WHY DOES PAPER TURN YELLOW?
by Laura Stirton Aust
As paper ages it often becomes yellowed or grayed. A drawing may look dingy because it is no longer bright white. Sometimes this a result of exposure to light, as was discussed in the last newsletter. But, even a print which is carefully stored in the best conditions can appear a different color from when it was new.
Paper discoloration is generally caused by acid. The paper manufacturing process is often to blame. Since the mid 19th century the largest source of acid is the wood pulp used to mass produce paper. There are additional sources of acid as well. Chemicals are added to make paper look brighter or whiter. Sizings are applied to make paper less absorbent to water and inks. Other materials make paper more rigid. Sizings, brighteners and other additives can break down with time and produce acids which discolor paper. The technology to manufacture paper with fewer acid-producing chemicals has only been adopted by paper companies in the last twenty years. It is a more expensive process, therefore we pay a premium for "acid-free" paper.
The discoloration of paper is a result of damaged paper fibers. Bleaching to eliminate the yellowing is counterproductive because bleaching also damages paper fibers. Over time bleached paper is even more likely to become discolored.
We know acid breaks down paper fibers and causes discoloration, so why not spray something on the paper to neutralize the acid? In fact, there are such products on the market. They were developed for libraries with shelves full of crumbling books. They are used today for books and ephemera such as newspapers, old letters or documents, and objects prized more for content than physical appearance. It is called deacidification spray or solution. Deacidification adds magnesium or calcium to the paper to neutralize acids and protects the fibers from additional acid. Deacidification sometimes reduces discoloration but can not rebuild the paper structure. It is available at archival supply stores. But, before considering your prints, drawings and watercolors as candidates for deacidification remember this procedure should be carried out by a professional because there are risks associated with deacidification. Sizing may be destroyed by the alkaline chemicals and the paper may take on a pink or green cast. Most paper is left gritty or coarse and the surface texture may change. There may also be media color changes. Finally it is not known how long the benefits of deacidification will last.
In light of these possible negative consequence I choose only to use deacidification as a last resort for artwork. Candidates for this process include pieces which may not be bathed, are dangerously brittle and may be lost completely without some action. Drawings on very thick acidic cardboard are sometimes sprayed from behind in hopes that deacidification will slow down the acid penetrating the artwork.
If deacidification and bleaching are not the answer to a discolored work of art on paper, what is? The most conservative approach is re-housing the artwork in alkaline materials; buffered tissue and/or rag mat board. Also, limiting handling and light exposure may slowdown acid degradation. Regularly changing buffered tissue will help to insure there is always alkaline material available to neutralize acids. This may be as often as once a month during the first year of storage. Fortunately, buffered tissue is inexpensive and available from archival suppliers. These measures probably will not change the appearance of a discolored paper, but may prevent the paper from becoming darker.
Whenever possible, bathing is the most beneficial way to reduce the acid content of paper and improve its appearance. Conservators use distilled or deionized water solutions to bath paper. This water may be treated with chemicals which neutralize acids. Bathing usually removes some, if not most of the discoloration and may prevent discoloration in the future.
Acidic paper is a huge problem for archivist, museums and collectors of works of art on paper. Although we must accept that some papers simply will not last and some prints will never look as good as they did when they were new, we can prevent a certain amount of discoloration and staining by using only acid-free materials for framing and storage and avoiding excessive exposure to light.
Laura Stirton Aust is a paper conservator at ARTcare
