EFFECTS OF AQUEOUS LIGHT BLEACHING ON THE SUBSEQUENT AGING OF PAPER
TERRY TROSPER SCHAEFFER, MARY T. BAKER, VICTORIA BLYTH-HILL, & DIANNE VAN DER REYDEN
1.1 PAPER PREPARATION
A 14½ × 21 in sheet of paper was misted with deionized water and submerged in a bath of 21 liters of Ca(OH)2 solution, initial pH 9.5, prepared by dropwise addition of a filtered, saturated Ca(OH)2 solution to deionized water until the desired pH was reached. The reverse osmosis water purification system at CAL consistently provides water of resistivity ≥ 18 Mohm-cm. After 4 hours, the paper was removed from the bath on polyester webbing, blotted between blotters, and then placed between blotters in a felt and Plexiglas press under ca. 1 psi weight. The blotters were changed after 5, 20, 40, and 60 minutes. A final blotter change was made the next day. Two sheets of each paper were so washed. The pH of the bath at the end of the washing procedure was 6.5 for the W56 paper and 7.3 for the W1. Each sheet of paper remained in the press 2 weeks, when it was removed, covered with polyester webbing, and allowed to equilibrate in the laboratory atmosphere for 1 day before being cut into 4 × 4 ½ in specimens with the machine direction in the short dimension. Cotton threads were sewn into two corners about ¼ in from the edges, so that each paper could be suspended vertically, with the machine direction horizontal.
Papers were suspended vertically in 600 ml polystyrene flat-sided culture bottles as described previously (van der Reyden et al. 1988), oriented so that the felt side of the paper would face the light source in the Weather-ometer. To each flask that contained a paper to be immersed during Weather-ometer incubation was added 580ml of a stock pH 9.5 Ca(OH)2 solution prepared as above. Enough stock solution was prepared immediately prior to loading the papers to be adequate for all immersed papers in an experiment.
2 AQUEOUS LIGHT-BLEACHING AND CONTROL TREATMENTS
Irradiance in the Weather-ometer chamber was maintained constant at 0.35 w/m2. Quartz tubes were used for the water jacket, which serves as a cooling filter for the xenon arc lamp. The walls of the polystyrene culture bottles filtered out most of the ultraviolet radiation (van der Reyden et al. 1988). The lamp power and the temperature in the Weather-ometer chamber were monitored throughout the experiment. The lamp power fluctuated between 7.1 and 7.4 kW during operation.
To prevent fogging by water condensation on the bottle surfaces, the Weather-ometer was operated without humidity in the chamber. Because the instrument has no refrigeration unit, it was not possible to keep the chamber temperature at room temperature while the xenon arc lamp was on. Dry bulb temperature in the chamber rose to 39 ± 1°C within a few minutes of starting the lamp and stayed in that range throughout the experiment. Immediately upon removal from the chamber, the temperature of the solutions in the bottles wrapped in foil were about 1°C above the temperature of the chamber. The temperature of the solutions in bottles exposed to light was 47–48°C. The temperature inside the “dry” control bottles could not be measured accurately. It is assumed to have been approximately the same as the chamber temperature.
The pH of the immersion solutions was measured when the solutions had cooled to approximately room temperature. As expected, the pH of all immersion solutions fell during incubations in the Weather-ometer. One W1 immersion solution and two W56 immersion solutions were assayed by a modified Lowry procedure to determine if protein had been solubilized from the W56 paper.
The dry condition controls were placed in archival polyester sleeves immediately upon removal from the Weather-ometer and stored in a binder. As each immersed paper was removed from its bottle, it was placed on a thick blotter and turned over after about 1 minute. These papers were then placed individually between 7 × 9 in blotters in a Plexiglas press and weighted under less than 1 psi. The blotters were changed twice at 10 minute intervals. The wet papers were then placed between blotters and felts in a larger Plexiglas sandwich and more heavily weighted overnight (less than 1 psi). The blotters were changed once the next day, and these papers remained in the press for 2 weeks, after which the dry papers were transferred to polyester webbing, covered with a fresh blotter, and allowed to equilibrate with the laboratory atmosphere for 1 day.
3 ARTIFICIAL AGING
All papers that had been in the Weather-ometer were cut in half along the machine direction, with a fresh scalpel blade, to give two pieces 4 × 2 ¼ in. The half of each paper that was to be artificially aged was sewn with cotton thread into a Plexiglas frame. The halves were positioned with the machine direction vertical and all four corners anchored, so that no papers were touching. The papers could not rotate or swing in the frames to come into contact with each other; however, they did vibrate in the oven draft.
At the end of the aging period, the papers were equilibrated overnight, still in the Plexiglas frame, in the dark at room temperature. After being cut out of the frames, they were stored in polyester film enclosures.
4 ANALYTICAL PROCEDURES.
The following tests were applied:
- Spot tests. The spot test for aluminum and the ninhydrin spot test for protein size were performed according to Browning (1977) on small pieces of selected specimens of W56 paper. Papers known to contain these sizes and W1 filter paper were used as positive and negative controls, respectively.
- SEM/EDX. Thin strips of papers to be examined by SEM/EDX were mounted on aluminum stubs and coated with carbon. The JEOL JXA-840A scanning electron microscope with a Tracor Northern TN5502 energy-dispersive x-ray analysis accessory was used to obtain representative micrographs of fibers and for elemental analysis. Dot map representations of Al distributions were recorded where appropriate.
- FTIR. Both reflectance and transmittance infrared spectra of some papers were recorded using a Mattson Cygnus 100 Fourier Transform Infrared Spectrometer with a Spectratech IR-Plan microscope accessory. These spectra were compared with those of hide glue and of cellulose standards.
- Protein measurement. Protein content of some 24 hour immersion solutions was measured using the procedure of Lowry et al. (1951), slightly modified. Gelatin (Knox), rather than the usual bovine serum albumin, was used as a standard. To 0.1 ml of immersion solution, or solution of gelatin standard, in a test tube, was added 0.25 ml of 1N NaOH. After incubation in hot tap water for 30 minutes, 2.5 ml of a freshly prepared mixture of: [100 ml 2% (w/v) NaCO3, 2 ml 1% Na(or K)tartrate, and 2 ml 0.5% anhydrous CuSO4] was added. The mixture was stirred vigorously. Ten minutes later, 0.1 ml of Folin reagent (Sigma Chemical Co.) was added, and the reaction mixture was immediately stirred vigorously. After 45 minutes, the extent of blue color development was determined by measuring the absorbance at 740 nm on a Beckman DU-64 spectrophotometer. A series of gelatin standard solutions of different concentrations were assayed with each set of samples.
- Surface pH was measured with an Orion #81-35 flat surface Ross-style combination electrode and Corning model 12 pH meter. The well-rinsed electrode, with a pendent drop of laboratory deionized water, was lowered onto a ½ inch square paper specimen resting on a polyethylene bag padded with blotters. The pH was recorded after 5 minutes. The electrode was calibrated with pH 7 and pH 4 buffers before each measurement session, and calibration was checked at the end of the session. As expected for this very stable electrode, calibration did not change detectably during any of the 2–3 hour periods over which measurements were made.
- Colorimetry. Paper color was quantitated with a HunterLab Ultrascan Spectro-colorimeter. Reflectance spectra were recorded at 10 nm intervals from 375 to 750 nm, using the visible light source, at three different locations on each side of each paper specimen. The instrument was set for a 10 degree observer, the small (¼ in diameter) area of view was used, and specular reflectance was included. The triplicate readings were averaged. CIE La∗a∗b∗ values and delta E∗ were computed automatically from these data.5 In most cases, values for recto and verso sides of each specimen, and of the papers treated identically in the two experiments, did not differ significantly from each other. Thus, the values were averaged, and standard deviations are reported above.
- Tensile measurements. Tensile properties of all papers were investigated using Mecklenburg relaxation tensometers6(Mecklenburg 1984), with a horizontal load applied in the machine direction to the paper strips. Narrow strips of uniform width were cut with a Dahle mat cutter. After measurement of paper thickness in five places with a micrometer, the paper strips were mounted horizontally in the apparatus exposed to laboratory atmosphere. Care was taken not to subject the papers to excess moisture as they were being mounted in the apparatus. Over the several months that tensile measurements were performed, the laboratory temperature fluctuated no more than 1.7°C and the relative humidity no more than 2%. After an initial equilibration period of 5–15 minutes at a gauge length of 2.5 in, the strip was stretched 2.5 × 10−3 in, and 1 minute later the stress sustained by the paper strip was recorded. This process was repeated once per minute until the paper strip broke. Measurements were made on three strips of each paper. From these data, nominal stress (force applied per nominal cross-sectional area of the strip) and strain (change in length divided by gauge length) were computed and corrected for bending of the strain gauge. Nominal stress was plotted as a function of strain for each paper strip. In the figures presented above, averages of the curves obtained from the measurements on the triplicate papers are shown. Where error bars do not appear, they would be no larger than the symbols used on the graphs. In almost all cases reproducibility was very good both among the triplicates for each paper in an experiment and between the two different experiments performed (tables 2 and 4).
1. Dr. David Martin of Whatman Paper, Ltd., London, advised us that Whatman records showed that the paper was made of 100% cotton rag with alum, gelatin, and soap flakes added. Further details of the manufacture of this paper are no longer available.
2. A major advantage of collecting spectral data is that any of the color measurement scales can be used to describe the results. CIE L∗a∗b∗ is considered to be one of the more useful and sensitive scales; however, many researchers use % reflectance or K/S values at certain wavelengths. These data can be taken from the tabulated spectral data recorded on the HunterLab instrument. For example, for measurements of some color standards, the following results were obtained:
The wavelength used for % reflectance and K/S was 457 (TAPPI Standard 452 os-77). In addition to its sensitivity in making color changes more evident, the spectral data give a complete picture of what is happening to the specimen. For example, the curves for the standard tiles are very similar at the 457 nm area, but the orange tile spectrum shows peaks at 510 and 620 nm, while the yellow-green tile spectrum shows peaks at 525, 600, and 675 nm.
3. Application of animal glue has also been shown to increase the strength of Japanese papers; for some of these papers this strength advantage is maintained upon artificial aging (Inaba and Sugisita 1986).
4. Although it provides a solution with greater neutralizing capacity, the common procedure of diluting a saturated Ca(OH)2 solution 1:1 with water was avoided because it can yield a solution of pH 12 initially and because it does not provide a reproducibly reliable buffer reserve. It is also likely to result in precipitation of CaCO3 during the 24 hour incubation, as far more CO2 is absorbed from the air.
5. Readers interested in more complete reflectance data or in brightness values can obtain these by contacting the authors at the Conservation Analytical Laboratory.
6. Details of the construction and use of these instruments may be obtained from the Conservation Analytical Laboratory.
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TERRY TROSPER SCHAEFFER received a B.A. in physics and biophysics (1961) and a Ph.D. in biophysics (1967) at University of California, Berkeley. Most of her research has been in the areas of interaction of photosynthetic pigments with other plant and bacterial membrane components, and ion transport across mammalian cell membranes. In 1989–90, she entered the field of conservation research when she was a Fellow-by-Courtesy, G.W.C. Whiting School of Engineering, Johns Hopkins University, and a Visiting Research Collaborator, Conservation Analytical Laboratory. She is a fellow in Paper Conservation Reseach at the Conservation Center of the Los Angeles County Museum of Art. Address: Conservation Center, Los Angeles County Museum of Art, 5905 Wilshire Blvd., Los Angeles, Calif. 90036.
MARY T. BAKER received her B.S. in chemistry in 1980 and her Ph.D. in 1986 in materials science with a specialty in polymer science from the Institute of Materials Science at the University of Connecticut. She has worked at the Conservation Analytical Laboratory, Smithsonian Institution, as a research chemist since 1987, collaborating with conservators on projects such as the effects of fumigation on materials; treatment and characterization of coated papers; light bleaching of paper; and methods development for analysis of microsamples of paints, varnishes, and other materials. Her current research is on the modern polymeric materials in air and space artifacts, their aging mechanisms, storage, treatment, and display. Address: Conservation Analytical Laboratory, Museum Support Center, Smithsonian Institution, Washington, D.C. 20560.
VICTORIA BLYTH-HILL is the senior paper conservator in the Conservation Center of the Los Angeles County Museum of Art. She is past chair of the Book and Paper Group of the AIC and has been a Fellow of that organization since 1988. She has presented lectures at AIC and other professional organizations and at universities on subjects ranging from general conservation awareness to the treatment of the Leonardo da Vinci “Codex Hammer.” Blyth-Hill has supervised many interns over the years at LACMA and has sponsored such research projects as accelerated aging of adhesives and pigment analyses on Persian miniatures. Address: Conservation Center, Los Angeles County Museum of Art, 5905 Wilshire Blvd., Los Angeles, Calif. 90036.
DIANNE VAN DER REYDEN received certificates in conservation from Harvard University Art Museums (1981) and the Conservation Center, Institute of Fine Arts, New York University (1980), along with an M.A. in art history (1979), serving internships at the Fogg Art Museum, the Library of Congress, and the Museum of Modern Art. She is senior paper conservator and co-head of the Paper Conservation Laboratory at the Conservation Analytical Laboratory, Smithsonian Institution, engaged in research in aqueous light bleaching and the effects of solvents on specialty papers and in the training of interns and professionals. She recently served as secretary of AIC and has been a Fellow for several years. Address: Conservation Analytical Laboratory, Museum Support Center, Smithsonian Institution, Washington, D.C. 20560.