Air bells

Owned by uneven development, bromide drag and surge marks.

What you see

Small, round, sharply bounded clear spots on the negative — a few, rather than many — often grouped towards one edge or in the region that sat highest in the tank. They print as black dots.

The two features that identify them are that they are circular and that they are completely clear: not thin, not weak, but as clear as the rebate, because nothing developed there at all. An edge that is sharp rather than graded rules out a flow fault.

On a print, the same thing appears as small white spots, from a bubble trapped under the sheet in the developer dish.

Likely causes

1. Air entrained during the pour, rising and lodging against the emulsion.
2. No rap on the bench after filling, or at each agitation.
3. A tank filled to the very top, leaving no air space for a bubble to escape into, or filled too little, so that part of the film is not covered.
4. Foam, from a wetting agent over-dosed or agitated too hard.
5. On a print, a sheet slid into the dish flat rather than edge-first, trapping air underneath.

What is happening: the chemistry and physics

Development is a reaction between a solution and a solid. Where the solution is not in contact with the emulsion, no reaction occurs.

A bubble of air held against the gelatin by surface tension is a patch of emulsion that is not in the developer. The halide there is never reduced, the fixer dissolves it in the ordinary way, and what is left is base — perfectly clear, in the shape of the bubble’s footprint.

The sharp edge is the diagnosis. A bubble has a definite boundary and does not diffuse, so the mark has a definite boundary too. Every flow fault on the flow-marks page has a graded edge, because a concentration gradient in a liquid has no edge. That single observation separates air bells from drag, mottle and surge in one look.

!The rap is a published step, not folklore

Both major manufacturers put the same physical action in their printed procedure.

ILFORD, in the guide to a first film: “each time you invert the tank tap it on the bench to dislodge any air bubbles which may have formed on the film.”

Kodak, in AJ-3’s small-tank agitation table, as the first instruction for both invertible and non-invertible tanks, before any agitation at all: “Tap tank on work surface to dislodge air bubbles.”

Two makers, independently, on two continents, in their own procedures. The rap is a step.

Diagnostic questions

• Are the spots round with a sharp edge? Round and sharp is a bubble. Soft-edged and streaky is a flow fault; irregular with company nearby is dust.
• Are they perfectly clear, or merely thin? Perfectly clear means no development at all, which is a bubble or a dust particle before exposure. Thin means less development, which is flow.
• Where were they on the film in the tank? Bubbles rise, so they collect on the uppermost part of the film and against the reel’s spiral.
• Was the tank tapped? Ask about the first fill specifically; most people who tap at each inversion forget the one that matters most.
• Is there foam on the surface of the used solution? A frothing bath is producing bubbles continuously.
• Every film, or one? Every film points at the routine; one points at that pour.

Corrective action

Nothing on the negative. There is no silver to recover, and the spot goes through to the print.

On the print, spotting works and is the standard remedy: an air bell prints as a black dot on a white ground, which is one of the few marks that retouching handles well. Spot the print, never the negative.

Prevention

Tap the tank on the bench immediately after filling, firmly, two or three times — and again at each agitation, as both makers’ procedures say.

Pour smoothly and quickly rather than slowly, which entrains less air than a hesitant pour, and fill to the maker’s stated volume rather than to the brim.

Use the wetting agent at the stated dilution and only in the final rinse. Kodak Limited’s 1944 formulary records that a wetting agent reduces the tendency to form air bells — it lowers the surface tension that holds a bubble against the gelatin — but ILFORD warn that foaming will occur if the solution is agitated excessively, and Kodak’s own note adds that with very alkaline developers any wetting agent “increases appreciably the tendency to froth”. More is not better.

Slide a print into the dish from one edge, so that the solution sweeps across the sheet rather than closing over it.