If the plate is allowed to dry before exposure there is a severe loss in emulsion speed. Therefore plates must be prepared just before use. Then exposed and developed immediately. Once the collodion dries it is no longer permeable to water. Wet plate photography required that you have a portable darkroom unless the photos were made in a studio. Matthew Brady who made many American civil war photographs used a wagon. There are photos of his setup. Once the dry plates were perfected the number of photographers using wet plates fell off dramatically.
Wet plates are only sensitive to near UV to blue wavelengths since they do not incorporate any sensitization dyes. If you look at wet plate prints of women their dresses are always rendered as completely black unless by chance they are wearing blue.
