The Large Glass

For me, one of the arguably most radical steps intellectually, in the evolution of answer to the question, "What Is Art?" was the series of pieces by Marcel Duchamp (1887 to 1968)  which he referred to as glasses.
They were of glass, images and ideas and process trapped between 2 panes of glass, deliberately discarding the idea of a traditional canvas or the traditional idea of a picture plane.

The Bride Stripped Bare By Her Bachelors Even

(or, The Large Glass)

This piece was started in 1915. The physical images were created by diverse techniques and processes. A portion of the glass was silvered, like a mirror, then the silvering scraped away to leave the resulting geometric figures in the right hand side of the lower pane.
Lead wire was used to create an "armature" which is the machine which has parts which are named, the glider, the chocolate ginder, the nine malic molds.
The figure in the top glass is referred to as "the bride".
Much of the shading in the pieces was created over a period of years, by the careful controlled accumulation of dust particles which were varnished over as the desired shade of opacity was achieved.
The dust accumulation process itself was documented in a stop action film by the artist Man Ray.
Duchamp himself wrote a guide for the piece which is a pseudo/dada/surreal technical work, a playful treatise, that is an amazing document in itself. Even the title, with it non sequiter ending reflects Duchamps fascination with language and the playfulness of nonsense and puns.
The work does not really lend itself to interpretation outside of Duchamps pseudo/satiric/dada intellectual treatise. It has been interpreted as a mockery of the criticism it inspires in the viewer who searches to find the key to understanding it. One interpreter poetically said:
  ”The Large Glass has been called a love machine, but it is actually a machine of suffering. Its upper and lower realms are separated from each other forever by a horizon designated as the ‘bride’s clothes.’ The bride is hanging, perhaps from a rope, in an isolated cage, or crucified. The bachelors remain below, left only with the possibility of churning, agonized masturbation.”

One very striking feature of the piece is the network or cracks running through the glass. They occurred in 1924, when the piece broke being moved for exhibition.
After repairing it, Duchamp said he believed the cracks enhanced the piece. The chance accident enhanced the piece, following the flow of energy he had created.
The piece today is in the Philadelphia Museum of Art. It is in a room in front of a window and it is my favorite reason to visit Philadelphia.