Whenever I specify framing for a customer I steer them to the rag mats. They are my preference for two reasons: they are the best product to protect your artwork and aesthetically I find them more attractive. I will start with a general description of the available types of matting.
Rag Mats (or Museum Board) are made from 100% cotton- thus their name. They are completely acid (PH) and lignin free. Lignin is a substance found in cellulose that can promote acidity. The color in Rag Mat is consistent through the whole sheet, meaning that the top will be the same color as the core which is exposed on the bevel cut. They come in a limited range of colors, the majority of which are some shade of white. Aesthetically I find this more attractive than other products.
Acid Free Mats or Conservation Mats are composed of cellulose (wood pulp) that has been deacidified or a mixture of cellulose and cotton. They have a lignin level below 1%. Over the long term this level can rise changing the acidity or PH level of the paper. Acid Free mats can be either consistent color throughout or white core with a colored paper on top.
Paper Mats are made from 100% cellulose and are the least expensive matting option. They come in a huge range of colors – which is an applied piece of paper over the top of the board. Over time the board will begin to yellow and become brittle because of the acid in the cellulose. This is most clearly seen in the discoloration of the bevel cut (figure 2). The decorative colored paper applied to the top will also fade dramatically.
In figure 3 the original color of the top mat can be seen at the outside edge of the matting where it was covered by the frame, thus not exposed to sunlight. UV protective glass is another post though! (figure 2 shows how a painting which fell off its mount shifted to expose the original color of the paper- bright rust.)
The framing industry has learned a lot about conservation framing in the last 50 years or so. We know that using cellulose based mats will cause damage to artwork. We have also learned that wood (or cardboard) backing is inappropriate. In figures 1 & 4 the effects of both are seen. Wherever a break in the wood backing was, air was drawn through and caused severe discoloration of the paper. It also becomes quite brittle. If you have ever held an old newspaper you are familiar with the effect. The paper cracks and easily crumbles. Figure 5 shows how an acidic mat discolored the image from the front. There is a line where the mat meets the photo.
In conclusion, while you may be saving money in the short run, non-conservation materials can cause long term damage. Some people are seduced by the array of colors available in non-rag mats but these will fade in 10-15 years. The vibrant colors are fleeting. The plainer and less obtrusive conservation style framing is an elegant, timeless but acquired taste.