During the picture framing process, matboard, also referred to as a mat, also serves as additional protection, separating the art from the glass.
This article describes various types of matboards...
A number of manufacturers make matboard for the framing industry. It is available in hundreds of colors and finishes in several qualities. The standard size is 32" x 40" but 40" x 60" is available in many colors. There are other sizes such as 48" x 72". Rag is available 4' x 8' and larger from conservation suppliers.
Standard or Regular Acid-Free Matboard
This is standard 14-play matboard that has been buffered, which means the center and the backing paper have been treated with calcium or magnesium carbonate to neutralize the pH level, with enough left over to provide an alkaline reserve, to help protect against future acid problems. But, because buffering delays but does not prevent deterioration, this board is not suitable for conservation framing and is intended for use of decorative arts only. These boards have an off-white core that will eventually darken. Perhaps these boards were standard in 1950 but not today. This matboard should exclusively be used for decorative framing.
White Core Matboard
Several manufacturers make matboards with colored cores ranging from subtle to bright to black. Check manufacturer information to determine the quality of these boards. Check for pH, buffering, and lignin content. Some, like Crescent BrightCores, are suitable for conservation framing.
These boards have a rag or purified wood pulp core, with acid-free surface and backing papers adhered to the core. The surface papers are available in many colors, designed to be highly fade and bleed resistant. For conservation framing this board is nearly as good as 100% cotton rag board. The boards made from purified wood pulp are sometimes called alpha cellulose boards. These boards made with a cotton core are generally called rag.
Conservation Board with Molecular Traps
Some rag and conservation boards are being manufactured with additives call "zeolites" which are "cage molecules" designed to attract, trap, and neutralize a range of pollutants beyond those managed by calcium and magnesium carbonate buffers. Lab tests have shown promising results in battling certain environmental hazards but some conservators are concerned about the long-range effects of active ingredients like zeolites within the frame package.
This "top-of-the-line" board is made of 100% cotton fibers. It is often called rag board. Cotton is naturally pH neutral but rag boards are usually buffered for added longevity. Museum boards are made from both short and long fibers. The short fibers cut easily and the long fibers require little processing, cooking, bleaching, beating, and washing so they remain strong. Rag boards are by nature more durable than boards made from other materials. Solid colours (in which the entire board is died one colour) are available in only a limited selection because the colouring agents must be pure, acid-free, fade resistant and able to colour cotton evenly. This is the proper board for highest level conservation framing.
Unbuffered Museum Board
Manufactured the same as other cotton rag boards except the buffering is not added. Since cotton is naturally neutral it is not necessary. Many items require a non-alkaline environments, such as some photographs.