AQUEOUS LIGHT BLEACHING OF PAPER: COMPARISON OF CALCIUM HYDROXIDE AND MAGNESIUM BICARBONATE BATHING SOLUTIONS
TERRY TROSPER SCHAEFFER, VICTORIA BLYTH-HILL, & JAMES R. DRUZIK
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SOURCES OF MATERIALS
PAPERWhatman 1, rectangular sheets 46 × 57 cm
local laboratory supply houseWhatman artists' paper, 1956
private collection of C. C. HillWaterleaf blotters, 112
Paper Technologies, 929 Calle Negosio, Unit D, San Clemente, Calif. 92673
CHEMICALSPurified photographic gelatin, code. 41555
Sanofi Bio-Industries, 620 Progress Ave., P.O. Box 1609, Waukesha, Wis. 53187P-dimethylaminobenzaldehyde (Ehrlich's reagent)
Sigma Chemical, P.O. Box 14508, St. Louis, Mo. 63178N-propanol, spectrophotometric grade
Aldrich Chemical, 1001 W. St. Paul Dr., Milwaukee, Wis. 53233NaOH, CuSO4, H2SO4, and H2O2, all reagent grade, and MgCO3·Mg(OH)2·xH2O, USP grade
local laboratory supply house
INSTRUMENTS AND SUPPLIESLaboratory glassware, plastic ware, and bench top apparatus
local laboratory supply houseVacuum filtration assembly
Millipore Corp., 397 Williams St., Marlborough, Mass. 01752, (other brands also available from laboratory supply houses)Fluorescent tubes
local electrical supply houseGilson Pipetman 100 μl–1.00 ml automatic pipettor
Rainin Instrument Co., Mack Rd., P.O. Box 4026, Woburn, Mass. 01888Japanese hole punch
Bookmakers, 6001 66th Ave., Ste. 101, Riverdale, Md. 20737
Ross-style flat surface combination electrode, 8135
Orion Research, Inc., (available from local laboratory supply houses)Diano Matchscan II spectrocolorimeter
Bausch and Lomb, (this particular model is no longer available)Instron Automated Testing System tensometer, model 4201
Instron Corp., 100 Royall St., Canton, Mass. 02021JDC Model .5–10 precision sample cutter
Thwing-Albert Instrument Co., 10960-T Dutton Road, Philadelphia, Pa. 19154Spectronic 20 spectrophotometer
Milton-Roy Corp., 820 Linden Ave., Rochester, N.Y. 14626, (available via some laboratory supply houses)Temperature Humidity Chamber Model no. 435304
Hotpack Corp., 10942 Dutton Rd., Philadelphia, Pa. 19154,
1.1 DETAILS OF TREATMENT PROCEDURES
Ca(OH)2: Stock saturated calcium hydroxide solution was stored over solid material at room temperature in a closed container. This solution was filtered through Whatman 1 filter paper to remove undissolved material and immediately diluted into deionized water to a final concentration of 0.4% saturation by volume (4 ml filtered solution per 996 ml H2O). The pH of the diluted solution was 10.1 ± 0.2 for different batches.
Mg(HCO3)2: A stock magnesium bicarbonate solution is always kept on hand in the LACMA Paper Conservation Laboratory. It is prepared by suspending 200 g USP grade powdered MgCO3·Mg(OH)2·xH2O (approximately 42% as MgO) in 15 gal deionized water, letting the suspension stand overnight, and then bubbling CO2 through vigorously for 6 to 8 hours, by which time the solution is clear. This solution is diluted 1:1 (by volume) with deionized water immediately prior to use. Calculation of the concentration of the resulting Mg(HCO3)2 solution based on the percentage of MgO in the powdered material gives 18.5 mM. The pH of this solution immediately after the dilution has been made varies slightly from batch to batch of the stock material, in the range 6.8 to 7.2.
Washing: Paper samples, ca. 7 × 21 in., were cut with the machine direction oriented in the long dimension of the piece. Samples to be washed were placed on a piece of Reemay and dampened with a fine spray of deionized water on both sides. Each piece was put into a 20 × 24 in. polyethylene tray containing 10 liters of the appropriate bathing solution and submerged gently with gloved hands. After 4 hours, the paper was removed and drained briefly on the Reemay, placed on waterleaf blotter, covered with another piece of Reemay and blotter, and blotted by carefully moving the hand over the top of the blotter. The Reemay sandwich was transferred to dry blotters and pressed under a sheet of Plexiglas and light weight for ca. 20 minutes. The washed papers were then transferred to fresh blotters and dried and flattened under Plexiglas and moderate weight for 3 days. They were stored between new blotters in the press until the next step of the experiment.
Aqueous Light Bleaching and Dark Controls: Each washed piece of paper was cut into three pieces ca. 7 × 7 in. One of these was reserved as the washed-only control, and the other two were dampened and submerged on Reemay in individual 9 × 12 in. white polyethylene dissecting trays containing 1,500 ml freshly prepared bathing solution. Each sample was placed in the same type of solution, Ca(OH)2 or Mg(HCO3)2, used for the first wash. One of the two trays was wrapped in heavy-duty aluminum foil. Four pairs of trays were prepared: two with W1 in Ca(OH)2, two with W1 in Mg(HCO3)2, two with W56 in Ca(OH)2, and two with W56 in Mg(HCO3)2. They were arranged in two rows of four on the bottom of a large stainless steel tub, under a bank of eight 40 W fluorescent tubes (Philips F40D Daylight, preheat, rapid start; no UV blocking materials are used in this fixture, so the small component of UV in the lamp output is not removed). The lights were positioned ca. 15 in. above the tops of the trays. A small fan (System Papst model 4600 X) placed on the rim of the tub constantly circulated room temperature air over the trays so that the temperature of the bathing solutions never rose above 27°C. The samples were bleached for 8 hours. Because the light was not completely uniform over the entire area of the samples, the trays were rotated at the end of each hour so that each one received the same exposure. (The average light level at the surface of the trays was approximately 1,450 ft-c.) At the end of the bleaching period, the samples were removed from the trays on the Reemay, blotted, and dried under moderate weight as described above. The dried paper samples were stored in the conservation laboratory in the dark in archival polyester photograph sleeves.
Humid Oven Aging: Every paper sample was cut in half with a sharp scalpel, so that the machine direction of the halves was in their long dimension. Cotton thread, 150 gauge, was sewn into the corners of one-half piece of each sample. These half samples were individually tied into Plexiglas frames with the sheet surface oriented in the vertical plane and the machine direction vertical. All four corners of each sample were tied so that the paper pieces could not swing around or come in contact with each other. In the first experiment, the samples were aged in the dark at 70 ± 0.5°C and 50 ± 4% relative humidity for 28 days in a Hotpack model 435304 temperature-humidity chamber. (The aging protocol was not duplicated precisely in the second experiment; the humidity fluctuated well beyond the limits used before, and the process was terminated after 24 days. The data for the two experiments have not been combined in the statistical analyses.) At the end of the aging period, the oven was turned off and the door left open until the Plexiglas frames did not feel hot to the touch. The frames with the samples tied into them were placed in a cupboard in the dark for ca. 3 hours to cool. The samples were then cut out of the frames and stored in polyester photograph sleeves.
2 B. ANALYTICAL PROCEDURES
Colorimetry: The appearance of all samples was monitored by colorimetry. Reflectance spectra from 380 to 700 nm were measured at 2 nm intervals on a Diano Matchscan II instrument with a white tile as the reference material and another white tile in back of the paper samples. This second tile was also used to calibrate the instrument before each use. Four scans at different positions on each side of each sample were recorded using a 2° observer and the small (.25 in.) aperture. Specular reflection was excluded. CIE 1976 L∗ a∗ b∗ values were computed for a D65 illuminant by the instrument from the data of each scan. The values for the four scans of each side of each sample were averaged. If recto and verso values were not significantly different, the average of all eight scans is reported; otherwise recto values are given. All average values are given ± population standard deviation.
Tensile Behavior: Tensile properties of the samples were determined by evaluation of stress-strain data obtained with an Instron 4201 instrument using Series IX Automated Materials Testing System. The sample papers were equilibrated in the laboratory at least 24 hours before they were tested. Strips of paper precisely 0.5 in. wide were cut from each sample using a JDC Model .5–10 precision sample cutter. The thickness of each strip was measured in five places with a Mitutoyo 505-637-50 micrometer. The third digit of each reading was interpolated from the micrometer scale. The average value of the thickness was used in stress-strain calculations. Unless otherwise indicated the strips were cut with the long dimension (ca. 7 in.) in the machine direction. Assignment of the machine and cross directions of the papers was confirmed by observing stress-strain behavior of untreated sample strips cut in each direction. Pneumatically controlled grips held the sample strips in place, with a 5 in. expanse of paper between the grips. The upper grip was attached to a strain gauge appropriate to the maximum strength of the papers. The force on the strip of paper as it was stretched was stored electronically. Stress-strain curves were generated from this information by the Series IX computer program supplied with the instrument. This program calculated the modulus (initial slope), maximum stress, and strain at the break point for each strip and determined the average values and standard deviations for all strips of each sample. Other features of the stress-strain curves were also available. The relationship of these properties to mechanical properties of the paper is described in the text.
pH: The acidity of the paper samples was measured by the cold extraction method for pH, TAPPI test method T 509 om-83, slightly modified as follows. Because of the limited amount of sample material available, one-half the weight of paper and one-half the volume of solution specified in the procedure were used for each determination. The sample was cut into small pieces, and 0.50 ± 0.05 g weighed on an analytical balance. The sample was placed in a stainless steel Waring blender minicontainer. Then 37.5 ± 0.5 ml deionized H2O were added, and the material was blended for 10 seconds. The sides of the container were scraped down, and the slurry blended for a further 10–20 seconds, depending on the extent of breakdown of the paper sheet during the first blending period. The slurry was transferred to a clean 150 ml beaker, covered with Parafilm, and allowed to stand 1 hour. The pH of the suspension was measured with an Orion Ross-style model 8135 flat surface combination electrode and a Markson model M-6071 pH meter, after N2 had been bubbled through the slurry for 10 minutes. This procedure generally provides pH values accurate to ± 0.2 pH units. The electrode was calibrated with commercial standard buffers of pH 7.0 and 4.0 before use each day and was checked for drift every few hours. This type of electrode is particularly stable, drifting ≤ 0.03 pH 7.0 and 4.0 before use each day and washing solutions and those used for reimmersion and aqueous light bleaching were also measured, both before and after their use, with the same electrode.
GELATIN ASSAY: The amount of gelatin in the Whatman 1956 samples was determined by a semiquantitative modification of TAPPI standard procedure T 504 om-84 for qualitative determination of glue in paper (Schaeffer 1995). Briefly, ca. 4-12 mg of paper sample was extracted in 0.5 ml 5 M NaOH at 100°C. The extract with the paper samples still in it was analyzed for hydroxyproline using Ehrlich's reagent, and the absorbance of the resulting colored solution was read on a Milton-Roy Spectronic 20 spectrophotometer. the amount of gelatin present was determined by comparison of the absorbance with a standard curve generated for each assay, using purified photographic grade gelatin (Sanofi Bio-Industries). The modified method has been shown to give results that are good to ± 10%, which was adequate for comparing the relative amounts of gelatin sizing remaining in the samples after each of the different treatment steps.
TERRY TROSPER SCHAEFFER received a Ph.D. in biophysics from the University of California at Berkeley in 1967. After more than 20 years of research in photosynthesis and membrance transport, and occasional university teaching, she entered the field of conservation science in 1989–90. Since that time she has studied the effects of aqueous light bleaching on paper and devised laboratory procedures to facilitate research projects in this area. Her interests include the accelerated aging of paper of experimental purposes and the effects of conservation treatments on modern and historic papers. Address: Los Angeles County Museum of Art, 5905 Wilshire Blvd., Los Angeles, Calif. 90036.
VICTORIA BLYTH-HILL is the senior paper conservator in the Conservation Center for the Los Angeles Country 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 meetings and at universities on subjects ranging from general conservation awareness to the treatment of the Leonardo da Vinci Codex Hammer and a large-scale decoupage by Henri Matisse. Blyth-Hill has supervised many interns at LACMA and has sponsored several externally funded research and survey projects. She also organized the Western Association of Art Conservation symposium, “Conservation Treatment of Tibetan Thankas.” Address: Los Angeles Country Museum of Art, 5905 Wilshire Blvd., Los Angeles, Calif. 90036
JAMES R. DRUZIK has been a conservation scientist at the Getty Conservation Institute since 1985. His principal research interests concentrate on environmental chemistry and paper conservation, and he is responsible for administering GCI's contract research activities. In 1988, 1990, 1992, and 1994 he, along with Pamela Vandiver, Edward Sayre, George Wheeler, and Ian Freestone, organized and conducted a series of international symposia entitled “Materials Issues in Art and Archaeology” for the Materials Research Society. Prior to joining GCI, he worked in the Conservation Center, LACMA, under both Victoria Blyth-Hill and Pieter Meyers from 1980 to 1985, and from 1970 to 1980 at the pasadena Museum of Modern Art and the Norton Simon Museum. Druzik has a B.S. in chemistry. Address: Getty Conservation Institute, 1200 Getty Center Dr., Ste, 700, Los Angeles, Calif. 90049–1684.