What is the proper amount of time a plate needs to spend in the silver bath? Clearly, there's a lower limit to this time, when the reacting chemistry hasn't had enough time to form the light sensitive silver iodides and silver bromides that make the plate "photographic". But is there an upper limit? It turns out there is, but we also find that the story is much more than just "too much" or "too little".

The conversion of the iodides and bromides in the film to silver iodide and silver bromide doesn't occur instantaneously; it's a process that occurs over time as more and more of the original metal salts are converted to silver halides which gradually accumulate in the film. What's the effect of this continual accumulation?

When light strikes a sensitized plate, it triggers a response in the silver halides that are then transformed into silver metal during development. If the density of silver halides (number of silver salts per unit area of the film) is low, then there are fewer image forming particles available to receive the incident light, and therefore the "image density" of the developed image will be correspondingly low.

On the other hand, if there is a high density of silver halides on the plate, the same intensity of incident light now has a greater number of photographically sensitive sites to trigger which result in correspondingly greater image densities after development. The equation would look something like this:

  • Given a constant incident light intensity
  • plus More silver halides on the plate. (i.e. longer time in the silver bath)
  • equals More image forming silver metal on the plate after development.

This is similar to saying: The longer the silver soak time, the greater the number of silver halides produced in the film, which results in an increased effective film speed. (Notice that we're altering the effective film speed by adding more photographically sensitive sites to the film. We're not, however, changing the sensitivity of any of the individual silver halides that reside in the sensitized film. This is a subtle but important distinction).

But there are limits. Too little time in the silver bath produces low density images and superficial streaking on the surface of the image. Too much time in the silver bath produces excessive image density and overall fogging of the image.