Works of art that express a sad/mournful tone might depict grieving characters and/or scenes of death or profound loss. Beyond representation, art elements such as cool/deathly colors, dark values, or minimal indication of form might contribute to a sad/mournful feeling.
An artist might impart a sad/mournful tone to a piece due to his or her own grief, or the artist might choose to evoke this tone as a reflection of the character(s) or scene being portrayed. A sad/mournful tone might also be adopted to suit a piece that will be shown in a setting that demands an expression of sorrow.
Featured: Death in the Sickroom by Edvard Munch