What is Acid-Free Paper

By Catalogs Editorial Staff

How do you preserve books to keep them from fading?

What is acid-free paper? Acid-free paper is paper that has a neutral or basic pH 7 (or slightly greater). It addresses the problem of preserving documents for long periods.

The Origins of the Problem

Paper can turn yellow over time. Because paper is made from wood-based pulp, if it has not had its lignin removed then over time it turns yellow and deteriorates. Exposure to light and heat break the molecules in the acidic paper even faster. This problem was recognized in the 1930s when a librarian, William Barrow, published a report about the deterioration of paper in the libraries.

How Is Paper Made?

Paper comes from trees. Trees are a composite of cellulose fibers bonded with lignin, sugars and other organic compounds. Depending on tree species, about 40 to 50 percent of the tree consists of cellulose suitable for papermaking.

Since only the cellulose is needed, the first step in papermaking is a process called pulping which separates the lignin and other materials from the cellulose. This produces wood pulp. There are two primary methods of making this pulp: mechanical and chemical.

Mechanical pulping involves finely grinding or chopping the wood to separate the cellulose fibers from everything else. Mechanical pulping is very efficient, and can convert 90 percent or more of the wood into pulp. However, the resulting pulp contains most of the lignin it started with, causing the resultant paper to turn yellow or brown when exposed to the sun.

Chemical pulping uses chemicals, heat, and pressure to dissolve the lignin in the wood, freeing the cellulose fibers. In the process, the wood and chemicals are cooked in a digester to remove the sugars, about 90 to 95 percent of the lignin, and anything else you don’t want in the final product.

For paper with a high level of whiteness, the pulp must be bleached. Pulp produced chemically is generally easier to bleach due to its lower lignin content.~

Producing Acid-Free Paper

Wood pulp has natural acids; during production, acid-free paper may be treated with a mild base – usually calcium or magnesium bicarbonate – which neutralizes the acids and prevents the formation of additional acids.

Much of the commercially produced paper today is acid-free. This is not due to the neutralizers added, but is largely the result of a shift from china clay to chalk as the main filler material in the pulp. Chalk is cheaper. It also reacts with acids and requires the pulp to be chemically neutral or alkaline.

Another feature is sizing. Paper can absorb ink. Sizing is used on fibers in order to reduce their tendency to absorb liquids; so ink stays on the surface of the paper, instead of being absorbed. The sizing additives mixed with the pulp must also be acid-free.

So, What is Acid-Free Paper?

In principle, papers which contain no free acid have a pH value of 7.0 or greater. In practice, papermakers consider a paper having a pH value of 6.0 or greater to be acid-free. Acid-free paper is commonly used for fine art prints and limited-edition printing, as well as permanent records where contact with paper acidity could harm the documents.