Wet Mounting
Wet mounting is a permanent mounting process that uses liquid glue as the

The glue is applied to a backing board; then the artwork is placed on top of
the glue surface. The piece is placed into a vacuum press, with the heat
turned off, to completely flatten the art. Since liquid glues are water based,
there is a chance of the backing board warping as the piece dries. To
counteract this effect, a second piece of kraft paper is mounted onto the back
of the board.

Since the process is permanent, you should not mount any original, limited
edition or collectible art that you would ever need to remove.