The last piece in the art package, it should be some type of stiff material to provide support. It is also what protects the back of the framed art package. Materials used for backing board include cardboard, foam board (also called foam or fome core), and gator board, and are available in a variety of thickness, from 1/8th inch up to 1/4 inch.
It is common to mount the artwork directly to the backing board. This is acceptable framing technique in most instances (see BACKING MAT). It is not acceptable to forgo a backing board when a backing mat is used. Framed canvases usually don’t have a backing board since they don’t require support.
Westfall Framing sells 3/16th inch Foam Core, both regular and acid-free. The regular is cheaper, and has a clay coated acid-free backing on both surfaces. The acid-free is just that, but more expensive. When using a backing mat, it is in most instances acceptable to use regular foam core. Artwork can be mounted directly to acid-free foam core with all confidence regarding acidity. Mounting directly to regular foam core? It is frequently done. We have no comment regarding its long-term safety.