[2][3] Mercer, however, never practiced law;[2] he was admitted to the Philadelphia County Bar on November 9, 1881, but departed for Europe the same month.

[5] Leaving his position with the Museum in the late 1890s, Mercer devoted himself to finding old American artifacts and learning about German pottery.

Mercer is well known for his research and books about ancient tool making, his ceramic tile creations, and his engineering and architecture.

He wrote extensively on his interests, which included archaeology, early tool making, German stove plates, and ceramics.

The series of four hundred mosaics trace the history of the Commonwealth of Pennsylvania from prehistoric times, and is the largest single collection of Mercer's tiles.

Other collections of tiles by Mercer can be found at Kykuit, the Rockefeller estate in Pocantico Hills, New York; Grauman's Chinese Theatre in Hollywood, California; the Casino at Monte Carlo in Monaco; the St. Louis Public Library; and the former Morton (Richmond) High School Building in Richmond, Indiana.