A thermophysical study on the freeze drying of wooden archaeological artifacts

Abstract

Abstract The protection of unearthed waterlogged archaeological artifacts is a heavy task when large-scale construction is carried out in today's China. Freeze drying offers an efficient way for preserving wooden artifacts. In order to rationally design the freeze drying process, thermophysical studies were carried out in this paper. The measurements of water vapor diffusion coefficient and thermal conductivity were made. The impacts of cooling velocity and grain orientation on effective diffusion coefficient of water vapor, D, in dried wood were investigated. At low cooling velocity, 0.2 K/min, D was (4.8 ± 0.4) × 10 −4 m 2 /s in parallel grain direction, or (1.9 ± 0.4) × 10 −4 m 2 /s in perpendicular grain direction; at higher cooling velocity, 3.0 K/min, D was (4.3 ± 0.2) × 10 −4 m 2 /s in parallel grain direction, or (1.2 ± 0.1) × 10 −4 m 2 /s in perpendicular grain direction. The thermal conductivity of the dried sample was in the range of 0.78–1.06 W/(m·K). With the values attained, one-dimensional pseudo-steady analysis were made for the primary drying process of a cylinder and a flat slab. It was found that the process was mass-transfer controlled and the difference between surface temperature and sublimation temperature was very limited. Thus only mass transfer equation needed to be solved with assumption of constant sublimation temperature. The primary drying of a two-dimensional object was also simulated to predict the time required. Monitoring the surface temperature of each artifact and handling artifacts with similar drying time in a batch are proposed for the freeze drying of archaeological artifacts. The freeze-dried wooden artifacts by the authors yielded very small deformations.