This is a cast taken from the original death mask of John Wesley. A death mask is a likeness of a person's face after their death, usually produced by taking an impression from the deceased person's body.
Death masks are often highly realistic. Before the days of photography, death masks served sculptors as models for creating busts and statues of the deceased individual. By the time John Wesley died, such masks were beginning to be valued and collected in their own right, especially if the person was of high social standing or well known.
An engraving of John Wesley on his death bed is displayed behind the mask.