American Breweriana Journal, Museum Commemorative Edition
The wood-pulp beer coaster (or mat) was patented by Robert Sputh of Dresden in 1892. It became a popular advertising medium in the early twentieth century and after World War II a common collectible.
Nelson, Max. (2008). The History of the Wood-Pulp Beer Coaster. American Breweriana Journal, Museum Commemorative Edition, 82-83.