Hand-coating a sheet with a rod and with a brush

Purpose

To put a measured volume of sensitiser evenly onto a marked area of paper, and to know afterwards how much went on.

When to use it

Whenever a sensitiser is applied to paper by hand, by rod or by brush. It stops at the dry sheet; exposure and wet processing belong to the process’s own page.

It does not cover floating a sheet on a bath, which is a different operation with its own timing, and it does not choose your volume: the drop counts and coating volumes come from the process you are running.

Before you start

You should already know that a rod uses roughly 30 cm³ of sensitiser per square metre and a brush roughly 40 to 60, and what that ratio is worth on an expensive metal, from the negatives and papers lesson.

Calibrate your own dropper before you count a single drop. A drop is not a unit: two suppliers of the same process publish drop volumes about 25 per cent apart. Deliver fifty drops of water into a tared beaker, read the mass, divide by fifty, and repeat three times.

Level B or higher, set by the sensitiser in your hand, not by the act of coating. Gloves throughout; splash goggles wherever the sheet for that substance asks for them; the process’s own rod or brush by the tool dedication rule. Weighing any noble-metal salt as a powder is outside this course’s sessions, which use bought solutions.

On the bench: a level glass plate or board with a spirit level; low-tack tape; blotting paper under and beside the sheet; a pencil; the negative or a mask; the measured volume in a small cup; and the sensitiser’s own waste container, open.

Procedure

Preparing the sheet

1. Read the hygrometer at the bench and inside the drying enclosure, and write both figures down with the temperature. Where the process names a working humidity band, and yours is outside it, deal with that before you mix anything.
2. Bring the paper and the solution to the temperature of the room and give them an hour to get there.
3. Cut the paper now, dry, because a coated sheet is not trimmed: a coated area about a centimetre larger than the negative, with margins beyond that.
4. Tape the sheet at its corners over blotting paper and check with the spirit level that it is level left to right. A rod finds any slope you leave it.
5. Lay the negative or the mask on the uncoated sheet and mark its four corners in light pencil. That concentrates the sensitiser where the image is and leaves a coated-but-masked border, which is the most useful diagnostic area on the finished print.

Coating

6. Mix or pour only the volume this sheet needs, and only once the sheet is taped, levelled and marked.
7. By rod: pour the volume in a line along one edge just outside the marked area, set the rod in the pool and rock it a few degrees each way so it loads by capillary action.
8. Draw the rod across with very light pressure, lift at the far edge, skip back over the bead and draw the other way. Five passes: the first two quick, the last three as slowly as you can manage.
9. By brush: wet the brush with distilled water and blot it hard first, so the sensitiser stays on the paper instead of wicking into the bristles.
10. Pour the whole measured volume into the centre of the marked area and spread it immediately, across the sheet and then down it, alternating. Stop brushing before the paper does: over-brushing abrades the sizing and lifts fibre that prints as texture you did not choose.
11. Blot the residual bead at the foot of the coating with clean blotting paper.
12. Rinse the rod at once, before anything crystallises on it, and wash the brush into its own process’s labelled waste container. Check your local regulations, which govern where that container may go.

Drying

13. Lay the sheet flat until the reflective sheen has gone, then hang it in the dark.
14. Give it about an hour, unattended. Warm air at a uniform low temperature is allowed; a hairdryer is not, because direct heating with one is uneven and an uneven dry is an uneven sensitiser.
15. Look at the dry sheet in room light and compare its colour against what the process publishes for a good coating. A sheet whose colour is wrong is not exposed; it is diagnosed.
16. Record the volume used, the passes, and the area covered, so the coverage figure is a measurement rather than an assumption.

How you know it worked

• The sheet was level, and the coating stops at the pencil marks with a masked border outside it.
• The volume was measured with a dropper you calibrated today, and it is written down.
• The rod took about five passes, or the brush finished spreading well inside half a minute.
• The bead was blotted and the rod rinsed before the sheet was moved.
• The dry sheet is even in raking light and the colour the process publishes.
• The humidity and temperature at coating and at drying are in the record.

If a step fails

Step 1, the humidity is far below the process’s working band. Put a tray of water in the drying enclosure, wait an hour and read it again. A defect caused by a dry coating appears in the tray after all the metal has been spent.

Step 4, the plate will not sit level. Shim it. Do not compensate with rod pressure: a slope moves solution to one edge and the print records it as a gradient.

Step 8, you can make more than six passes, or a line of solution is still standing after ten. The volume is too high. Blot the standing line off with the corner of a paper towel, and reduce the volume by a quarter next sheet.

Step 10, you are still spreading after thirty seconds. The same fault by brush. Reduce the volume by a quarter and record the change.

Step 11, the bead dried in place. Inspect the negative before the next exposure rather than after it. Crystals at the foot of a coating are what damage a negative in a printing frame.

Step 12, the rod or brush went unwashed. Retire the brush from that process rather than rescuing it, and inspect the rod against the light before it is used again.

Step 15, the dry sheet is the wrong colour. Do not expose it. On a cyanotype a blue cast means iron(II) is already present, by light or by contamination, and the answer is to find out which of the three causes it was rather than to print through it.

What to record

Paper and its lot; the sensitiser and its own batch, strength and date; your measured drop volume with its spread; the volume delivered and the area covered, converted to cubic centimetres per square metre; rod or brush, and the number of passes; the temperature and relative humidity at coating and at drying; the drying time and method; and the colour of the dry sheet. Those are the fields a coating fault is diagnosed from a week later, and none of them can be recovered from the print.