The method of using the glass plate that Jim describes above is limited in some cases such as our lab bench scale emulsions. A plate coating for a 4x5 takes about 6 ml of emulsion and over a 60 minute finish will therefore require about 36 ml of emulsion from a 200 ml batch as one example. That is a lot to use before you even start. To add to this, often we are making slow speed emulsions that cannot be easily printed by projection. Some print film and paper emulsions could not be tested this way due to speed and volume being made.
Just some thoughts. It is useful but there is a better way.