Pinholes

Owned by the emulsion under stress.

What you see

Small clear spots in the developed image, printing as black dots.

That description fits at least three unrelated faults, which is the point of this entry and the reason the single name is misleading. What separates them is the edge of the spot and the company it keeps.

Cause	The spot	Its company	Reflected light
Gas from carbonate meeting acid	A crater with a rim, in the plane of the layer	Usually many, distributed over the whole sheet	A dimpled surface that catches a grazing light
Bubble in a hand-coated layer	A clean round hole right through to the support	Often with brush or rod marks nearby	Flat; the hole is an absence, not a deformation
Dust before exposure	An irregular outline, the shape of the particle	Usually friends nearby, in the same region	Flat
Air bell in development	Round, sharp-edged, perfectly clear	Few, grouped where the film sat highest	Flat

Likely causes

1. Carbon dioxide evolved in the layer, where a carbonate-bearing developer met an acid bath. Kodak’s 1928 primer calls the result a blister and gives the dry appearance as “tiny crater-like depressions when examined by reflected light” — which is what most people call a pinhole.
2. A bubble or a dust particle in a hand-coated emulsion, trapped before the layer set. This is a coating fault and belongs to Part V; its own entry is coating bubbles and pinholes.
3. A dust particle on the plate at the moment of exposure, which masked the light so that no latent image formed under it.
4. An air bell in the developer, which has its own entry because its prevention is a physical action rather than a chemical change.

What is happening: the chemistry and physics

Three different ways to arrive at no developed silver here.

Gas. Carbonate plus acid gives carbon dioxide, and gas evolved inside a soft swollen gel has nowhere to escape quickly. It lifts the layer locally; when the film dries, the lifted gelatin collapses into a small crater. This is the only one of the three that deforms the layer, which is why it is the only one you can see in reflected light at a grazing angle. Kodak Limited later marketed an alkali, Kodalk, on precisely this mechanism: it “does not evolve carbon dioxide on acidifying”, so “risk of blistering is thus eliminated”.

A bubble in the coating. Where the wet emulsion carried a bubble, the dried layer has a hole in it right down to the support. There was never any halide there, so there is nothing to develop and nothing to fix. The edge is the shape of a bubble — round, clean, and the same all the way through.

A dust shadow. A particle lying on the sensitive surface at exposure masked the light; no latent image formed under it; the developer had nothing to work on and the fixer removed the halide in the ordinary way. The hole is the shape of the particle, which is generally irregular, and there are usually more of them nearby because dust arrives in company.

!The reflected-light test separates the gas case from the other three

Tilt the dry film against a strong light so that the light grazes the surface, and look at the surface rather than through it.

A crater catches the light — you can see a dimple and a rim, because the gelatin has been physically displaced.

The other three are flat. A coating bubble, a dust shadow and an air bell are all an absence of silver in an undisturbed layer, so the surface is smooth across them.

That single observation splits the four into one and three, after which the edges and the company separate the remaining three.

Diagnostic questions

• Is the layer deformed at the spot, in reflected light? Gas, if yes; one of the others, if no.
• Is the hole perfectly round with a clean edge, or irregular? Round is a bubble or an air bell; irregular is dust.
• Is the material hand-coated? Then a coating bubble is available as a cause, and it is not otherwise.
• How many, and how distributed? Gas craters are many and general; air bells are few and grouped where the film was highest in the tank; dust comes in local clusters.
• What alkali is in the developer, and what is between it and the fixer? A carbonate developer with no rinse into an over-acid fixer is the gas condition.
• Was the tank tapped after filling? Both ILFORD and Kodak put the rap in their published procedure.
• Was the plate or the holder clean, and was the darkslide drawn in a dusty place?

Corrective action

None on the negative in any of the four cases. There is no silver to recover.

Spotting the print works, and works well: all four print as a black dot on a lighter ground, which is the easiest of all retouching jobs. Spot the print, never the negative.

Prevention

It differs by cause, which is why getting the diagnosis right matters more here than the appearance suggests.

Against gas: rinse or use a stop bath between developer and fixer, keep the fixer at its intended acidity, keep the session and the baths from running warm — and, where the fault persists, change to an alkali that does not evolve carbon dioxide on acidifying, which was Kodak’s own answer.

Against coating bubbles: Part V’s coating pages own it — pour and settle rather than stir, and let a made emulsion stand before coating.

Against dust: clean holders, a blower on the darkslide, a still dust-free place for loading, and — for drying — the still, dust-free atmosphere ILFORD’s own guide asks for.

Against air bells: tap the tank on the bench at filling and at every agitation, as both manufacturers’ procedures require.