A METHOD FOR THE AQUEOUS DEACIDIFICATION OF OXIDIZED PAPER
JOHN BOGAARD, HANNAH R. MORRIS, & PAUL M. WHITMORE
APPENDIX 1: EXPERIMENTAL DETAILS
Whatman no. 42 filter paper, a pure cotton linter paper, was first photo-oxidized by exposure to ultra-violet-A lamps, specifically the Q-Panel UVA-351 bulbs. During exposure the samples were held 4 in. from the light source, producing approximately 6 mW/cm2 intensity in the 300–400 nm wavelength range. The paper studied in the final experiment was also Whatman no. 42 that had been exposed to UV-A lamps, then stored at room temperature for about seven years. Thermal aging was performed by hanging individual sheets in an oven at 90°C, 50% RH. After exposure or aging, samples were conditioned in a constant environment room at 23degC, 50% RH prior to testing.
Viscometric degrees of polymerization of single samples of each test sheet were determined by the standard method (ASTM 1962) in a solution of 0.5M cupriethylenediamine, following overnight (approximately 18-hour) treatment in unbuffered 0.01M sodium borohydride to stabilize the alkali-sensitive linkages. The concentration of chain scissions is derived from this DP by first converting to the number-average DP and then calculating the concentration of chains, which is expressed in the same units as the carbonyl and carboxyl analyses (mmol/100 g). The difference in the concentration of chains between unaged and aged samples gives the concentration of scissions that have occurred. For a more detailed explanation of these calculations, see Whitmore and Bogaard (1994). The precision of the viscosity measurements (reported by ASTM to be about 3%) is such that changes in scissions of more than 0.1 unit are considered significant.
Carbonyl contents were measured using the hydrazine method (Albertsson and Samuelson 1962). Reported values are the average of duplicate measurements made on two replicate samples taken from the same test sheet. This procedure serves to increase the precision of this technique to approximately plus or minus 10% of the value. Carboxyl contents were assayed using the standard methylene blue test (ASTM 1963). Reported values are results for single analyses, which are said by ASTM to have a precision of about 2-8%, with the higher values being slightly more precise than the lower. Neither the hydrazine nor methylene blue tests are able to detect oxidation on soluble fragments of the cellulose polymer.
Measurements of the cold extract pH were made using the standard method (TAPPI 1988) with the modification of using 0.1M sodium chloride, except when the ion contents were also being measured; in this case deionized water was used to soak the samples. Precision of these results is about 0.1 pH unit. Measurements of the calcium and chloride ion contents of the paper samples were made with ion-selective electrodes using the ratio of 0.5 g paper to 50 ml deionized water. Since the samples were run concurrently, the same ionic strength adjustor, a solution of sodium nitrate, was used for both. First the chloride contents of all the samples were measured. Then the paper/water slurry was acidified to pH 3.5 using drops of 0.1 N hydrochloric acid; this procedure took from 0.2 to 0.4 ml. The acidification ensured that the calcium ions were fully released from the paper fibers prior to their measurement, which followed.
Treatment solutions were prepared from degassed water that had been deionized in a reverse osmosis system. The solutions used in the final, multistep treatment were made with deionized water that was put through an ion-exchange column to remove trace minerals (primarily sodium chloride) that are retained after the reverse osmosis purification. The contents of calcium and chloride ions in the treatment solutions were measured with ion-selective electrodes, using a sodium nitrate ionic strength adjuster. Since a reducing solution can interfere with measurement by the chloride electrode, the reducing power of the sodium borohydride solution was quenched by dropwise addition of a solution of potassium permanganate prior to measuring its chloride content.
Reflectance spectra of single sheets backed with Millipore were measured on a MacBeth ColorEye 7000 spectrophotometer. Brightness (%R at 460 nm) is reported. The reflectance changes at this wavelength track the yellowing of the papers (decreased reflectance at blue wavelengths), and no other color changes besides yellowing were observed.
1. Calcium chloride may seem an unusual choice for a conservation treatment, and conservators may be understandably wary of introducing chlorine into their papers. However, chlorine is a common impurity found in most papers, especially those that have been bleached. Even ashless Whatman filter paper shows chlorine as the trace element found in the greatest abundance (Whatman Inc. 1983). It should be noted that there is a vast difference in reactivity between the different forms of chlorine. The chlorideion (Cl ) generally forms inert salts that are found in common foodstuffs, road treatments, etc. By contrast, the oxygen-containing chlorite (ClO2-) or hypochlorite (ClO) ions are highly corrosive and are used primarily for bleaching and disinfecting.Chloride salts of different metals, such as iron, copper, and magnesium, are all highly soluble in water, so if the chloride ion reacts with another trace metal in the paper during treatment, it should be easily washed away. The only exceptions are silver and lead—chloride will react to form sparingly soluble compounds that may be difficult to remove—so this salt solution is not recommended for any papers that may contain these metals in significant quantities (such as artworks with lead pigments).
This work was performed at the Art Conservation Research Center (formerly the Research Center on the Materials of the Artist and Conservator) at Carnegie Mellon University. The financial support of the Andrew W. Mellon Foundation is gratefully acknowledged. John Bogaard thanks Dr. Robert L. Feller, emeritus director of the Center, for helpful discussions of this paper. Parts of this paper were presented at the American Institute for Conservation's 29th Annual Meeting in Dallas in June 2001 and were summarized in the Book and Paper Group Annual 20.
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SOURCES OF MATERIALS
Cupriethylenediamine GFS Chemicals Inc. P.O. Box 245 Powell, Ohio 43065 (800) 858-9682 www.gfschemicals.com
Lab supplies; Whatman no. 42 filter paper (large sheets) Fisher Scientific Co. 585 Alpha Dr. Pittsburgh, Pa. 15238 (800) 766-7000 www.fishersci.com
Other chemicals Aldrich P.O. Box 2060 Milwaukee, Wis. 53201 (800) 771-6737 www.sigma-aldrich.com
Benchtop pH/ISE meter Thermo Orion 500 Cummings Center Beverly, Mass. 01915-6199 (978) 232-6000 www.thermo.com
Spectrophotometer (ColorEye Model 7000) Macbeth Division Kollmorgen Instruments Corp. 405 Little Britain Rd. New Windsor, N.Y. 12553
Temperature and humidity chamber Blue M Electric 2218 W. 138th St. Blue Island, Ill. 60406 (708) 385-9000
UV-A fluorescent lamps (UVA-351) Q-Panel Corp. 26200 First St. Cleveland, Ohio 44145
JOHN BOGAARD has a BS in chemistry from Carnegie Mellon University. He had worked at the Art Conservation Research Center (formerly the Research Center on the Materials of the Artist and Conservator) since 1978, where his primary area of research was paper chemistry. Address: Department of Horticulture, Penn State University, University Park, Pa. 16802
HANNAH R. MORRIS has a PhD in analytical chemistry from the University of Pittsburgh, where her research focused on materials characterization in complex polymer blends using spectroscopy and chemical imaging techniques. Since 2000 she has been deputy director of the Art Conservation Research Center (formerly the Research Center on the Materials of the Artist and Conservator). Address: Art Conservation Research Center, Carnegie Mellon University, 700 Technology Drive, Pittsburgh, Pa. 15219
PAUL M. WHITMORE has a PhD in physical chemistry from the University of California at Berkeley. Following an appointment at the Environmental Quality Laboratory at Caltech studying the effects of photochemical smog on works of art, he joined the staff at the Harvard University Art Museums. Since 1988 he has been director of the Art Conservation Research Center (formerly the Research Center on the Materials of the Artist and Conservator) at Carnegie Mellon University, where his research has been directed toward the study of the permanence of modern art and library materials. Address: As for Morris