Black spots

Lifted from Break/Fix — emulsion coating defects, entry 9 of twelve.

What you see

Fine black dots scattered through the image and, crucially, through the unexposed areas too.

They are frequently invisible to the naked eye. Wall’s 1929 instruction is exact and worth obeying — the specks are sometimes so fine that they cannot be seen by ordinary visual examination, so test plates should always be examined with an eyepiece, particularly in the areas protected from light.

Likely causes

Two faults with one appearance.

1. Pepper — a characteristic coarse grain distributed through the emulsion and reduced to the metallic state without any exposure to light at all.
2. Slugs — undissolved halide that developed out, named in modern practice as the cause of black slugs or sunspots.

A third, less common, is metallic contamination. Rust from cheap stainless steel in contact with silver nitrate is specifically warned against in the emulsion practice this course follows.

What is happening: the chemistry and physics

Both faults are silver that was reduced without a latent image to justify it, and both therefore appear in the parts of the sheet that saw no light — which is why the unexposed area is where you look.

Pepper is a property of the make. It is described as a coarse grain reduced to the metallic state without exposure, and the three published preventions do not agree on a single mechanism, which is itself informative.

A slug is a property of the mix. An undissolved fragment of halide is a locally enormous concentration of developable material sitting in the layer, and development finds it.

⁇Three preventions for pepper, and no settled mechanism

The literature offers three separate remedies and does not reconcile them, which is a good sign that the fault is real and the mechanism is not settled.

Wall in 1929 gives two avoidances — reduce the quantity of water during the mix, or add the silver nitrate dry to the salted gelatin. Modern working practice gives a third — warm the silver solution immediately before precipitation.

The course records all three and prefers none, because none of the three sources argues against the others and no source it has read explains why any of them works. Try one at a time, in a batch record, and you will at least have your own evidence.

Diagnostic questions

• The loupe, on an area that was never exposed. This is the observation the fault is defined by, and it separates black spots from anything caused by exposure or by the safelight.
• Does every sheet from the melt have them? Every sheet points at the melt, which points at pepper. Only some sheets points at the coating, which points at slugs picked up from the jug or the rod.
• Are the marks a range of sizes with hard edges, or a fine even sprinkle? A slug is a fragment and looks like one; pepper is uniform in character across the sheet.
• Was any part of the silver route in contact with a metal you have not verified?

Corrective action

None on that sheet. Both faults are developed silver in the layer and there is nothing to remove.

Where the fault is slugs, strain the remaining melt and record that the batch was split.

Prevention

For slugs — grind the halide to a powder, dissolve it completely, hold the solution up to the light before it goes in, and strain every melt.

For pepper — pick one of the three published routes above and test it as a single change against a control, rather than adopting all three at once and learning nothing.

For contamination — keep silver nitrate away from unverified metals, and use glass or a verified plastic for every vessel on the silver route.

Examine one test plate from every melt with an eyepiece, in the unexposed area, before you commit a session to it.