The Myth of "Acid Free"
By Jim Miller, MCPF, GCF

"Acid free" is a common term in the picture-framing industry, but it is often misunderstood and misused. Many consumers - and some framers - believe that mat and mount boards described as "acid free" are the best available. The logical assumption is that "acid free" boards are entirely free of acid; but that's not entirely true.

First, it's important to know about the pH scale. Here's a good description given by the U.S. Environmental Protection Agency (www.epa.gov/airmarkets/acidrain/ph.html):

"The pH scale measures how acidic or basic a substance is. It ranges from 0 to 14. A pH of 7 is neutral. A pH of less than 7 is acidic, and a pH greater than 7 is basic. Each whole pH value below 7 is ten times more acidic than the next higher value. For example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times (10 times 10) more acidic than a pH of 6. The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value. For example, a pH of 10 is ten times more alkaline than a pH of 9. Pure water is neutral, with a pH of 7.0."

As you can see, a slight difference in pH value represents a significant difference in acidity or alkalinity.

The wood pulp from which our "acid free" framing boards are made contains lignin, an acidic, organic substance that bonds the wood fibers together. Some lignin is extracted during most automated paper-making processes. But some is also left behind, which would cause discoloration and embrittlement of the final product. In order to counteract the lignin's harm, a high-alkaline buffer is added to the paper pulp. When the "acid free" buffered framing board is new, it's on the alkaline side of the scale, with a pH value of 8 or higher. But as the board ages and the lignin uses up the buffer, the pH value eventually diminishes - past the neutral value of ph 7, and continues further to the acidic side of the pH scale.

The buffer commonly used in framing boards is calcium carbonate, which is also a main ingredient in antacids like Tums and Rolaids. Because calcium carbonate is high alkaline, it counteracts acid in our stomachs; and it works similarly in our boards, where the acid comes from lignin. The buffer added to framing boards is proven to be a good defense against acid deterioration for a decade or more, but it eventually becomes depleted. As the buffer wears out, the acid from lignin (which was in the paper all along), may cause the board to discolor and become brittle.

"Acid free" is probably not the best term to describe these boards. A better term would be "temporarily acid-neutralized" or "acid-delayed." And these are not the best boards to use for framing.

Our best boards are made from alpha-cellulose: highly purified pulp of wood or cotton, which contains no lignin. Most of these boards are also buffered, to neutralize acids that might migrate from other sources, such as the frame's raw wood rabbet. But some alpha-cellulose boards are also available without the buffer. When organic items are being framed (sheepskin, leather or seashells, for example), an unbuffered pH-neutral board usually is preferred, because some organic items react unfavorably to high-alkaline environments.

When we want the best framing boards, "acid free" isn't good enough. Instead, we should specify "lignin free."

Jim Miller, MCPF, GCF