Quote Originally Posted by hrst View Post
Hmm, why mixing two or three emulsions and coating in one layer won't work as well? There must be some reason to do multilayer coatings.
You can do it either way.

If done in one layer, then the exchange of ingredients must not take place. If it does take place then more than one layer should be used to minimize exchange.

Imagine a fast 10% iodide emulsion mixed with a slow 1% iodide emulsion. They will exchange iodide resulting in a "mess".