Foam core was originally designed for the silk screen industry. It has been "adopted" by the framing industry because it is such an excellent back--sturdy, stiff(resists warping) and lightweight.

Standard foam core is a plastic foam like center sandwiched between paper on top and bottom. Standard foam core has a clay-coated surface on the paper that was orginally designed to make a better surface for printing. This "clay coat" provides a very smooth, uniform surface.

Archival foam core was designed for the needs of the framing industry. It uses archival paper on the top and bottom and a more archival foam in the center.

Almost all mats and backs have been buffered--calcium carbonate (the main ingredient in many antacids for your stomach) has been added to the board. This ingredient retards the accumulation of acid. Paper based mat boards have calcium carbonate added in order to retard the natural acidity of paper.

Most cotton rag mat boards also have calcium carbonate added even though they are naturally "acid free". The calcium carbonate combats environmental pollutants that can increase acidity.