You wrote in your article:
…, if the different grains had different makeup such as iodide content, then the grains could equilibrate during keeping in some fashion….
What do you mean by “equilibrate”, and how could that happen in a set gelatin?
And could you point out more detailed what the difference would be between blending several different emulsions into one and coating them onto each other?
In a set emulsion some grains are in contact with others before and after coating. There is also salt present. If a 10% iodide grain is adjacent to a 1% iodide grain, there can be a tiny exchange of ion in the moist emulsion which leads to a tiny change.
Coating emulsions in one vs two layers increases internal reflections and also the cost of coating.
More internal reflections reduce sharpness, a single thicker emulsion will have a higher number of internal reflections. If you use two different layers of emulsion, you can tailor them to contribute different qualities to the final emulsion, much like modern films. It's more complicated than just the gelatin being the same, there is the Ag content, Iodide concentration, etc. Gelatin is not a solid, if it can absorb chemicals (think developer, fixer) then the chemicals in it can move around as well. Since the older films have varied sizes of grains, chemical action will try to find an equilibrium of distribution (like a drop of ink in an aquarium of water). Newer films have grains sizes that are much more even, hence they are already close to equilibrium, and will change less over time.
Now, did I get that right?
Yes, you did. I just want to add that a single thicker emulsion will have more internal refelctions, but two layers often act as if they have a huge partially reflective mirror between them. So it is a tossup as to which works better. The fast component is usually coated first, then the medium, with the slow one on top if it is done. Usually, B&W films are done in single layers, and color films are done in mulitilayers.
Thanks PE. Fascinating. Encore! Encore!
Thanks, projectbluebird and PE for the clarification.
I may have been a bit too much into ultra-fine grain (Lippmann) emulsions - grain size < 30nm - which are completely transparent. Assuming two layers of the same type (same gelatin and same AgX concentration), you wouldn't see any internal reflections. One practical application might be coating a blue-green sensitive layer on top of a red sensitive layer. By the way thickness differences between the two layers wouldn't matter since the refractive index is the same.
Interestingly, a grain has to be absorbant to light at the wavelength needed, in order to have imaging take place, so the grain is not 100% transparent. In fact it cannot be. So, if you were to observe holographic emulsions at the wavelength being used, the emulsion must have some absorption, even if it is UV or IR.