Bubbles and pinholes in a coating

Lifted from Break/Fix — emulsion coating defects, entry 4 of twelve.

What you see

Clean round holes in the dry layer, or clean round clear spots after processing, with no rim. They come in a range of sizes rather than in one size.

A bubble sitting at the surface breaks as the layer dries and leaves a hole. One trapped underneath holds the emulsion off the support and leaves a clear disc, which reads as a white spot on the finished print.

Likely causes

1. Air whipped into the melt during stirring.
2. A bottle of emulsion shaken to remix it.
3. Emulsion poured from a height, so that it entrained air on the way down.
4. Air trapped under the layer during the stroke and never chased out to the edge.

What is happening: the chemistry and physics

Gelatin at coating temperature is viscous enough to hold a bubble in place and setting fast enough that the bubble has no time to rise out on its own. Duffin’s mechanism for setting is the formation of hydrogen bonds between gelatin chains as the temperature falls, and it begins within seconds of the layer leaving the warm jug.

So a bubble that would have surfaced and burst in a beaker of warm emulsion is simply frozen where it is in a coated layer. Where it sits at the surface it leaves a hole when it goes; where it sits at the interface it acts as a spacer, and there is no emulsion at all above it.

That second case is why the mark survives processing. It is not a surface blemish — the silver was never there.

Diagnostic questions

• Is there a rim? No rim means a bubble. A repellency crater has one.
• Is there a range of sizes? Whisking produces a distribution rather than a single size. Spots that are all the same size and evenly spread point at the support rather than the melt.
• Did the melt stand before it was used, or was it poured straight from the stirrer?
• Are the marks concentrated at one end of the stroke, where trapped air would have been pushed?

Corrective action

None on that sheet, and do not rework a plate to try — a second pass over a layer that has begun to set leaves ridges of its own.

For the rest of the melt, let it stand covered in the warm bath for ten minutes. The bubbles rise and break, and the emulsion is no worse for the wait.

During the stroke, chase the bubbles that do appear out towards the edge, where they can burst against the dam bar rather than in the picture area.

Prevention

Stir, do not whisk, once the emulsion is made. Pour down the inside of the vessel rather than from a height. Do not shake a bottle of emulsion to remix it — warm it and swirl it.

Build the ten-minute stand into the coating routine rather than saving it for a session that has already gone wrong, and note the standing time in the coating record so that a batch coated in a hurry is identifiable afterwards.