Volume 9, Number 1
May 1996

Effect of Metals on Paper: A Literature Review

Iron and copper have given manuscript curators a lot of grief over the centuries. Iron-gall ink has eaten its way through important documents, turning them into lacy souvenirs of what they used to be, and the blue and green copper pigments in manuscript illuminations have caused whole sections of pages to drop out. Research has pointed to one solution after another, but nothing has worked very well. Deacidification helped, but it did not stop the corrosion.

In recent years, however, conservation scientists and others may have found a way to arrest the destruction of paper and parchment by iron-gall ink. Johan G. Neevel concludes that the damage is caused by two processes--acid-catalyzed hydrolysis and metal-catalyzed oxidation--which under certain conditions can proceed independently even when the other process is arrested. (We usually see the effects of oxidation disappear when hydrolysis is discouraged by deacidification, and conversely, we see the effects of hydrolysis disappear if an artifact is stored in a dark, low-oxygen environment. This is because the two processes are normally symbiotic, so to speak, feeding off each other's degradation products.) (Iversen, 1989)

Currently, Neevel and his co-workers are working on a treatment for iron-gall ink corrosion. It may involve only one compound, such as magnesium phytate or sodium phytate, that appears to be able to inhibit both hydrolysis and oxidation.

The effect of copper on paper has also been investigated recently, but there has not been as much progress.

Many metals can affect the permanence of paper, or interfere with the paper manufacturing process. The following metals are often found in the wood from which the pulp is made: iron, manganese, copper, aluminum, calcium, and magnesium. Zinc, cobalt and barium are also found in wood sometimes.

In the mill, metals can come from the supply water, or build up as a result of corrosion. Metals can also be contaminants in additives; iron has been a contaminant in alum. Iron has even been investigated recently as a mordant for rosin soap size! (Jhuang, 1993)

In the pulp mill, metals (especially iron, manganese and copper) may interfere with hydrogen peroxide or ozone bleaching, or darken the pulp and paper.

It takes only a very small amount of iron or other metal to have an effect, because when it acts as a catalyst it is not used up. The effect, a darkening of the paper, can be localized (foxing spots), superficial (a result of flattening paper sheets by putting them through a roll press between two steel plates in hand paper mills) or in the printing or writing ink, artist's pigments, or elsewhere. Some metals are photosensitizers, especially Fe(III), Pb(II), Ag(II) and Cu(II).

However, not all metals are harmful. Zinc, magnesium, calcium and barium compounds have all been used for deacidification.

This review is based on materials in the Abbey Publications library and files, and includes citations from annotated bibliographies on the shelves. It is not exhaustive, but it may be useful because it is interdisciplinary (drawing upon the literature of conservation as well as papermaking), international, and up to date.

Banik, Gerhard and Johann Ponahlo, 1981. "Some Aspects Concerning Degradation Phenomena of Paper Caused by Green Copper Containing Pigments." Preprints, ICOM [International Council of Museums] Committee for Conseration, Graphic and Photographic Documents Working Group, Ottawa, 1981. Paper # 81/14/1, pp. 1-14. 15 refs.

Ten kinds of copper compounds were aged with dry heat and with light, and the degradation products analyzed with infrared spectroscopy. The compounds reacted differently to cellulose. General oxidation processes were not seen as responsible for damaging effects caused by natural aging.

Bicchiere, Marina, 1996. "Ferric and Cupric Ions: Interactions with Cellulose as a Function of pH." Paper presented at the International Conference on Conservation and Restoration of Archive and Library Materials, Erice, Italy, April 1996. Pp. 599-609 in the Preprints.

Two mechanisms have been proposed for the cellulose-metal interaction: a free radical mechanism in which the metal catalyzes homolytic scission of the cellulose peroxide, and a Lewis mechanism. In a low acid medium, iron ions catalyze the cleavage of the cellulose 1-4-b-glucosidic bond, whereas copper ions catalyze the oxidation on the anhydroglucose ring. Results are in agreement with the Lewis mechanism. In low-alkaline medium, both metallic ions seem to be active on the cleavage of the glucosidic bond. Reducing compounds are being investigated to find one that can impede oxidation catalyzed by metals and also bleach the paper.

Farber, Eduard, 1954. "Chemical Deterioration of Wood in the Presence of Iron," Industrial and Engineering Chemistry 46/9, p. 1968-1972. There is a definite reduction in mechanical strength, carbohydrate content and lignin solubility after wood has been exposed to moisture and air in contact with iron. (This was a study of old railroad ties.)

Harrison, James E., 1989. "Ferric Iron in Mill Water can Affect Paper Brightness," American Papermaker, Nov. 1989, p. 33. TAPPI water quality standards for turbidity, color, iron and manganese are summarized for fine paper, bleached kraft, unbleached kraft, groundwood kraft and soda and sulfate pulp in a little table, which by the way does not say what the units of measurement are. A graph from TAPPI February 1982 is reproduced here, showing how G.E. brightness falls when pulp contains more than 0.01% of iron.

Iversen, Tommy, 1989. "Oxidative Decomposition of the Polysaccharide Components of the Paper," in Ageing/ Degradation of Paper: A Literature Survey, p. 43-47. (FoU-projektet för papperskonservering, Report No. 1E) [Riksarkivet], Stockholm, Sept. 1989. ISSN 0284-5636. "During the hydrolytic degradation of cellulose, new low molecular products… are formed which can increase the paper's sensitivity to oxidation. This means that the oxidation breakdown of paper can have a greater importance the farther the hydrolytic ageing process has reached.… The autoxidation, however, also introduces acid carboxyl groups into the different components of the paper, which means that oxidation can accelerate the hydrolytic degradation of the cellulose."

Jhuang, Jenfeng and Christopher J. Biermann, 1993. "Rosin Soap Sizing with Ferric and Ferrous Ions as Mordants," Tappi Journal Dec. 1993, p. 141-147. The authors hypothesize that iron ions are effective mordants only in the sizing of lignin-rich unbleached pulps. For this use iron is much better than alum. Iron ions are recommended for sizing of dark unbleached or mechanical pulp papers [where presumably the darkening caused by the iron will not be noticed].

Kolar, Jana, and Gabrijela Novak, 1996. At the International Symposium on Book and Paper Conservation in Ljubljana, Slovenia, these authors will present their paper entitled "Investigation of the Effect of Various Metal Ions on Ageing Stability of Cellulose." Proceedings will be published. For information contact Ms. Jedert Vodopivec, Arhiv Republike Slovenije, fax (386 61) 216 551.

Lindström, Tom, 1989. "Effect of Metal Ions." Pp. 108-111 in Ageing/Degradation of Paper: A Literature Survey. (FoU-projektet för papperskonservering, Report No. 1E) This is a compact review of the ways in which metals can affect cellulose. Metal hydroxides/oxides catalyze cellulose hydrolysis, even in the absence of oxygen, especially at high temperatures. In an earlier chapter (p. 67-70) he points out the fact that Fe, Cu and Mn strongly accelerate the absorption of SO₂ and SO₃ in paper.

Neevel, J.G., 1995. "The Development of a New Conservation Treatment for Ink Corrosion, Based on the Natural Anti-oxidant Phytate." Paper presented at the September 1995 meeting of IADA in Tübingen, and printed on p. 95-100 of the Preprints (edited by Mogens S. Koch and K. Jonas Palm, published by the Royal Danish Academy of Fine Arts in Copenhagen; ISBN 87 89730-19-4). The author is at the Central Research Laboratory in Amsterdam.

During accelerated aging, deacidification failed to protect the paper against new acidity; therefore, acidity cannot be the only cause of ink corrosion. In simulated ink corrosion tests, lignin-free paper degraded faster than the lignin-containing paper, even when it was sized with acid alum. Lignin acts as an anti-oxidant because it is oxidized faster than cellulose and forms stable radicals; this shows that the process is oxidative. Iron II ions are seen as catalysts. Attempts to prevent oxidation by use of EDTA only accelerated the degradation process. Only a few complexing agents block the Fenton reaction (formation of hydroxyl radicals); one of them is phytic acid, found in seeds. Neevel's report is detailed and well-presented. The research was also reported in Restaurator, 1995, v.16, p. 143-160, with 45 references.

Shahani, Chandru J. and Frank H. Hengemihle. "Effect of Some Deacidification Agents on Copper-Catalyzed Degradation of Paper." Distributed as a separate by the Preservation Directorate, Library of Congress, November 1995 (Preservation Research and Testing Series No. 9501). 13 pp. Originally presented at the conference on Conservation of Historic and Artistic Works on Paper, organized by the Canadian Conservation Institute, Ottawa, Canada, Oct. 3-7, 1988, where it appeared in the program as "The Effect of Deacidification on the Aging of Paper Contaminated with Copper"; however, it did not appear in the postprints.

Aqueous solutions containing magnesium, calcium (in the Barrow two-step process, after CaOH) and zinc bicarbonate were used to deacidify copper-doped paper, in an effort to find the cause and control of corrosion by copper-based pigments. The work revealed the reason for the effectiveness of magnesium bicarbonate and the relative ineffectiveness of the Barrow two-step treatment in inhibiting the catalytic effect of copper on the oxidative degradation of paper. A simple qualitative test demonstrated that complexation of the adsorbed copper is indeed the key to its removal from the paper, but the complexation effect takes place with bicarbonate ions rather than with magnesium species. Unlike free copper ions, the soluble bicarbonate complex of copper has little affinity for sorption sites on the cellulose matrix; possibly some of the adsorbed copper can be washed out. But if the paper was treated first with calcium hydroxide, subsequent bicarbonate treatment was ineffective.

Shahani, C.J. and F.H. Hengemihle, 1986. "Influence of Copper and Iron on Permanence of Paper." Historic Textile and Paper Materials: Conservation and Characterization. (Advances in Chemistry Series No. 212; ACS, Aug. 27-29, 1984). p. 387-410. Neutralization of bleached kraft paper slowed the degradation rate, decreasing Fe-catalyzed degradation but increasing the efficiency of Cu-catalyzed degradation. The effect of chelation was also studied.

Williams, John C., C.S. Fowler, M.S. Lyon, and T.L. Merrill, 1977. "Metallic Catalysts in the Oxidative Degradation of Paper." Advances in Chemistry Series, ACS #164, p. 37-61. Even alkaline buffered paper can fail by oxidative degradation if it contains transition metal catalysts (compounds of copper, cobalt, or iron). Magnesium carbonate offers better protection against aging than calcium carbonate. Paper was treated with copper acetate at various pH levels and oven aged. Folding endurance declined more in the humid oven than in the dry oven, indicating that dry-oven aging has little predictive value for oxidate degradation.

Wilson, L.P., 1920. "Catalytic Action in the Oxidation of Cellulose." Journal of the Society of [the?] Chemical Industry, [Transactions, 39 #13, July 15, 1920]. [No page number.] In the viscose process, by which rayon is made, cellulose is gelatinized by oxidizing it in caustic soda, with the aid of catalysts, which are usually oxides or hydroxides of metal which exist in two states of oxidation, e.g., iron, nickel, cobalt, cerium, vanadium or manganese.