Document Type

Report

Publication Date

2007

Pages

1-57

Abstract

This study was undertaken to evaluate the physical effects of drying waterlogged archaeological wood using supercritical carbon dioxide as compared to air drying and the popular polyethylene glycol (PEG)/freeze drying method. Previous studies have shown supercritical drying to be a potentially advantageous alternative to current methods of drying waterlogged wood through reduced processing time, minimal shrinkage, and increased success for reversibility. Samples in the form of blocks and planks of a few centimeters in size were prepared from two archaeological waterlogged wood sources and grouped into three treatment sets: one for air drying, one for PEG/freeze drying, and the other for supercritical drying. Supercritical drying was carried out by replacing water in the wood with methanol, and then removing the methanol in a 250 ml chamber containing supercritical carbon dioxide. Shrinkage was tracked by measuring pins inserted in the samples before and after treatment and by 3D measurement via laser scanning. The scanning allowed a more accurate measure of shrinkage through total volume determinations and 3D distortion comparisons. Subsamples were taken from randomly selected samples for characterization and treatment. Analyses were carried out to determine the degradation state of the wood prior to treatment. These analyses included density and maximum water content measurements, and the identification and quantification of foreign inclusions in the wood, such as iron corrosion products. The subsamples were examined using low-pressure scanning electron microscopy before and after treatment to determine their state of preservation and the microscopic effects of the imposed treatments. The wood sources were found to vary in degree of degradation, but all were classified as moderately to highly degraded. Elevated levels of sulfur and ash and very small amounts of iron were found in the subsamples. Overall, PEG/freeze drying gave the best treatment results with virtually no collapse of the wood cell structure and the least amount of shrinkage. Air drying did not produce acceptable shrinkage results. Supercritical drying gave shrinkage results close to the estimated expected shrinkage, but cracking occurred on sets of samples from one of the wood sources. Cracking of these wood source samples also occurred in the air-dried set.

Original Publication Citation

Schindelholz, E., Blanchette, R., Cook, D., Drews, M., Hand, S., Held, B., Jurgens, J. & Seifert, B. An evaluation of supercritical drying and PEG/freeze drying of waterlogged archaeological wood (MT-2210-05-NC-10). National Parks Service, U. S. Department of the Interior. https://www.nps.gov/subjects/ncptt/upload/2007-04.pdf

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