Wood pulp (or paper mat) is naturally acidic but can be buffered with calcium carbonate to temporarily shift the ph balance to alkaline. Some companies will market this as acid free but it will return to being acidic and start to harm the image in about 5 years. Paper mats are by far the least expensive and are considered by the manufactures to only be suitable for “temporary framing”.
Alpha cellulose and Cotton are more naturally inert (acid free) and may or may not be buffered which allows the mat board to absorb harmful acidic components from within the frame environment such as a wood frame which is very acidic. In general you do want the board to be buffered since it will help absorb acidic compounds being admitted from a wood frame and from the wall and reduce the time it takes the mat board to become contaminated.
In addition to looking for acid free, you also want to look for lignin free which is nearly as critical for achival. Paper mats typically aren’t but alpha cellulose and cotton usually are.
Although the vast majority of framing mat and mount boards are buffered a small selection of types and colors are available “unbuffered”. This is for a handful of much less common acid photographic printing processes such as Cyano and dye transfer which can react to the buffering.
Mat board composed of cotton is typically refereed to as “Rag” board and is generally composed of separate “ply’s” of cotton board dyed for color and layered to together in 4, 6, or 8 ply’s. 4-ply is generally about 1/16 of inch thick and 8-ply is about 1/8 of an inch thick. Paper and alpha cellulose boards are not separate ply’s but will often be referred to as 4-ply to give a relative indication of their approximate thickness.
Another term thrown around a lot is museum quality mat board. The highest quality cotton rag boards will often have much smaller color selection since they are manufactured with dye’s that are less likely to introduce harmful components into the frame package. All of the ply’s are dyed with this color so the mat board will have a consistent color all the way through. All Bainbridge rag board is like this, but Crescent has two levels of rag board. The standard rag board which is more equivalent to the Bainbridge alphamat, and then they also have their Ragmat Museum which is manufactured like this.
Most other mat board consists of a core made of cotton, alpha cellulose or paper that is typically cream colored or white and then laminated with the mat color. Most manufactures will offer a few other choices of core board color such as black. The color of the core will be visible in the bevel cut of the mat surrounding the image.
Another excellent protection is a coating of zeolite molecular traps which allow the mat board to absorb harmful environmental pollutants. Some types of prints are believed to be more susceptible to fading from environmental pollutants than they are from UV light. Bainbridge is the only company manufacturing mat board with this technology and they market it as treated with “Artcare”. Due to patent restrictions no other company can use this technology. In a few years this patent will run out and you will see it picked up by other companies. Artcare process also helps increase the time it takes for the mat board to become contaminated.
Bainbridge and Crescent are two of the largest and most common mat board manufacturers. Most custom frame shops around the country will offer a selection of mat boards from both of these companies. Most companies have many different levels of products from the 100% cotton rag board that is purified, colored with archival dye, buffered and treated with zeolites, all the way down to laminated paper mat board for decorative use. At the lowest level of paper mat board there will be differences in the color of the core (cream vs white) and the thickness. The least expensive mat boards being a just a little thinner (less material) with no option for white core or black core.
In short, you can look for buffered, acid free, lignin free, Artcare process, thickness and core color.