AQUEOUS LIGHT BLEACHING OF PAPER: COMPARISON OF CALCIUM HYDROXIDE AND MAGNESIUM BICARBONATE BATHING SOLUTIONS
TERRY TROSPER SCHAEFFER, VICTORIA BLYTH-HILL, & JAMES R. DRUZIK
The results presented here indicate that aqueous light bleaching in either Ca(OH)2 or Mg(HCO3)2 solutions, performed under conditions normally used by paper conservators in the laboratory, decreased the natural discoloration in both the unsized and the gelatin-sized cotton cellulose papers, usually to a greater extent than washing and dark immersion did. These papers were not markedly discolored initially, so the magnitudes of the bleaching effects were small. Although the decrease in yellowness was statistically significant in each experiment and detectable by eye for the sized paper, caution should be exercised in generalizing these results.
Artificial, humid oven aging caused some color reversion in all samples. However, the samples that had undergone aqueous light bleaching were in no instance significantly darker than the washed samples. No deleterious change in paper appearance or mechanical properties could be attributed to light exposure itself, either immediately after treatment or after humid oven aging. A similar conclusion was reported in an earlier study that used a harsher bleaching process in the Weather-o-meter and dilute Ca(OH)2 as the immersion solution (Schaeffer et al. 1992).
The relative efficacies of Ca(OH)2 and Mg(HCO3)2 immersion solutions for aqueous light bleaching to improve paper appearance are harder to evaluate. For example, no visible difference in appearance among any of the treated, unsized W1 paper samples was detected before or after aging. Although those samples bleached in Ca(OH)2 were measured to be slightly less yellow after humid oven aging, the differences in b∗ were too small to be discerned by eye. Many more repetitions of the entire experiment would be required to impart major significance to these data.
The observation has been made that papers deacidified or bleached in Mg(HCO3)2 tend to appear more yellow than those bathed in dilute Ca(OH)2, especially upon aging (Hey 1979; Eldridge 1982), but that effect was not observed in this study. Rather, in both experiments, the W56 samples that were bleached in Mg(HCO3)2 appeared slightly less yellow than the samples bleached in Ca(OH)2. This result may be attributed, at least in part, to the presence initially of (slightly discolored) gelatin size and to removal of a larger amount of it by immersion of the paper in Mg(HCO3)2 than in Ca(OH)2.
When the efficacy of aqueous light bleaching in the two immersion solutions is compared with respect to other paper properties, a different pattern emerges. The strength of the gelatin-sized paper is better maintained if it is washed and bleached in Ca(OH)2. The difference appears to be due to effects of immersion and not to light exposure itself.
The stress values calculated in these experiments are inversely proportional to the thickness of the paper sample being tested. The thicknesses were measured with calipers (appendix B) and are thus nominal thicknesses. The washed papers had slightly increased thicknesses—ca. 6% for the unsized W1 paper and 15% for the W56 samples—increases that, even in the absence of loss of material from the papers, would lead to an apparent decrease in the calculated strength. However, the changes in the tensile properties of the treated papers were larger than could be accounted for by the thickness increases. The additional decreases in stress could be due to loss of soluble material that contributed to the paper strength and/or to chemical alterations of the components of the paper that shortened the fibers or decreased the fiber bonding. The present study was not designed to address this question.
As expected, the effects of immersion on paper pH were different for the two solutions. Mg(HCO3)2 provided a good buffer reserve to the initially acidic gelatin-sized paper but caused the unsized W1 paper to become highly alkaline. In contrast, the dilute Ca(OH)2 solution did not provide to the W56 paper a buffer reserve that persisted upon humid oven aging. As a result, the aged W56 samples that had been immersed in Ca(OH)2 were distinctly acidic.
Removal of significant amounts of gelatin size from the artists' paper by both immersion steps occurred throughout the investigation. This side effect of the conservation washing and deacidification procedure has been noted previously (Schaeffer et al. 1992; Schaeffer 1995). In the present experiments, significantly more gelatin remained in the W56 paper immersed twice in Ca(OH)2 than in Mg(HCO3)2. Because gelatin size contributes to the strength of the paper, the differences in the amount of gelatin lost to the immersion solutions could be partly responsible for the differences in the strengths of the treated samples. Paper conservators may want to consider the ramifications of immersion in Ca(OH)2 or Mg(HCO3)2 on the size content when they plan a treatment for a work on gelatin-sized paper.
The sample papers used in this study were modern, new, or naturally aged papers made with 100% cotton α-cellulose fiber furnish. They did not contain lignin. There were no media or stains on them. Because of their content and condition, caution must be exercised when conclusions reached on the basis of these experiments are extended to stained materials, to papers with other sizing materials, to 19th- and 20th-century papers that may contain lignin, or to artworks on paper. The effects of treatment on the image materials or the chemicals composing the stain, and possible interactions among these substances and the components of the paper, all need to be considered. The effects of aqueous light bleaching on stained cotton cellulose papers (Schaeffer and Blyth-Hill 1993) are currently being investigated; findings will be reported separately.
The results of the research presented here emphasize the fact that the side effects of a treatment procedure may be as important to the condition of the treated object as the primary result of the treatment. In the case of aqueous light bleaching, the choice of immersion solution—Ca(OH)2 or Mg(HCO3)2—may be influenced more by the pH, sizing, and strength of the paper than by the desired result of decreasing discoloration. As with other treatment considerations, the choice of immersion solution for aqueous light bleaching will need to be made on an individual basis.