Paper marbling

Paper marbling is a method of which can make patterns. The patterns are the result of paint floated on water. The colouring is transferred to an absorbent surface, such as paper or fabric.

Endpaper from a book published in Scotland in 1842. Encyclopædia Britannica, 7th edition

It is often used as a writing surface for calligraphy, and especially book covers and endpapers in bookbinding and stationery.[1]

It is rarely used today, but was common in the 19th century. The reason for its lack of use today may be that it adds to the expense of the finished book.