Wetting agent

Description

A few drops in the final rinse and water sheets off the film instead of beading. The cheapest way to avoid drying marks on a negative.

What it does

It lowers the surface tension so water cannot form droplets. A drying droplet leaves a ring of dissolved salts on the emulsion — those rings are the marks you see on a scan and then have to retouch. A wetting agent removes the cause rather than the symptom.

How to use it

A few drops in a tank of distilled water as the last rinse, then hang the film without wiping it. Too much is worse than too little: an excess leaves streaks of its own, so the dose is drops, not a capful.

Questions and answers

Could washing-up liquid do instead?

Many people do it, but household detergents carry fragrances and salts that stay on the film. The saving is not worth a ruined negative.