Experimental Study On Vacuum Freeze-drying Of Archaeological Wooden Artifacts

Archaeological wooden objects are the witness of a country's or region's history. Along with our country's large-scale infrastructure construction, how to protect the unearthed archaeological wooden artifacts is urgent. Freeze-drying removes water by freezing it first then subliming the ice. This method has been used widely in food and biomaterial preservation, drug production, ultra-fine powder preparation, etc. In the field of archaeological artifact dehydration, freeze-drying is a kind of relatively novel method. Compared with other physical and chemical methods, it can effectively eliminate the cracking phenomena happened in the drying process, basically maintain the original features of the objects. The drying time is much shorter and the treatment has no weather and seasonal restrictions.In order to offer guidance to the batch freeze-drying of archaeological wooden objects by a large device adopting radiative heating, this paper studied the feasible process parameters for the freeze-drying archaeological wooden samples on a small self-made device adopting radiative heating. The experimental results indicated that wooden object with the size up to 72mm×62.5mm×124mm can be successfully freeze-dried after treating with 50% PEG2000 solution. The operation parameters are as follows:pre-freezing temperature at -35℃; pre-freezing time for 2h; vacuum at 7～10Pa; drying end point set as when the quality change in 1h is less than 0.1%.In order to obtain fundamental knowledge of the freeze-drying process of archaeological wooden objects, this paper also carried out tests on a freeze-drying device using conductive heating. Water vapor diffusion coefficient in the dried part was fitted using the experimental data. Error analysis was also conducted. While the vacuum was set at 10Pa and the samples were slowly pre-frozen, the water vapor diffusion coefficient at three primary drying temperature -30℃,-33℃,-35℃were 1.665×10-4m2/s,1.141×10-4m2/s,1.133×10-4m2/s respectively. While the vacuum was set at 10Pa and the primary drying temperature was kept at -33℃, the water vapor diffusion coefficient for the case of quick freezing was 49 percent of the case of slow freezing. These data can provide the help for optimizing the freeze-frying process, estimating freeze-frying time and improving the process economy.