Art of the Marbler

Paper marbling is a method of aqueous surface design, which can produce patterns similar to smooth marble or other kinds of stone. The patterns are the result of color floated on either plain water or a viscous solution known as size, and then carefully transferred to an absorbent surface, such as paper or fabric. Through several centuries, people have applied marbled materials to a variety of surfaces. It is often employed as a writing surface for calligraphy, and especially book covers and endpapers in bookbinding and stationery. Part of its appeal is that each print is a unique monotype.
— via wikipedia