Bronzing and reversal from cyanotype over-exposure

Owned by Break/Fix — the cyanotype that went wrong, whose sixth case reached this entry’s conclusion independently and says so: bronze before the wash is the expected appearance of a fully exposed sheet, reversal is documented over-exposure, and the corpus carries no account of a metallic sheen on a finished, washed cyanotype. That page adds the mechanism for the reversal, which is Ware’s selection of Murray’s proposal over Suzuki’s together with the experiment behind it, and it names the nearest documented neighbour — the “burning out” of commercial blueprint papers, which is grey rather than metallic. This entry was written before Part XXI and left the bronzing question open for it; the answer, on this corpus, is that there is no source.

What you see

Two appearances, and they are not the same fault.

Bronze. A metallic bronze or grey-bronze surface on a fully exposed, unwashed print. Jacquard’s own instructions state it as the expected condition: “The fabric will look bronze in color once fully exposed”, and “with wetting, the print will change from a bronze to blue color.” That is what a correctly exposed cyanotype looks like before the wash, and a reader who stops the exposure to avoid it will under-expose.

Reversal. The shadows — the parts that received most light — going paler than the tones beside them, so that the deepest areas of the print are a pale grey-blue. This is solarisation, and it is a real, observable, describable phenomenon that can spoil a print or be used deliberately.

✕What the course could not find, and says so

The register’s title names bronzing as a defect. The course has searched its corpus and has found no source describing a cyanotype defect called bronzing — no metallic sheen on a finished, washed print analogous to the over-inked bronzing of a printing plate, and nothing in Ware’s monograph, which is the most detailed technical account of the process in the literature and which does not use the word at all.

What the corpus does establish is the two things above: bronze is the normal appearance of the print-out image, on a manufacturer’s own instruction sheet, and reversal is a genuine over-exposure phenomenon, named by Herschel in 1842 and explained chemically by Ware.

So this entry documents reversal and records bronze as an expected stage rather than a fault.

Part XXI has since been written and searched the same corpus to the same end. Its break/fix page states that a metallic sheen on a finished, washed print is an observation for which it asserts no mechanism, and it names the nearest documented neighbour as something else entirely: the greyish “burning out” Ware describes in the commercial blueprint papers of the reprographic era, which he attributes to an excess of iron(II) bleeding into and degrading the shadows rather than to anything at the surface. The register’s row therefore stays half unfilled, and it is not being filled from general knowledge.

Likely causes

1. Exposure carried well past maximum. The reversal is a property of exposure, not of a mistake in chemistry.
2. A variety that reverses strongly. Ware’s comparison table gives the brown ferric ammonium citrate a relatively slight reversal and the green salt a stronger effect with a higher maximum density.
3. Judging by the print-out image. The wet print is not the dry print and the unwashed print is not the washed one.
4. An incomplete return to blue. Prussian white is colourless; if it is not re-oxidised, the tones that reversed stay pale.

What is happening: the chemistry and physics

Prussian white is the answer to both halves.

Ware gives the reaction and the consequence. Under continued light, and in the presence of ferrous ions, some of the Prussian blue already formed is further reduced to Prussian white — ferrous ferrocyanide — which is colourless, because the mixed valence that makes the pigment blue is gone and there is no intervalence transition left to make. The deep blue image begins to pale in its regions of greatest exposure, and that is solarisation.

He is careful about one thing and the course repeats the care: the transformation is not brought about solely by light. The chemical reduction of Prussian blue to Prussian white must be accompanied by an oxidisable substance — so the same exposure on a different paper does not necessarily reverse the same way.

And reversal is often wanted. Ware’s account is that it is desirable for a print-out image, because the self-masking action of Prussian blue in the shadows is diminished by the reversal to white, which lets more light in and forms more product — so the finished print reaches a higher maximum density than it could have without it. His own New cyanotype instruction tells you to expose until “the high values appear green, the mid-tones are blue, and the shadow tones are substantially reversed to a pale grey-blue, giving the image a solarized appearance.”

The blue comes back. After exposure, Prussian white is oxidised back to Prussian blue, slowly by the oxygen of the air over hours, or rapidly in a bath of an oxidising agent — Ware’s is hydrogen peroxide at 0.3 per cent, and Jacquard include a peroxide step in their own sheet.

!The confirming test: wait, or oxidise, before you judge anything

Put the washed print in the dark for several hours, or give it a brief bath of dilute hydrogen peroxide, and look again.

• The pale shadows fill in and the print deepens: nothing was wrong. You were looking at Prussian white on its way back to Prussian blue, and the exposure may well have been right.
• The pale shadows stay pale after full oxidation: the exposure genuinely reversed past the point of return for this paper and this sensitiser, and the fix is less exposure.

And separate reversal from the other pale-print fault. A print that is weak everywhere, with separated shadows and blue wash water, is weak maximum density from peptisation. A print that is dense in the mid-tones and pale only in the deepest areas is reversal. The shadows are the discriminator, and they point in opposite directions.

Diagnostic questions

• Have you waited? Ware describes the shadows darkening and the colour intensifying over several hours after the wash, as atmospheric oxygen does its work.
• Which salt, green or brown? Ware’s own table makes reversal a property of the variety.
• Was the bronze on the unwashed print, or on the finished one? Bronze before the wash is expected; anything metallic on a finished, dried print is outside what this course can source.
• What was the exposure judged against? Ware’s New-cyanotype instruction is a description of the wanted appearance, and it includes reversal.
• Was a peroxide bath used, and at what strength? Ware’s is 0.3 per cent w/v for about half a minute.

Corrective action

Oxidise and wait before deciding anything. Most prints reported as reversed are prints judged too early.

If the reversal is genuine and unwanted, expose less — and note that Jacquard’s own advice runs the other way for ordinary work: over-exposure is almost always preferred to under-exposure, which is a statement about where the process is forgiving.

Do not attempt to bleach a reversed print back. Ware records that under no circumstances should a bath of potassium or ammonium dichromate be used for re-oxidation, although it has been recommended in the past, and the course carries that prohibition rather than inventing an alternative.

Prevention

Make a step-tablet exposure series once for your own sensitiser, paper and light source, and read the point at which the shadows begin to reverse. That number is worth more than any published exposure time.

Judge exposure on the finished, dried, oxidised print, never on the print in the frame.

Include the peroxide step if you need to see the final result the same day, and record whether you used it, because a print oxidised in a bath and a print oxidised in air are not the same experiment.