Monday, October 05, 2009

Silver and filamentary growth

The above is picture of a developed silver crystal that during reduction has transformed into a filamentary mass. This is quite common with most modern developers and film types, and is especially true of fine grain developers of the MQ and PQ type.
What I'm often asked is what causes the filamentary growth? why don't the grains always keep their original post exposure shape?
I'll try to explain.
When a silver halide grain is exposed to light electrons move within the crystal with extreme rapidity. Some of these electrons are 'trapped' in lower energy areas of the crystal such as defects or impurities, the trapped electron makes a still more efficient trap for other electrons and so on.
In this state the silver is said to be a 'latent image' and needs a developer to magnify it.
During development the charge that has been concentrated at the traps is highly negative and especially so at the tips which attract positive ions from the solution.
A needle like protuberance is formed which rapidly turns into a filament, such filaments have a large surface for their volume giving plenty of opportunity for developer ions to be adsorbed along its sides forming a flat ribbon like structure.
This results in a very strong negative charge at the tip of the filament which attracts silver ions, which when they come in contact with the tip are neutralised to give metallic silver and therefore even more rapid filamentary growth.

I hope that this explanation helps those who have asked how these filaments are formed.
© Photo Utopia 2009


Noons said...

Developers said to "dissolve the silver" will affect these filaments, won't they?

Photo–Smith said...

I think you're talking about developers that contain solvents? In which case no, not really. The levels of solvents commercial contain is quite minimal, you may find higher concentrations encourage a very small degree of 'plate back' making the grains appear smaller but in reality these developers are normally ones which give lower speed, which in turn means more exposure.
Modern MQ based developers have relatively small amounts of solvent, the image on this page was developed in such a developer, so filamentry growth is pretty much the normal these datys