1 INTRODUCTION

When in 1980 Keyes first presented her conservation procedure of aqueous light bleaching to lessen discoloration of paper, she immersed the objects to be bleached in ca. 0.2% Mg(HCO3)2 solution (1987). Since that time, various concentrations of either Ca(OH)2 or Mg(HCO3)2 solutions have been used as immersion solutions (Baker 1982; Branchick et al. 1982; Eldridge 1982; van der Reyden et al. 1988; Lienardy and van Damme 1989; Schaeffer et al. 1992). Some conservators prefer one solution over the other. However, no direct comparison of aqueous light bleaching in Ca(OH)2 or Mg(HCO3)2 has been reported for controlled experiments in which long-term as well as immediate effects of the treatments were quantitated.

We have now undertaken such a comparison, emphasizing the use of experimental conditions similar to those used by conservators in actual treatments. A gelatin-sized artists' paper and an unsized cellulose paper used as a control were each subjected to several treatments in this study. Half of every sample was aged artificially in a humid oven, so that direct comparisons could also be made of long-term effects of each treatment on the sample papers.

Ca(OH)2 and Mg(HCO3)2 solutions are also employed by paper conservators as deacidification solutions (Lienardy and van Damme 1990). It has not been the intention of this investigation to compare the effectiveness of these two solutions for deacidification; a substantial literature already addresses this question (Hey 1979; Calvini et al. 1988; Bredereck et al. 1990; Lienardy and van Damme 1990). We have instead compared the efficacy of these two solutions for use as immersion baths for the aqueous light-bleaching process. The results of our experiments are presented below. No clear-cut preference for one immersion solution over the other emerged from the data, in part because the sized and unsized papers responded differently to the treatments. These results have also served to emphasize that some gelatin sizing is lost from papers when they are bathed and to illustrate the influence of this sizing material on some physical properties of the papers.