Until the digital age, photography was mainly based on silver emulsions. Here’s how it works.
First, the emulsion is usually a silver halide, like silver chloride, or silver iodide, but most low solubility silver salts1 work. This silver salt is in the form of crystals. The grain size of the crystals determines the ASA index (like ASA400) or speed of the film.
When a photon of light strikes a molecule of silver salt in the emulsion, it knocks free an electron. This free electron can then reduce another silver atom.
A single silver atom is not stable for very long though, and will recombine with another anion in a short while. However, if enough photons strike enough silver salt molecules, more silver atoms will be reduced. If there are at least 8 silver atoms nearby, they will combine to form a stable silver seed particle. 8 silver atoms become stable because they can share their outer electrons making each atom see a full outer electron shell. (Some say it only takes 4 silver atoms to be stable because silver can also share 3 electrons, so 2 atoms sharing 3 electrons together with 2 atoms sharing one would make up the required 8 electrons for stability. It doesn’t matter to this discussion whether 4 or 8 are required. )
So the stronger the light impinging on the emulsion, the more silver seeds are produced within each crystal of silver salt.
At that point, the emulsion contains various amounts of silver seeds where exposed to light, and none where no light struck.
Now comes the fun part, development.
Silver metal is a catalyst for many chemical reactions. A catalyst is a substance that aids other chemical reactions, enabling them or speeding them up without being changed or consumed in the process. When a weak reducing agent is introduced to the silver salts in the emulsion, the silver salt crystals which have been exposed to light, and contain silver seeds will reduce to pure silver metal much more quickly than silver salt crystals which do not contain silver metal seeds. So the development must be timed correctly as eventually all the silver salts would be reduced destroying the picture.
Once the development is complete, the reducing agent is destroyed by the acid ‘stop bath’, and then the remaining silver salts are washed away with the ‘fixer’ leaving only the silver metal particles in the film.
Now, applying that knowledge about photography to making colloidal silver, what happens?
If the ionic silver solution (silver salt) is exposed to strong light, it will create silver seed crystals. These seeds will catalyze the reduction reaction in their vicinity causing the ionic silver in their vicinity to quickly reduce and combine with the seed. This makes a larger particle. The more seed crystals, the more larger particles will be produced. This is not what we want. We want all the silver to reduce uniformly and fairly slowly. Its also the reason why commercial ionic silver is sold in brown glass bottles.
Silver is most sensitive to ultraviolet and blue light. That’s why darkroom lights are red. Therefore, its best to keep ionic silver out of strong light, especially sunlight and fluorescent lights because they both have strong ultraviolet content.