Washing Films

After fixing, fixing salts (thiosulfate) are present in the gelatin layer of the film.

When this layer is brought into contact with fresh water, a concentration gradient is created between the layer and the water, which causes the salt to diffuse out of the layer into the water to create a concentration equilibrium. The greater the gradient (i.e., the more fixing salt is still in the film), the faster this diffusion occurs.

A freshly fixed film therefore loses half of its fixing salts when it comes into contact with water.

So the basic idea is to leave the film in the can, keep changing the water, and keep tilting it. Due to the fact that the salt comes out of the film slower and slower, it is necessary to tilt more often or longer with each pass.

Diffusion is also temperature dependent, so cold water should not be used. Ideally, water at 20 °C should also be used for washing (as for development), so that the film does not get a temperature shock.
Process

The whole process looks like this:

1. Add water to the can, tilt 3 times, empty out water
2. Add water to the can, tilt 6 times, empty out water
3. Put water into the can, tilt 12 times, empty water out
4. Put water into the can, tilt 24 times, empty water out

As this was mentioned in Ilford data sheets, the whole soaking method is now colloquially called the Ilford method (although it was not invented by Ilford).

After that, the film is archivally watered out. The water consumption is minimal (approx. 2 liters of water for a roll film spiral).