EFFECTS OF DILUTE CALCIUM WASHING TREATMENTS ON PAPER
JOHN BOGAARD, & PAUL M. WHITMORE
Conservation treatments of individual paper artifacts, such as works of art or archival materials, often involve steps of washing, brightening, and deacidifying with aqueous solutions. Deionized water has been used in these steps, so that harmful iron and copper salts are not introduced into the sheet, but it has been found that washing with this ion-free water may also remove beneficial components (Tang and Jones 1979). To counter this effect, small amounts of calcium salts are often added to the deionized water used in the washing step.
Although this enrichment of the wash water with calcium is generally accepted as beneficial, there is still some uncertainty as to which calcium salt and what strength of solution are appropriate in different situations (Burgess 1986). Most papers requiring treatment are acidic and thus subject to degradation by hydrolysis reactions that can be prevented by an alkaline wash treatment. However, papers that have been oxidized by light exposure or bleaching treatments may also be susceptible to alkaline damage during such a treatment and in subsequent aging. Assessing and minimizing these two risks are necessary for the design of the most effective treatment for each object.
The subject of washing or deacidifying using aqueous solutions of calcium compounds has been examined for many years beginning with work by Barrow (Barrow and Sproull 1959), in which he suggested using successive solutions of calcium hydroxide and calcium bicarbonate. Subsequent work has examined the effects of calcium treatments on different types of paper (Hey 1979; Burgess and Boronyak-Szaplonczay 1992) or the use of different concentrations of calcium (Tang 1981).
Studies have also been made that compare calcium and magnesium treatments (Calvini et al. 1988; Kolar and Novak 1996; Bansa 1998). Schaeffer et al. (1996) examined the use of calcium and magnesium solutions in aqueous light bleaching. Lienardy and Van Damme (1990) looked at the effects of various deacidification agents on the inks and pigments found on paper. In general, these studies evaluated papers using measures such as pH, brightness, and folding endurance, before and after a set period of aging, but they did not examine the chemical details of these treatments. They generally support the notion that alkaline calcium treatments are effective at stabilizing cellulose in papers, but the choice of calcium salt solution, concentration, and treatment times are still left to the conservator.
The work described in this article explores the relative stabilizing effects on paper of a number of calcium salts applied as dilute aqueous solutions. Chemical measurements were made in order to examine the interactions of these solutions with cellulose and to quantify small amounts of damage and deterioration before they would be manifest as a loss in physical properties. Accelerated aging of the sample sheets was performed so that long-term effects of these treatments could be distinguished. Of special interest was studying the extent of problems associated with the use of alkaline treatments on oxidized papers.
The salt concentrations used are probably closer to ones employed in a conservation washing step than to those employed in a deacidification treatment. Solutions of a variety of calcium salts were used to separate the effects due to the calcium itself from those due to the alkalinity. To examine the interactions of the calcium solutions with cellulose alone, tests were run on a pure cellulose filter paper—Whatman no. 42, which is unsized and free of the additives found in commercial papers. This paper is made from cotton linters and is slightly acidic due to its carboxylic acid content. The effects of the treatment on both unaged and photo-oxidized paper sheets were studied. For the photo-oxidized sheets, treatment effects were examined both with and without a prior chemical reduction. The chemical tests employed included determination of the degree of polymerization (DP) as well as the carbonyl and carboxyl group contents. Accelerated aging was carried out with multiple samplings, so that both deterioration rates and mechanisms could be determined. Treatments were assessed for their effectiveness in slowing deterioration of paper and for their possible risks of damaging it.