Investigations on pre-treatment prior to freeze-drying of archaeological pine wood with abnormal shrinkage anisotropy

Abstract

The current research is aimed at determining the nature of changes in the dimensions and hygroscopicity of archaeological Scots pine ( Pinus sylvestris L.) wood after treatment with polyethylene glycols and vacuum freeze-drying. The research was undertaken on mature sapwood with narrow annual rings and a small percentage of latewood. The experimental material used in a comparative assessment is characterised by a considerable degree of wood degradation, but low tangential and radial shrinkage and relatively high longitudinal shrinkage. The waterlogged samples were treated with aqueous solutions of PEG 300 with final concentrations: 8, 12, 16 and 20% (v/v), to which PEG 4000 with final concentration: 15, 20 and 25% (w/v) was added. After freeze-drying and seasoning of samples in the air at RH 30, 40, 50 and 65%, the water content of the samples and the degree of shrinkage of wood in three anatomical directions were determined. The dimensional stability of the samples was also determined on the basis of ASE values at RH 50%. After treatment of the samples with a mixture of PEG 300 and PEG 4000, vacuum freeze-drying and seasoning in the air, radial and longitudinal shrinkage of the pine wood was very low and stayed at the same general level, independent of the concentration of solutions and uptake of impregnating agents. More visible dimensional changes in wood were observed only in a tangential direction. On the basis of the results of the research on dimensional stability and the hygroscopicity of pine wood, it is suggested that the optimum method of pine wood conservation is vacuum freeze-drying, preceded with treatment with aqueous solution of 8–12% PEG 300 and 15–25% PEG 4000.