EFFECTS OF AQUEOUS LIGHT BLEACHING ON THE SUBSEQUENT AGING OF PAPER
TERRY TROSPER SCHAEFFER, MARY T. BAKER, VICTORIA BLYTH-HILL, & DIANNE VAN DER REYDEN
2 MATERIALS AND METHODS
The papers used in this study were:
- Whatman 1 (hereafter W1) filter paper, manufactured from 100% cotton seed hairs without sizing, used as an appropriate unsized control. An analysis of this paper was reported previously (van der Reyden et al. 1988).
- English artists' rag paper bearing the watermark “J. Whatman 1956” (hereafter W56), made of 100% cotton rag sized with alum and gelatin.1
Analytical procedures, including spot tests, optical and scanning electron microscopy (SEM), energy-dispersive x-ray analysis (EDX), and Fourier transform infrared spectrometry (FTIR), were used to assess the overall chemical composition and condition of the paper specimens and to monitor changes in these properties. Not all the tests were applied to all paper specimens at each stage of the experiment, especially in the case of the W1 unsized control paper. Details of these procedures are given in the appendix.
The information obtained from analyses of W1 and W56 papers “as received” (untreated) and after washing confirmed the manufacturer's specifications.
Elemental analysis of the untreated W56 paper by EDX documented the presence of aluminum and sulfur. After washing, the presence of these elements was barely detectable. The observations are consistent with removal of a major portion of the alum and other aluminum salts during the washing step.
Scanning electron micrographs of washed papers showed a significant increase in the cross-sectional area of most fiber lumens. Greater variation in interfiber spaces was also evident. Both these observations may be related to the ca. 15% increase observed in nominal thickness of the washed W56 paper, from 0.0165 ± 0.0007 cm before to 0.0196 ± 0.0005 cm after washing (n = 5).
2.2 EXPERIMENTAL DESIGN
All the W1 and W56 papers were handled as works of art on paper would be, with the additional precaution that plastic gloves were always worn when the papers had to be manipulated by hand. The overall experimental protocol is diagrammed in figure. 1. Details of the procedures are given in the appendix.
2.2.1 Washing and paper preparation
A piece of each paper was reserved as an untreated control. Other sheets of the W56 and W1 papers were washed by soaking them individually in a Ca(OH)2 solution, initial pH 9.5, and then drying them between blotters, felts, and Plexiglas under weight. This preliminary step was undertaken to simulate the usual conservation procedure of washing a discolored paper object in an alkaline bath before bleaching as well as to provide washed controls to isolate the effects of alkaline water alone. The untreated papers and the washed papers were cut into specimens of appropriate size for exposure to aqueous light bleaching or the other, control, conditions described below.
2.2.2 Aqueous light bleaching and control exposure conditions
Aqueous light bleaching was carried out in an Atlas Ci35 Weather-ometer, with each individual paper specimen suspended in a 600 ml flat-sided polystyrene culture bottle as described previously (van der Reyden et al. 1988). Thirty such bottles could be placed in the Weather-ometer chamber at one time, permitting 15 specimens of W1 paper and 15 specimens of W56 paper to be exposed simultaneously. In practice, 3 specimens of each paper were exposed to the same conditions, but for three different time periods: 2, 6, or 24 hours. Thus, five different experimental conditions were used for each of the two papers (see fig. 1). These were: (1) unwashed control paper, kept dry in the dark; (2) washed control paper kept dry in the dark; (3) washed control paper immersed in Ca(OH)2, initial pH 9.5, in the dark; (4) washed control paper kept dry and exposed to light; and (5) washed sample paper immersed in Ca(OH)2, initial pH 9.5, and exposed to light, which is equivalent to aqueous light bleaching. All dark controls were in bottles that had been wrapped securely in heavy-duty aluminum foil.
One specimen of each paper subjected to each experimental condition was removed from the Weather-ometer in its bottle after each of the specified exposure times. The immersed papers were blotted and dried in a felt and Plexiglas press under weight. After the incubations the pH of all—and the temperatures and protein content of some—immersion solutions was measured. Details of these procedures are included in the appendix.
The entire experiment was performed twice.
2.2.3 Artificial aging
All papers that had been in the Weather-ometer were cut in half. One set of these halves was reserved for analysis. The other set was sewn into Plexiglas frames for humid oven aging. They were artificially aged for 20 days in the dark at 90°C and 50% RH in an Associated Environmental Systems HK-4116 temperature-humidity chamber. These conditions have been used previously for artificial aging studies (Lee et al. 1989b; Erhardt et al. 1987). Washed papers that had not been in the Weather-ometer were aged at the same time. Aged papers were subsequently analyzed using the same procedures as those applied to the un-aged portions of each specimen.
2.3 EVALUATION OF EFFECTS OF TREATMENTS AND AGING ON PAPER PROPERTIES
In addition to the qualitative chemical analyses described in section 2.1, each paper specimen was characterized physically by surface pH, colorimetry, and relaxation tensometry. Experimental details of these analytical procedures are provided in the appendix.
In this study, changes in surface pH have been regarded as a qualitative indicator of changes in the acidity of the paper. Because of the recognized inaccuracy of the measurement, only changes of at least one-half pH unit are considered significant.
Colorimetry was used as a quantitative, objective measure of paper appearance. Reflectances throughout the visible range were recorded, and CIE L∗a∗b∗ values were generated from these. Comparisons of these values for different specimens will indicate not only changes in lightness-darkness, but also shifts in red-green and yellow-blue color coordinates.2 The CIE L∗la∗b∗ data for all papers are given in table 1 and table 3.
TABLE 1 COLORIMETRIC DATA FOR WHATMAN 1 PAPER
TABLE 3 COLORIMETERIC DATA FOR WHATMAN 1956 PAPER
Tensile data obtained with sensitive relaxation tensometers were used to monitor mechanical properties of the papers before and after treatments and artificial aging. To use these data, nominal stress (force on the paper strip divided by cross-sectional area of the strip) was plotted as a function of strain (percent change in length). In these graphs, the relative strength of the paper is indicated by the stress withstood at a given strain; the slope of the curve is an indication of the stiffness; and decreased strain at failure implies increased brittleness of the paper. A summary of tensile data for all papers (ultimate stresses and strains to break) is presented in table 2 and table 4.