Effect of Some Deacidification Agents on Copper-Catalyzed Degradation of Paper
Table of Contents - Introduction - Experimental
Description - Testing and Analysis
Results and Discussion - References - Supporting
Documents
Results and Discussion
Praskievicz and Subt, as quoted in Williams et al.,3 found that new papers contained an average of 4 parts per million (ppm) of copper, and recycled paper contained an average of 12 ppm of copper. The highest amount of copper they found in a commercial paper sample was 77 ppm. By comparison, the average copper content of the copper-doped test samples in these experiments was 725 ppm. A high copper content was desired so that the paper samples would degrade mainly through copper-catalyzed reactions, which form the focus of this study. Also, it has been observed that, at lower concentrations, copper deposits in paper appear to be less uniform, with localized areas of high concentration. Figure 1 shows the devastating effect that copper exerted on the aging of paper samples in these experiments.
For a comparison of the effects of different deacidification methods to be relevant, the alkaline reserve concentrations imbibed during these processes must be of a similar order. The alkaline reserve concentration sought from the deacidification treatments in this work was 0.8% ± 0.1% (as grams of calcium carbonate per 100 grams of paper). An alkaline reserve of close to 1% can be easily attained after a single immersion in saturated magnesium bicarbonate solution, or after one two-step sequential treatment with calcium hydroxide and calcium bicarbonate. However, zinc bicarbonate solution, even when fully saturated under ambient conditions, is so dilute (0.01 mol/l) that to obtain an alkaline reserve of a corresponding order would have required repeated immersion and drying steps.
To avoid physical stress on the test samples due to repeated handling while moving them in and out of a deacidification bath, a spray technique for zinc bicarbonate deacidification was used. The sample sheets were alternately sprayed with zinc bicarbonate solution and were air-dried until the desired level of alkaline reserve was achieved. Magnesium bicarbonate treatment was applied by both immersion and spray techniques, so that this treatment would form the central reference point with which both calcium hydroxide bicarbonate (immersion) and zinc bicarbonate (spray) deacidification treatments could be compared. As will become clear shortly, this choice of experimental conditions was most fortunate.
The deacidified test samples were aged at 90°C and 50% RH, and their fold endurance, brightness, and pH values were determined at intermittent intervals. The fold endurance data are presented in Figure 2. To facilitate an evaluation of the fold endurance data, the fold endurance values for different experimental systems have been compared at a single, arbitrarily selected point. For each treatment, the time taken for the fold endurance to fall to 1/8 of its initial value has been computed from the data in Figure 2. These data, along with the respective solution concentrations and alkaline reserves of the treated samples, are presented in Table 1. The corresponding brightness and pH data are presented in Table 2 and Table 3, respectively. An examination of the fold endurance data in Figure 2 and Table 1 immediately provides new insight into the role of magnesium-based deacidification in copper-doped paper.
In conformity with the findings of Williams and his co-workers,3 deacidification by the Barrow two-step treatment with calcium hydroxide and calcium bicarbonate was mildly effective in inhibiting copper- catalyzed oxidative degradation of paper, while the magnesium bicarbonate bath was much more effective. However, the same magnesium bicarbonate solution was surprisingly ineffective when applied as a spray. The zinc bicarbonate spray deacidification was also ineffective in stabilizing copper-doped paper. The data for the spray-deacidified samples and the untreated copper-doped control are so close together on the plot that, in the interest of clarity, only a single regression line has been drawn to fit the data for the control sample points.
What role does magnesium bicarbonate deacidification play in retarding copper-catalyzed oxidative degradation? The deactivation of copper by magnesium through complexation or any other mechanism can be ruled out, because the sample deacidified with magnesium bicarbonate spray was loaded with magnesium but still aged at about the same rate as it did before the deacidification treatment. An examination of the data on the copper content of the treated samples, which are shown in Table 4, provides the solution.
Table 4 also presents the copper contents of similar copper-doped samples after immersion in aqueous calcium, zinc and sodium bicarbonates, a calcium hydroxide solution, and a non-aqueous methyl magnesium carbonate solution. The samples deacidifled by immersion in a magnesium bicarbonate bath lost most of their copper. The samples subjected to the two-step calcium hydroxide/calcium bicarbonate treatment lost a small fraction of the adsorbed copper, while the samples which had been spray-deacidified retained practically all of their copper content. All bicarbonate solutions were seen to be effective in dislodging the sorbed copper species from paper.
On the other hand, calcium hydroxide, magnesium acetate, and methyl magnesium carbonate were all equally ineffective. Calcium hydroxide must react with the adsorbed and/or exchanged ionic copper species to form insoluble calcium hydroxide within the paper matrix. Aqueous magnesium acetate and nonaqueous methyl magnesium carbonate were both ineffective, since magnesium does not complex copper or react with it in any other manner to deactivate it. The calcium bicarbonate treatment was much more effective if it was applied without a prior calcium hydroxide treatment. Precipitation of the sorbed copper species by calcium hydroxide renders it inaccessible to complexation by a subsequent bicarbonate treatment. This observation suggests that not all copper species in paper are susceptible to complex formation with bicarbonate ions.
A simple qualitative test demonstrated that complexation of the adsorbed copper is indeed the key to its removal from paper, but the complexation effect takes place with bicarbonate ions rather than with magnesium species. If any of the bicarbonate solutions were added to a dilute solution of a copper salt, a precipitate was first formed. This precipitate dissolved on further addition of the bicarbonate solution as a soluble bicarbonate complex formed. Bicarbonate complexes of copper have been well characterized.16,17,18,19
The copper content data in Table 3 suggests that, unlike free copper ions, the soluble bicarbonate complex of copper has little affinity for sorption sites on the cellulose matrix. Earlier work reported a slower rate of copper-catalyzed oxidative degradation after neutralization with a sodium bicarbonate solution.13 It is possible that washing out of some of the adsorbed copper by the sodium bicarbonate solution might have been at least partly responsible for this observation.
In summary, aqueous bicarbonate solutions of magnesium, calcium, and zinc are all effective in dislodging adsorbed and/or exchanged copper species from paper. In the Barrow two-step treatment, on the other hand, the calcium hydroxide treatment fixes the sorbed copper species, making their removal by lata treatments more difficult. Paper conservators who have a preference for this deacidification treatment can do better by employing only the second step, as originally recommended by Schierholtz.20 The alkaline reserve imbibed from a calcium bicarbonate treatment alone is appreciably smaller.
In conservation treatments where color changes are a concern, changes in pH need to be minimized. In such cases, a reduced reserve of calcium carbonate may even be advantageous. Deacidification treatments applied as sprays or from nonaqueous media, even if they contain magnesium, do not serve to inhibit copper-catalyzed degradation of paper.
Table of Contents - Introduction - Experimental Description - Testing and Analysis - Results and Discussion - References - Supporting Documents