Wet mounting is one of the oldest known methods of adhering artwork to a backing board. This method is normally used for paper or fabric artwork.
This process is almost like applying wallpaper. A paste substance is rolled onto the chosen backing board and then the back of the artwork is lightly dampened, then the artwork is placed onto the backing board that has the paste on it and pressure is applied. Normally, you would just apply a weight to them and let them dry, but they can also be placed into a vacuum press machine until all moisture is drawn out.
This type of mounting is not recommended for high end artwork or valuables. It is a very strong irreversible bond that cannot be undone without causing severe damage to the artwork.