| Abstract: | CONCLUSION. A study of aqueous deacidification including solutions of Mg(HCO3)2, Ca(HCO3)2 and Ca(OH)2 showed that it is generally preferable if duration of immersion is long (>20 min) and concentration of the alkali high (>0.01 mol L-1). The alkaline reserve obtained after a single treatment of Whatman paper is up to 2-3% CaCO3, however, Ca(OH)2 precipitates much more efficiently than the two hydrogencarbonate analogues. The pH of Whatman paper, achieved after deacidification, does not depend on the content of CaCO3 if higher than 0.2%. The same study using historical paper samples showed that deacidification is not achieved as easily. Unless high concentrations of Mg(HCO3)2 or Ca(HCO3)2 are used (achievable in CO2-pressurised bottles), deacidification will not take place at all. The uptake of alkaline reserve seems to depend also on alkalinity of the deacidification solution. The pH after deacidification is similar for both Ca- and Mg-containing papers (with comparable alkaline reserves), indicating that the remaining non-cellulosic compounds in the rosin-sized real papers may have a buffering role. By ageing the samples as loose sheets at several temperatures, and by application of the Arrhenius model, a comparison of extrapolated rates of degradation at room temperature is possible. This provides comparative data on the suitability of a particular stabilisation treatment. A study using Whatman paper showed that the most favourable stabilisation is achieved using iodide in Ca(HCO3)2-deacidified papers, however, these samples are also expected to discolourate at a more pronounced rate. Bromide and thiocyanate also exhibited a stabilizing effect, although all samples are expected to yellow at a faster rate than the non-treated, slightly acidic sample. The study on historical samples showed a different picture. Generally, both deacidification treatments and all antioxidants are expected to perform decisively better than the non-treated sample. During thermo-oxidative degradation, the formation of colour proceeds at the same or smaller rate in deacidified and additionally stabilised samples than in the non-treated samples. A combination of iodide and CaCO3 mostly gave the best results, the factor of stabilisation (prolongation of expected lifetime) being up to 60. Other treatments mostly performed comparably. In one real-life paper, all samples containing MgCO3 were significantly better stabilised than CaCCVcontaining samples, the achieved maximum factor of stabilisation was found to be 660. Contemporary papers containing CaCOi as a filler do not benefit from an addition of antioxidant, however, no harmful effect was found either. Despite the strong degradation during irradiation of factual paper samples made from bleached chemical pulps, the yellow component decreased. The determination of b* or brightness after irradiation, as proposed by the recent ASTM standard is thus of no practical value. Instead, DP or mechanical properties have to be determined. In all samples, a subsequent thermal ageing step introduced strong yellowing of paper. It is proposed that such a step is included in standard evaluation of light stability of paper. While Whatman filter paper is an extremely valuable reference model, as it is generally available and of comparative quality, real-life paper samples may exhibit different properties. The differences in the ageing behaviour between unsized Whatman paper and historical paper may originate in addtitives, which decrease the pH of paper deacidified using Mg(HCO3)2. Any proposed stabilisation treatment must therefore be optimised and checked on a variety of such samples. The Arrhenius approach, although time-consuming, may provide comparative data on the suitability of treatments and their performance at room temperature. |
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