JAIC 2002, Volume 41, Number 3, Article 6 (pp. 279 to 290)
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Journal of the American Institute for Conservation
JAIC 2002, Volume 41, Number 3, Article 6 (pp. 279 to 290)




Pulps are the sources of cellulose fibers that make up paper and paperboard. There are two primary sources for pulps: cotton and wood. Historically, cotton has been considered the more stable of the two pulps and, therefore, by default, the most appropriate for use in contact with photographic images. Unfortunately, papers and paperboards made from cotton pulps can be very expensive. The alternative, wood pulp, is much cheaper.

Historically, wood pulp developed a bad reputation as a result of negative experiences with papers made from nonpurified wood pulps. These woodpulp papers quickly became acidic, discolored, and brittle. The degradation was attributed to lignin, the primary noncellulose component of the wood, as well as to the acidic alum-rosin sizes used. Similarly, lignin-containing envelopes and boxes used to house photographs have been blamed for the degradation of enclosed images.

As a result of the concern over lignin in wood pulps, the pulp and paper industry developed alternative wood-pulping processes that removed most of the lignin. The use of these delignified pulps along with the introduction of alkaline size has led to wood-pulp papers of much greater stability. These purified papers have come to be accepted for many preservation applications, from permanent documents to envelopes and boxes for housing photographs and other archived items.

It has never been shown experimentally, however, that lignin is a cause of photo degradation (fading, mirroring, red spots, or staining). It is the purpose of this study to examine the effects of lignincontaining paper and paperboards on photo stability.

There are a large number of pulping processes. The simplest involves mechanically grinding logs to separate the fibers. The resulting product is referred to as groundwood. The pulping process can be speeded up by the application of heat, a process known as thermo-mechanical pulping. Both processes leave most of the lignin in the pulp. Pulping processes that remove or reduce the lignin use chemical reaction to dissolve the lignin. This process is referred to as digestion.

Lignins vary in composition among tree species, and even among individual trees, and are more correctly referred to in the plural. Lignins are complex polymers that are amorphous and threedimensional in structure. They function chiefly as a filler or cement substance to impart rigidity to wood tissue. In North American woods, lignins content is approximately 18–25% for hardwoods and 25–35% for softwoods (Biermann 1996). They are difficult to study because there is no known method for isolating them that does not alter them chemically and quantitatively. Lignin molecules are extremely large, having molecular weights in the thousands, and their chemical structures can take innumerable forms.

A concept important to the pulp and paper industry is the yield. Yield refers to the ratio of the weight of the wood going into a pulping process to the weight of the pulp produced. When lignins are removed, the yield is reduced and less paper can be produced. So, just as there is a price connection with pulp sources (cotton versus wood), there is a price connection to level of delignification. The more lignins that are removed, the smaller the yield and the greater the final paper cost. By-products from the chemical pulping processes are not considered waste. Other industrial products can be extracted from the pulping residuals, and whatever is left can be burned for production energy. However, these savings are not enough to offset the loss in pulp yield.

Various methods are used to reduce lignins content during pulping. Some lignins can be removed in an acid environment and some in an alkaline environment. Both approaches have been used, although alkaline pulping is currently the more common. Not every pulping process fully removes lignins. There is a continuum ranging from highlignin pulps such as groundwood down to fully purified pulps such as kraft. In between are the semipulps. These pulps are produced through various combinations of mechanical grinding, heat, and chemical actions by which the cellulose fibers are freed from wood and lignins content is reduced. The advantage is that the papers produced from semipulps are somewhat stronger and longer lasting than those produced from groundwood pulps but still less expensive than those produced from fully purified pulps.

Beyond the cellulose fibers and lignins, wood also contains small amounts of a variety of other compounds. These are referred to collectively as the extractives, because either organic solvents or water can extract them. Like lignins, the composition of extractives within wood varies from species to species and even within a given species, depending on growth conditions. The extractives can impart color, taste, and decay resistance to wood. In North American woods, the extractives content is approximately 1–5% for hardwoods and 3–8% for softwoods. There are also traces (less than 1%) of inorganic substances in wood (Biermann 1996), but they are not under consideration.

In the paper industry there has been a movement to establish papers made from semipulps as appropriate for permanent documents. Experiments examining paper performance characteristics, such as tensile strength and fold endurance, have suggested that pH is the prime indicator of paper permanence, not lignins content (Bégin et al. 1998). The suggestion is that lower-cost semipulps could be used to make permanent papers if an alkaline buffer is used. Some experimenters suggest that the presence of lignins may actually enhance paper permanence by acting as an antioxidant (Schmidt et al. 1995). While these are still questions to be debated, it seemed prudent at this time to determine if alkaline buffers in papers containing lignins reduce any potentially adverse effects on photographs.

Buffering has been included in papers for use in photographic enclosures to absorb any acid from the environment, the photograph, or the enclosure itself. It has been shown that acids can attack photographic film supports (Adelstein et al. 1995) and paper supports and that some acids can directly oxidize silver images (Carroll and Calhoun 1955). Unfortunately, in an effort to put those all-important words “acid-free” on their packages, some producers have been adding buffers to paperboards made with acidic groundwood pulps. The result is that pH tests may show a high alkaline reading, while the board may, in fact, contain acids or potential acid sources. It is worth noting that pH cannot be used as an indirect method of determining lignins content. Acid-free does not equate to lignin-free.

Finally, as manufacturers have striven to produce inert storage enclosures for use in direct contact with photographs, there has developed a question as to whether paper or paperboard products that do not come in direct contact with the photograph are required to be free of lignins as well. Many boxes, album covers, and slipcases are made with ligninscontaining cores covered by cloth on the exterior and paper liners on the interior. There is anecdotal evidence suggesting that lignins-containing boxes degrade photographic materials stored inside. However, no study has been completed to demonstrate the effect.

Given the above discussion, the specific questions associated with this problem and addressed by this study are the following:

  • What are the potential harmful effects of ligninscontaining paper and paperboard enclosures on photographs?
  • Are lignins really the cause of those effects?
  • Can alkaline buffering agents mitigate the effects of lignins on photographs?
  • Can lignins-containing paperboards adversely affect photographs across an air space in a box?

It should be noted that the results of this study apply to the storage of photographs only and not to the storage of other archived materials. This study also examines the effect of lignins on black-andwhite silver images only and not their glass, metal, paper, or plastic supports.

Copyright © 2002 American Institution for Conservation of Historic & Artistic Works