The framing industry has improved the methods and materials used these past few decades and it’s more evident over time as older works come in to the shop for reframing.
At one point, it was thought that corrugated cardboard (D) would suffice as a backing board for framed art. As you can see in the photo, the previous framer still had a concern with the texture of the board against the art and elected to place a barrier paper between the art and the corrugated cardboard. That idea came from a common practice of regular mats being lined with an acid free barrier (C) to prevent contact with the art and cause any “burning”. What wasn’t common knowledge then was that the bevel cut of the mat allowed the acidity from the degrading core to burn the art as seen in the darker border around the image (A).
Acid burn from materials that aren’t pH neutral generally occur from lignin, essentially the “fiber” in plants. Lignin are not acids, but do contain certain carboxylic acids. As the wood pulp degrades in lesser quality mats, it releases these acids. You can see this where crop mark like lines that extend past the burned area of the art on all four corners (A). That is a result of the acid leaching through the overcuts in the corners of the mat. More evidence of acid burn is in the discoloration of the back of the mat (B) from the corrugated backing board. You can actually see that the art and tape hinges also acted as a barrier, preventing the leeching of acid onto the back of the mat (C).
There is debate on mats that contain lignin as they do contain a small amount (up to 7.5% is allowed) but are bleached and treated before being turned into a board. These boards tend to be marketed as “Acid Free”, but most purists still believe there is still a chance of further digression and outgassing of acids despite the bleaching of wood pulps and additives such as calcium carbonate which supposedly nullifies the damaging effects of lignin.
The best way to prevent this kind of damage is to use museum board as a mounting board and mat. Made of 100% cotton rag, the board comes in different plies without wood based face or backing papers. We highly recommend the use of museum quality boards for precious, one of a kind works that are irreplaceable or are of immense value.
MH Fine Art Framing will inspect your work and discuss all available archival options with you.