Platinum print stain from incomplete clearing

Owned by Lab: exposing, developing and clearing a palladium print, whose troubleshooting section sends yellow highlights here and gives the supplier’s own remedy — a stronger clearing agent or a longer bath — with the observation that water hardness sometimes affects clearing time. Cost, permanence and when a platinum print earns its price supplies the reason it matters: the permanence of the print is decided in the third clearing bath, on the evening it is made, by a difference nobody can see at the time. This entry was written earlier from the reagents’ encyclopaedia entries and from Ware’s clearing chemistry.

What you see

Yellow in the highlights, and it is often not there when the print is made. Bostick & Sullivan put the working test in one sentence: if the print shows any yellowing in the highlights, it is not being properly cleared.

The delay is the cruel part. Ware’s account of the mechanism explains it: the stain can appear slowly, long after the print is dry and framed, because the iron that was left behind changes into something more strongly coloured over time.

Likely causes

1. Iron(III) chemisorbed onto the cellulose, which is Ware’s “chief problem for effective clearing”.
2. The wrong pH. Above pH 4, iron(III) hydrolyses to a polymeric colloidal hydroxide which lodges in the fibres.
3. Hard water or a buffered paper. Calcium precipitates insoluble calcium oxalate and promotes aquation and hydrolysis of the iron(III) complex — so a hard supply attacks the clearing from two directions at once. Bostick & Sullivan note in plainer language that water hardness will sometimes affect clearing time.
4. A sequence too short, too weak, or in the wrong order.

What is happening: the chemistry and physics

Almost everything left in the paper after exposure washes out easily. Ware lists them — aquated iron(III) oxalatocomplexes, free oxalate, ammonium or potassium, tetrachloroplatinate or tetrachloropalladate, chloride — and most are simply soluble.

The exception is the iron, and it has two ways of staying. It can bind chemically to the hydroxyl groups of the cellulose itself, which Ware argues is a genuine coordination — cellulose competing as a ligand — and which would account for the very strong binding of iron(III) to paper and the persistent yellow staining of platinotypes and palladiotypes. Or it can hydrolyse in place.

And hydrolysis has a clock on it. Freshly formed iron(III) hydroxide can be redissolved in dilute acid. Left, it transforms irreversibly into iron(III) oxyhydroxide — the mineral goethite — which is quite insoluble in dilute acids. Hence Ware’s flat instruction: it is essential to remove all the iron(III) at the wet processing stage, before the print dries.

Why the stain arrives late. Ware’s explanation is that the monomeric iron on the cellulose is only weakly coloured, but that with time adjacent units can cross-link into a μ-oxo bridged binuclear iron(III) species, which absorbs strongly in the visible by metal-to-metal charge transfer and is brown. The iron was always there; the colour was not.

⚗Why the three baths are in that order

Ware’s sequence is chemistry rather than custom, and each bath does a different job.

1. Disodium EDTA, about 5 per cent w/v, acidic at pH 3–4. The acidity suppresses hydrolysis and the chelate takes most of the iron(III) out.
2. Sodium sulfite or disulfite, about 2.5 per cent w/v, made up fresh each session because it does not keep — air oxidises it to sulfate. Its job is to reduce residual iron(III) still bound to the cellulose to iron(II), which is less strongly bound and less extensively hydrolysed, and therefore easier to remove.
3. Tetrasodium EDTA, about 5 per cent w/v, alkaline at pH 9–10, which is the optimum pH for chelating the iron(II) the second bath just made — and which leaves the sheet alkaline, desirable for the paper’s own preservation.

And Ware’s warning, which is a disagreement between two sources the course records rather than resolves: “It is important not to use tetrasodium EDTA for the first processing bath — in spite of the fact that it has been recommended by Bostick & Sullivan” — because at pH about 10 the iron(III) is not effectively complexed but hydrolysed, with the eventual result of yellow or brown staining. Bostick & Sullivan’s own kit sheets supply an EDTA clearing agent with sodium bisulfite and give a working procedure that clears successfully in practice; Ware’s objection is to the alkaline salt in the first bath specifically. The course reports both positions and does not adjudicate between the products.

The sequence has been measured. Matthew Clarke and Dana Hemmenway compared clearing procedures by XRF and found this one left less residual iron than any other combination tested — comparable with, or less than, the original iron content of the uncoated paper — and accelerated ageing of those sheets produced no perceptible yellow stain.

Diagnostic questions

• Are the highlights yellow, and were they always? A stain that appeared later is the μ-oxo mechanism and the iron has been there since the wash.
• Which EDTA, in which bath, at what pH? The single most consequential answer on this list.
• Is the sulfite bath fresh? Ware says it must be made up for each session because it oxidises in air, and a stale bath is water.
• How hard is the water, and is the paper buffered? Calcium is a cause twice over on Ware’s own equation.
• How many prints through this clearing bath? Bostick & Sullivan give fresh clearing agent as good for up to a dozen 8 × 10 prints.
• Did the print dry before clearing finished? If it did, the clock Ware describes has started.

Corrective action

Clear harder while the print is still wet. Bostick & Sullivan’s own instruction is to increase the concentration of the clearing agent or increase the time in the bath, and their standard is one or two successive five-minute baths.

Ware records an energetic treatment for a stain that has already appeared, and the course records its limits. Where the advice above was not followed, he says a more energetic clearing procedure using the powerful reductant sodium dithionite may remove the stain. Sodium dithionite has no entry in this course’s chemical encyclopaedia and no hazard record in its corpus, so the course reports the treatment and does not give it as a procedure.

And there is a treatment that looks like a cure and is not. Ware records that acetate will bleach the colour by splitting the μ-oxo dimer — but that XRF shows the iron is not removed, and that this is the likely explanation for Steichen’s treated Stieglitz palladiotypes having re-yellowed. A stain that comes back was never taken out.

Prevention

Follow the three-bath sequence in order, at the stated concentrations and pH values, and never let the print dry between them.

Finish in soft water. Calcium is on Ware’s own reaction path to the stain.

Choose the paper knowing this. Ware observes that alum-rosin sized papers such as Weston Diploma Parchment and Cranes show less tendency to iron staining than modern papers, and suggests the aluminium ions may be competing for the same binding sites.

Include a coated, unexposed margin on every print. Ware’s own test targets do exactly that, because that is where the stain will be most evident — and finding it there is finding it before it is in a picture.