Mottled coating from uncontrolled humidity

Owned by Lab: preparing the sensitiser and coating for palladium, which makes relative humidity a recorded variable of the process rather than a condition of the room, works at Ware’s 55 per cent ± 5, and names a coating dried below about 30 per cent as the first of five factors promoting tonal reversal in palladium; its troubleshooting section sends a mottled sheen here. Part V’s coating, drying and hardening owns drying as a variable for a gelatin emulsion, and this entry was assembled from it and from Ware before the alternative-process parts were written.

What you see

A print that differs from the one you made last week with the same negative, the same sensitiser and the same exposure. The evenness of the coating, the amount of print-out visible in the frame, and — on the noble-metal processes — the colour of the finished image all move with the weather.

At the humid end there is a second symptom that is unmistakable: the dried coating is tacky, and it can stick to the negative in the printing frame.

Likely causes

1. The paper had not equilibrated. Ware allows at least one hour for coated paper to come to equilibrium with the atmosphere before printing, and describes that as typical of hand-coated practice.
2. A deliquescent sensitiser in a humid room. Ware’s third shortcoming of the classic cyanotype is exactly this: poorly absorbed, and being deliquescent, it causes a tackiness which can damage negatives if the humidity is high. The solid salt itself is highly deliquescent and can compact into an intractable mass in store.
3. Very dry conditions. These are not automatically better. Ware’s preliminary experiments indicate that the print-out effect in palladium may only be seen with dry sensitizers, below about 30 per cent RH — which is a variable that changes what you see in the frame while you are judging exposure.
4. No controlled storage between coating and printing. Ware notes that sensitised platinotype paper does not keep unless it is well dried, and that complete desiccation was a commercial imperative: Willis sold his papers in solder-sealed tins containing anhydrous calcium chloride.

What is happening: the chemistry and physics

Paper is hygroscopic and so is the sensitiser, so the coating’s water content is set by the room. That would be a nuisance rather than a fault if the chemistry did not depend on it — but it does, in three separate ways.

Water is a reagent and a medium. The reactions that form the image happen in whatever water the sheet is holding, and their rates and their products depend on how much there is.

Ware demonstrates the consequence for colour directly. Section 11.16 shows the effect of relative humidity on the colour of a 1:1 platinum-palladium print-out, with the same sensitiser solution on the same paper at 80, 56 and 15 per cent RH — three different results from one formula. That is a controlled experiment showing humidity as an image variable, not an inference.

And relative humidity is not the quantity that matters, which is where most of the confusion lives. Ware prints a table of the absolute water content of air — 17.30 mg per litre at 100 per cent RH at 20 °C, 8.65 at 50 per cent — and a conversion showing which relative humidity at another temperature corresponds to the same absolute content. So a room at 55 per cent RH in winter and one at 55 per cent in summer do not hold the same water, and paper equilibrates against the absolute quantity.

The mottle itself is unevenness in that equilibration. A sheet that is coated while still coming into equilibrium, or dried unevenly, or handled while tacky, carries different water in different places, and therefore reacts differently in different places. The AIC make the general point for albumen prints and it applies to any layer on paper: it is variations in humidity that cause the trouble, through differential dimensional changes between the image layer and its support.

!The confirming test: coat two sheets and change only the equilibration

Coat two sheets from the same mix. Print one immediately after drying; hold the other for at least an hour in the room, then print it identically.

• The rested sheet is more even, or a different colour: humidity is your variable, and the fix is procedural rather than chemical.
• They match: the mottle is a coating or absorption fault, and blotchy and streaked hand coating is the entry.

Then measure rather than judge. A thermohygrometer costs very little and it converts “it was a damp day” into a number you can write next to the print. Ware works at about 55 per cent ± 5, monitors both the ambient and container values, and notes that drying and rehumidification are reversible but show hysteresis — so a sheet that has been dried hard and then rehumidified is not identical to one that was never dried.

And test the tacky end specifically. Press a clean sheet of the negative’s own material against a coated, dried sheet for a minute. Anything that sticks is a coating that will mark a negative.

Diagnostic questions

• What was the humidity, and was it measured? The first question and usually the unanswered one.
• How long between coating and printing? An hour is Ware’s minimum for equilibration.
• Is the dried coating tacky? That is the humid-end signature and it puts a negative at risk.
• Has the sensitised paper been stored? If so, in what — Ware’s own regime is a desiccant chamber at about 9 to 27 per cent RH after a rest and a warm air dry.
• Which process? This matters more than the shared title suggests, and the owning parts are explicit about it. Ware’s statement for the cyanotype is flat — prevailing relative humidity has very little effect on it, though it can profoundly affect some of the other iron-based processes — so on a cyanotype the humidity fault is the deliquescent citrate coating going tacky and marking a negative rather than the image itself moving, and chasing the hygrometer for a mottled blue print is chasing the wrong variable. A palladium print-out is the opposite case and is sensitive at the dry end.
• Has the paper been dried hard and then re-humidified? The hysteresis Ware records means it is not back where it started.

Corrective action

Wait. An hour’s equilibration before printing costs an hour and fixes most of this.

Dry deliberately rather than incidentally: Ware’s own regime is a 10-minute rest to let the sensitiser into the fibres, then 45 °C in an air stream for 10 minutes, then a controlled store.

Do not print a tacky sheet against a negative you value. Interleave a clear sheet, or wait.

Record the humidity with the print. A colour you cannot reproduce is a colour you did not measure.

Prevention

Work in a stable room and, if the process warrants it, in a constant-humidity box — Ware lists the saturated salt solutions that hold particular values, with lithium chloride at 15 per cent and calcium chloride at about 9 or 32 per cent depending on the preparation.

Store coated paper dry and sealed, which is what Willis’s tins of desiccant were for.

Log the hygrometer alongside the exposure on the exposure log worksheet. It is one more column and it is the difference between a repeatable print and a lucky one.