Oil & Acrylic:
Both of these art forms require only a frame. If painted on a canvas or paper, stretching them onto stretcher bars will make them “frame ready”. Glass, mat boards, paper backing on the backside of the frame – all not required. These forms of art need to breathe over time. Giclee images printed on canvas also fall into this category and parameters.
Water Colors/Guache/Pastels/Etchings/Sketches/Giclees' on Paper/
For these media, a frame and mat board(s) with glass is recommended. If a mat board is not desired, the use of plastic spacers will provide the air space necessary to keep the art piece from coming in contact with the glass. This will prevent it from adhering to the glass over time leading to degradation of the art. A mat board achieves the same purpose.