Menthol-Based Extraction of Fragile Wooden Coffin Lid (7–10th Centuries CE) in Laboratory Archaeology Excavation

Block lifting is a key step in stabilizing and removing fragile remains at archaeological excavation sites. Due to its favorable working properties and adhesive effect, menthol has recently been proposed as a volatile binding medium for temporary consolidation in archaeological conservation. This paper presents a case study on the use of menthol in the extraction and restoration of a large wooden coffin lid, approximately 1.9 m long and 0.9 m wide, from tomb 11 (M11) at Xie’ertala, located east of a Xie’ertala town in Hailar City, Inner Mongolia, dating to the 7th to 10th centuries CE. This coffin lid had fragmented into numerous wooden pieces, and was preserved in a relatively arid steppe environment, necessitating the extraction of the lid as a consolidated block. The use of menthol for consolidating and lifting the highly fragmented wooden coffin lid was intended to preserve critical archaeological information while avoiding damage to the underlying objects. An analysis of the physicochemical properties of these wooden remains suggests that the timber used for the coffin lid belongs to a common pine species from the Hulunbuir region. The degradation of the coffin lid was relatively mild, as shown by Fourier Transform Infrared Spectroscopy (FT-IR) and Scanning Electron Microscope (SEM) results. Dynamic Vapor Sorption (DVS) tests indicated that the hygroscopicity of the archaeological wood was 23.4%, compared to 21.1% for the reference sample, demonstrating good environmental stability. The safety of menthol as a treatment for fragile wooden remains was evaluated by comparing changes in the morphological and porosity characteristics of the coffin lid before and after menthol treatment. After treatment, the widths of the fissures remained largely unchanged, with all relative variations being less than 1%, and the porosity as well as pore size distribution of the wood showed negligible changes. Gas Chromatography–Mass Spectrometry (GC-MS) results showed that only 0.6% of menthol residue remained after 8 days of sublimation. This pilot study demonstrates that menthol is a safe temporary consolidant for block lifting and offers a promising alternative to the widely used cyclododecane. In conclusion, this research provided a new approach for conservators to safely lift similarly large and fragile wood remains during archaeological excavations.