Coating mottle and drying marks

Lifted from Break/Fix — emulsion coating defects, entry 7 of twelve.

What you see

A soft irregular texture across the sheet, like a watermark, visible in reflected light. It is usually noticed only after drying, and often only after the sheet has been printed on.

It has no direction and no edge, which puts it in the texture family rather than with the stroke faults or the gradients.

Likely causes

1. A draught blowing directly on the sheet as it dried.
2. A warm spot — a radiator, a lamp, a patch of sun on the drying box.
3. Contact with the drying rack, which conducts heat away and dries the sheet faster where it touches.
4. A change in the support that you did not make. A practitioner records a lot of a named paper whose sizing recipe had been altered by its maker, producing blistering during processing, and a later lot of a different paper that gave mottled fading late in the wash.

What is happening: the chemistry and physics

The layer goes down at about 85 per cent water and finishes at about 5 per cent, and gelatin holds on to 10 to 12 per cent of its own weight of water and will not release it. Everything in between is a drying front moving through a gel.

Where a part of the sheet dries faster, the gelatin in that part sets to a different density than its neighbours. The difference is permanent, because the network is fixed by the time the rest of the sheet catches up. What you are looking at is a map of how the sheet dried.

That also explains why the fault cannot be corrected. Wetting the sheet again does not return the gelatin to the state it was in at 85 per cent water.

Diagnostic questions

• Was it there wet? This is the discriminating test and it costs a torch. Look at the sheet in raking light while it is still wet, and again when it is dry. If it was not there wet, it is drying.
• Is the pattern related to anything physical — the bars of the rack, the direction of a vent, the edge nearest a window?
• Does it appear on every sheet in one drying box and not in another?
• What paper lot was it? A change you did not make is still a change, and without the lot number it is invisible.

Corrective action

None on that sheet.

The batch, though, is diagnosable. Dry two sheets from the same melt in two places and compare, which separates the drying space from the melt in one operation.

Prevention

Dry with the box shut, at an even temperature, with airflow that does not blow directly on the sheets. The published arrangement is a drying space near 24 °C with fabric taped over the air vent, so that air moves and dust does not.

Keep the paper brand and lot in the coating record. This is the prevention that catches the cause you did not create, and it is the difference between a fault that took weeks and half a dozen emulsion batches to find and one that took an afternoon.

📓Mottle has a second, unrelated home

The word covers two different faults in this course, and they are not to be confused. Mottle in a coating is a drying fault and lives here. Mottle in a developed negative is an agitation fault and lives in mottled development. The test that separates them is whether the texture was visible in raking light on the dry, unexposed, unprocessed sheet.