Comets

Lifted from Break/Fix — emulsion coating defects, entry 1 of twelve, where it belongs to the family of faults that have a direction.

What you see

A short dark or light head with a tail trailing behind it in the direction of the stroke. There is often more than one on a sheet, and when there is, they are all parallel to each other — which is the first thing to notice, because emulsion left to itself has no preferred direction.

Where the particle was a halide fragment, the head has developed out black. Where it was a fibre or a lump of gelatin, the head is a thin place and reads light on the finished print.

Likely causes

1. Undissolved halide. A potassium bromide pellet that never went into solution is named in the published practice as the cause of black slugs or sunspots.
2. Undissolved gelatin, a lump that survived the soak and the melt.
3. A fibre or grit from the room, the vessel or the coating rod itself.

What is happening: the chemistry and physics

The rod rides over the particle instead of through it, so it is held a fraction higher off the sheet for as long as the particle is under it. The layer laid down behind it is therefore thinner, and the particle ploughs a furrow the length of the remaining stroke. That is the tail.

The head is the particle. If it is silver halide, development reduces it and it is dense; if it is inert, it is simply a hole in the coverage and it is thin. Both share the same geometry, because the geometry is the rod’s and not the particle’s.

The coating weight in the tail is genuinely lower, so the fault survives into the print as a real density difference rather than as a surface mark.

Diagnostic questions

• Look at the head with a loupe in raking light, before processing. A comet has a physical particle at its head. A stroke fault does not. This is the discriminating test and it takes seconds.
• Are the marks parallel to one another and to the stroke? A fault with an axis is the tool.
• Is the head dark or light after development? Dark points at halide; light points at anything inert.
• Do they appear on every sheet from the melt, or on some? Every sheet points at the melt or the jug; some sheets point at the moment.

Corrective action

None on that sheet. A comet cannot be coated over, and reworking a plate is forbidden for its own reasons — the emulsion sets too fast for a second pass to merge with the first.

Strain the remaining melt through a fine mesh into the coating jug before coating anything else from it, and record that you did, because a batch that was strained mid-session is two batches.

Prevention

Dissolve the halide completely before the gelatin goes in — ground to a powder, and the solution held up to the light to check.

Strain every melt into the coating vessel as a matter of routine, not as a response to a fault.

Keep the coating rod and the jug clean and covered between sessions, and handle the sheet by its edges, so that the room contributes as little as it can.