Here's a link to the whole article
http://www.gustavstickley.com/tooledcopperart/about.htm
and this is the basic jist of it.
The technique used to create his art is a form of copper repousee. Self taught, his designs are drawn by hand from the back of the copper with a pointed stylus. No molds or stamps are used. The metal is turned over and paint is laid within the resulting raised lines. The remaining copper surface is oxidized and then lacquered. The finished art is mounted on a sand colored board, matted and framed under glass in a quartered oak mission style frame. An accomplished woodworker, he also makes his own framing. He takes great pride in the fact that his art is true to the Arts and Crafts ideal being totally handcrafted by the hand of the artist.

Basically tooling copper.
Using various tools and textures to create a work of art from sheet copper. I remember doing this stuff when I was a kid...kinda fun actually.