4 DISCUSSION

The results of this investigation demonstrate that aqueous light bleaching decreases discoloration immediately after treatment of both the sized and unsized papers. In addition, the extent of color reversion caused by humid oven aging of these papers is significantly less than that caused by aging controls that had been exposed to light while dry. After aging, the aqueously light-bleached W56 paper remained less discolored than the controls that were immersed and kept in the dark during Weather-ometer incubation.

The W56 cotton rag paper used in this study is a very strong paper that is also very deformable. Its strength is undoubtedly due in part to the presence of gelatin sizing.3 The disadvantageous effect of the aqueous light-bleaching treatment for the W56 paper was the marked decrease in its stress to break; however, similar decreases occurred in controls that were immersed in the dark during Weather-ometer incubation. This result suggests that the reduction in stress to break was caused by immersion of the paper for a second time and it was not directly owing to a photochemical reaction. The removal of some of the gelatin sizing could have contributed to this result; the paper lost some of its reinforcement with the size and also may have swelled as a result, causing an increase in cross-sectional area and a subsequent apparent decrease in stress to break. Two factors might have enhanced protein solubilization during Weather-ometer incubation. Concomitant loss of alum during the washing step may have softened the originally hardened gelatin, so that it was more easily dissolved out of the paper during the next immersion. Second, the unavoidably elevated temperatures in the Weather-ometer chamber (see appendix) would have increased protein solubility. Additional experiments to determine the effect of temperature on this process are under consideration. It should be noted that in an early study of aqueous light bleaching (Branchick et al. 1982), the samples that suffered the greatest loss of strength were 18th-century rag papers incubated at the highest temperature used in the investigation (40.5�C).

The reduced strain to break of the W56 papers that were immersed during Weather-ometer incubation may also be due in part to loss of gelatin size. The difference between the strain to break of the 2 hour light- and dark-immersed papers could be attributable to a temperature discrepancy, since the solution exposed to light may have heated up more rapidly than the solution kept dark. This difference becomes statistically insignificant with increase in immersion time (and increased solution temperature) and upon aging. It appears, however, that the initial effect of exposure to light during immersion and the effects of prolonged immersion may be comparable to the effect of artificial aging, exhibited by reduced strain to break. Because this similarity is not exhibited in the unsized paper, the possibility of interaction between light and size components (such as alum) must also be considered. Additional investigation of this effect is required before the phenomenon can be adequately explained.

The surface pH of the W56 paper was increased slightly by washing, but it fell back approximately to its original value upon artificial aging. The surface pH of the W1 paper was lowered slightly by the initial wash, and lowered much further by artificial aging. These observations suggest that the dilute Ca(OH)2 bath did not adequately neutralize acidic moieties in the paper and did not provide a buffer reserve that could successfully counteract acid groups formed during aging.4

The conditions to which the papers were exposed in the Weather-ometer were rather extreme. The W56 papers survived exposure to these conditions in surprisingly good shape. That the strong light itself—as distinct from immersion—was not responsible for significant deleterious changes in W56 papers that had been immersed is encouraging. Furthermore, no significant detrimental effect of light was discovered upon humid oven aging of the W56 papers immersed during Weather-ometer incubation. All papers suffered color reversion and embrittlement, but in no case was an aqueously light-bleached sample significantly more damaged by the artificial aging process than the corresponding control paper that was kept in the dark under otherwise identical conditions.

The results of this study indicate that aqueous light bleaching decreases the extent of discoloration on subsequent aging of the W56 paper, even for the shortest time of exposure under the present experimental conditions. The reduced color reversion in aqueously light bleached W56 samples might be partly attributed to removal of gelatin size, which could discolor upon artificial aging. However, this loss is unlikely to account for all of the decreased color reversion since the controls immersed in the dark lost about the same amount of gelatin as those that were aqueously light bleached, yet the former developed significantly more yellow chromophores during humid oven aging than did the latter samples.

It is interesting to note that the aqueously light-bleached W1 samples discolored the least after aging, discoloring less even than untreated papers. These results, for a paper composed solely of cellulose fibers, combined with the above results for sized W56, suggest not only that aqueous light bleaching removes chromophores from the paper directly but also that immersion during light exposure prevents colorless but potentially deleterious moieties from being present in the paper following that exposure. Whether this result is achieved by prevention of formation of the precursors that discolor upon aging or by removal of them into the immersion solution must be determined by more detailed chemical studies.

That sizing, and the chemical composition of the paper fibers themselves, will have a major effect on the response of any paper to every step of the aqueous light-bleaching process cannot be overemphasized. For example, the above results demonstrate that the effects of immersion on naturally aged, protein-sized papers must be carefully considered before the paper is washed, as well as reimmersed in preparation for the aqueous light-bleaching step (e.g. Burgess 1985). In general, a more complete basis for making the decision to use aqueous light-bleaching is required. More research must be performed with careful control of as many variables as possible. Not only must the effects of solution temperature, pH, and buffering agent be studied, but the composition of the paper and the nature of the chromophores responsible for the discoloration should also be considered (Lee et al. 1989a). These factors can be expected to have a major influence on the effectiveness of the treatment.

If loss of sizing is experimentally controlled, more subtle effects of light upon the mechanical properties of the paper may become evident. For documentation of these types of changes, chemical analyses and measurement of degree of polymerization are expected to be appropriate (Burgess 1985). It will also be desirable to use as a starting material a paper that is well defined chemically—that is, a new paper—and to discolor it by preaging it artificially under controlled conditions before subjecting it to further experimentation (Hofmann et al. 1991). Once the effects of the aqueous light-bleaching procedure in the long as well as the short term have been thoroughly documented, those papers for which it would be an efficacious conservation treatment can be identified, and appropriate treatment conditions can be determined.