Bromide drag

Owned by Uneven Development, Bromide Drag and Surge Marks, which derives the whole family from one mechanism — developer consumed and bromide released fastest where density is highest, leaving a layer of loaded solution that is denser than the bulk and sinks — and which places drag at the too-little end of the agitation lever, opposite surge. That page also supplies the observation that identifies it: drag follows the tank rather than the photograph. This entry was assembled from diffusion, swelling and the journey into the emulsion and the agitation arm of the solvent series.

What you see

Streaks running downward from the dense steps or the dense areas of a negative — lighter than they should be, in the direction gravity acted while the film was in the bath.

On a step wedge strip it is unmistakable, because the dense steps are regular and so are the streaks beneath them. On a pictorial negative it shows below a bright sky or a white wall.

Likely causes

1. Reduced or absent agitation.
2. A long, dilute, nearly unagitated development. Here the effect is not an accident but the intended mechanism arriving as a fault — the same flows that produce compensation produce this.
3. Long standing periods between agitation cycles.

What is happening: the chemistry and physics

Development produces by-products, and a layer of spent developer loaded with them is denser than the bulk solution. Denser fluid sinks.

As it sinks it flows over whatever is below it, and it retards development there, because it is spent solution carrying the halide released by the area above. So the streak is a record of where the exhausted solution went, and it always points the way gravity was pointing.

The same physics is productive at a smaller scale. Let the depletion diffuse sideways across a single edge and you get the border and fringe effects that make a Mackie line and raise acutance. Let it happen across a whole frame and you get bromide drag. It is one mechanism at three scales — adjacency, drag and mottle — and which one you get is decided by agitation.

Diagnostic questions

• Do the streaks run downward from dense areas? That direction, relative to how the film hung in the tank, is the diagnosis.
• Is there a direction at all? No direction is mottle, which is the same lever at a different setting. A pitch that matches the perforations is surge marks, which is the lever at the other end.
• What was the agitation script, and what was the standing interval?
• Was the development deliberately long, dilute and unagitated? Then this is a known risk of the method rather than a mistake, and it belongs in the report as such.
• Are the streaks on the emulsion side and absent from the rebate? They must be, because the rebate had no dense neighbour above it to spend the developer.

Corrective action

None on the negative. The streak is a real density difference in the silver, not a surface deposit.

Prevention

Agitate to a published script, at a stated interval, and record it. Kodak’s small-tank procedure is 5 to 7 inversion cycles in 5 seconds repeated at 30-second intervals, and it exists to keep the boundary layer thin without organising a flow.

Where the method is deliberately low-agitation, expect drag and design for it — decide before the session whether the compensation is worth the streaking risk on that subject, and say so in the log.

Vary the direction of agitation rather than repeating one motion, so that a flow pattern is broken up rather than reinforced.