversed shadows is bronzing and reversal instead.

Diagnostic questions

• Is the wash water blue? The first and best question.
• What is the tap water like? Ware’s table makes this a first-class variable, not a refinement.
• Was a surfactant used? Absorption into the fibres is the difference between an image that stays and one that leaves.
• What is the ferricyanide concentration? Below about 5 per cent, a pictorial maximum density is not available on Ware’s own account of the commercial papers.
• What is the negative’s density range? The classic process has an exposure scale of about 0.9 — three stops — comparable with a grade 3 silver paper. A long-range negative made for platinum will not print on it at all.

Corrective action

Wash in purer water, and finish in distilled or deionised. It is the cheapest large improvement available and Ware’s numbers support it directly.

Over-expose, if you are staying with the classic formula. Ware is blunt that this is the only remedy the classic process offers for the peptisation loss — “to overexpose it grossly, which affects the quality adversely and the ease of judging exposure” — and that some workers resort to double coating. The course reports his verdict along with his remedy.

Add a surfactant — Ware’s two to five drops of 20 per cent Tween 20 per 10 cc — to get the sensitiser into the fibres.

Or change the chemistry. Ware’s own answer to all seven shortcomings is to replace the citrate with ammonium ferric oxalate, which gives a Prussian blue more resistant to peptisation, little image loss on processing, and an exposure scale of about 2.2 that matches a long-range negative. That is a different process rather than a fix for this one, and it belongs to the part that will teach it.

Prevention

Choose the paper for absorption, not only for surface. A gelatin-sized sheet also reduces the edge-etch effect, which Ware attributes to inhibited ion mobility.

Match the negative to the process, not the process to the negative you have.

Measure your own maximum density once with a step tablet, so that “weak” is a number rather than an impression — the process atlas comparison fields exist for exactly that kind of claim.