Untitled by Robert Irwin - 1969 - Acrylic lacquer on cast acrylic disc

What is material and what is immaterial?

Robert Irwin and a group of California artists aimed to remove the boundaries between painting, sculpture and architecture, and made use of new materials like plastics, light, space, and color to do it.

http://www.sculpture.org/documents/scmag98/irwin/sm-irwin.shtml