Paper marbling

Endpapers of a 1735 book made in France
Wikipedia - "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. There are several methods for making marbled papers. A shallow tray is filled with water, and various kinds of ink or paint colors are carefully applied to the surface with an ink brush. Various additives or surfactant chemicals are used to help float the colors. ..."
Wikipedia
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Paper marbling from a book bound in England around 1830