Blotchy and streaked hand coating

Owned by Break/Fix — the cyanotype that went wrong, whose fifth case reads the geometry of the fault before reaching for a cure and gives the same pattern table, and which adds a chemical route this entry did not have: Ware’s suggestion that brush marks may themselves be due to friction promoting microcrystals in the coating, which makes the streak row and the crystal row less independent than they look. Printing a Van Dyke brown and coating for palladium both name this entry as the general treatment for their own coating faults. It was written earlier from Part V’s coating work and the suppliers’ sheets.

What you see

Four different appearances, and they are worth separating before anything else:

What you see	What it points at
Streaks in the direction of the stroke, evenly spaced	The brush or rod, and how much solution it carried
Blotches with hard, rounded edges, like drops that did not spread	The sensitiser not wetting the paper
Small white spots, sharp, scattered	A gritty sensitiser: surface crystals
A general unevenness with no direction	The paper’s sizing, or contamination from handling

Likely causes

1. The sensitiser is not wetting the paper. Ware’s account of the classic cyanotype names this as a shortcoming of the formula itself: the traditional sensitiser is poorly absorbed by cellulose fibres and tends to remain on the surface or in the pores.
2. The sensitiser is gritty. Ware is specific: when a sparingly soluble salt crystallises out of the mixture, “a gritty sensitizer produces small white spots on the print, typical of surface crystals” — and he raises the possibility that brush marks and other defects may be due to the friction promoting formation of microcrystals in the coating itself.
3. Too little solution, or a stroke that ran dry. Photographers’ Formulary’s kallitype instruction is the clearest statement of the target: brush first across and then up and down until the paper is completely wet, then work the surface with a second clean brush until it loses its gloss.
4. The paper, or the handling of it. Sizing varies between sheets and batches, and a fingerprint is a patch of grease.

What is happening: the chemistry and physics

Coating an iron-process paper is not painting a surface; it is loading a fibre. Ware puts the requirement plainly: these processes depend critically on absorption of the sensitiser solution into the interfibrillar space within the surface cellulose fibres, because the image substance is often in nanoparticle form and must be securely trapped there. A coating that sits on the paper is both uneven and fragile — see weak maximum density for what happens to it in the wash.

Wetting is a surface-energy problem and the standard remedy is a surfactant. Ware’s classic formula takes two to five drops of 20 per cent Tween 20 per 10 cc of sensitiser to help the solution into the fibres. The kallitype route gets at the same thing mechanically: a second, clean, dry brush worked over the surface until it loses its gloss is a way of pushing the solution off the surface and into the sheet.

Grit is a solubility problem. Where two concentrated solutions are mixed, a sparingly soluble product can come out on the paper as the water leaves. Ware’s case is potassium ferric oxalate, and the visible result is white spots where a crystal sat and no image formed.

And humidity governs the whole operation. Ware works at about 55 per cent RH ± 5 and allows at least an hour for the paper to equilibrate before printing; the citrate sensitisers are deliquescent, so at high humidity the coating stays tacky — which mars the coating and can damage a negative pressed against it. Mottled coating from humidity is the entry for that half.

!The confirming test: coat a sheet with no negative and read it in raking light

Coat a whole sheet, dry it, expose it fully without a negative, and process it. An evenly coated sheet prints an even tone; every fault above prints as itself, at full strength, with nothing to hide behind.

Then look at it in raking light before processing as well as after. Grit and beading are visible in the dry coating; a difference in absorption often is not.

Two follow-up tests split the remaining causes.

• Filter the sensitiser and re-coat. If the white spots go, they were crystals.
• Add the surfactant and re-coat. If the beading goes, it was wetting.

And keep a coated, masked border on every working print: it is the same reference the fogging algorithm needs, and it shows coating faults where they do not cost a picture.

Diagnostic questions

• Is the pattern directional? Streaks follow a tool; blotches do not.
• Was the solution filtered, and how old is the mix? Grit implies something came out of solution, and a cloudy sensitiser is the same event seen in the bottle.
• How much solution, over what area? “Liberally coated but not running wet” is the supplier’s own target.
• Was a surfactant used, or the surface worked until it lost its gloss? Gloss on a drying coating means solution still sitting on top of the paper.
• What is the humidity? Both extremes cause coating faults, and 55 per cent is where Ware works.
• Is the brush dedicated to this process? Bostick & Sullivan’s instruction is a brush used only for one process, with a glass rod as the shareable alternative if it is properly washed — a cross-contamination rule as much as a coating one.

Corrective action

Filter the sensitiser if there is grit, and discard it if filtering does not clear it.

Add the surfactant at the point of use, and re-coat. Ware’s dose is per 10 cc of working sensitiser, not of stock. And where the sensitiser carries a precipitate, Photographers’ Formulary’s instruction for kallitype is the general rule: the precipitate itself does no harm in the bottle, but be sure not to transfer it to the paper you are coating.

Mark the image area and mix the solution properly. Bostick & Sullivan’s instruction is to pencil the negative’s four corners onto the uncoated sheet so that the solution is concentrated inside the image area, and to make sure the two stock solutions mix thoroughly before coating — an incompletely mixed sensitiser coats unevenly by composition rather than by technique.

Re-coat rather than rescue. A badly coated sheet is cheap; a badly coated print is a wasted negative and a wasted afternoon.

Prevention

Test each new paper before buying a box, with the blank-coated sheet above.

Work at a stable, moderate humidity and let the paper equilibrate.

Dedicate brushes and rods to one process, both for coating quality and because of cross-contamination.

Protect the bench. Bostick & Sullivan warn that these solutions stain wood, metal and many plastics — which is a hint about what they will do to anything else they touch, including the next sheet.