Repellency spots

Lifted from Break/Fix — emulsion coating defects, entry 3 of twelve.

What you see

Small round areas where the emulsion pulled away, leaving a bare or nearly bare patch. The patch has a raised rim of emulsion around it, and the bare centre is clean.

They are visible in raking light on the dry, unprocessed sheet, which is where they should be caught.

Likely causes

1. A fingerprint. A bare finger on the support before coating is the commonest single cause.
2. Silicone, from a release agent, a polish or a spray.
3. Aerosol overspray, from anything used in or near the room.
4. Hand cream.
5. Detergent residue left by an inadequate rinse after cleaning.

What is happening: the chemistry and physics

A liquid layer on a solid minimises its own surface. Where a hydrophobic contaminant lowers the surface energy of a small patch of the support, the wet emulsion is no longer wetting the surface there, and it retreats — and it has to go somewhere, so it heaps up at the boundary it retreated to.

That heap is the rim, and the rim is the whole of the diagnosis, because it is the signature of a wetting failure rather than of an inclusion. A bubble does not push emulsion outwards; it simply holds the layer off the support and leaves a hole with nothing piled around it.

Diagnostic questions

• Is there a rim? A crater has emulsion piled up at its edge; an air bell leaves a clean circle with no rim. Read it in raking light before processing, when the relief is still there to see.
• Are the spots a range of sizes and scattered, or all one size and evenly spread? Identically sized, evenly spread spots point at the support rather than at the melt or the hand.
• Do they cluster where a hand would have held the sheet?
• Did anything get sprayed in the room that day?

Corrective action

Clean the support and recoat. On glass, a hot 2 per cent caustic soda clean removes what detergent will not; on paper there is no cleaning route and the sheet is lost.

The sheet itself cannot be saved by coating over the crater, because the second layer meets the same low-energy patch and does the same thing.

⚗The wetting agent, and the dose the course does not have

A wetting agent in the melt raises the layer’s ability to wet a marginal surface, and it is standard practice. This course cannot tell you how much to use. The source that supplies the coating formula the emulsion part works from names a wetting agent in its tools list and gives no dose anywhere in the corpus. Add one drop to a batch, record it in the coating record as your own variable, and find your own figure — and do not treat a number you were given by somebody who also could not cite one.

Prevention

Cotton gloves on clean glass, nitrile on wet work, and no aerosols in the room.

Clean supports properly and rinse them properly. A detergent left on the glass is itself a contaminant of exactly the kind that produces this fault.

Handle sheets by their edges from the moment they leave the packet, and coat two sacrificial sheets at the start of every session — coating is a motor skill and the first sheet is always the worst.