Technical quality of oak (Quercus sp.) wood from archaeological excavations in Elbląg Old Town

Abstract

Experiments were conducted to determine the chemical composition and the structural, physical, and mechanical properties of historical oak wood from archaeological excavations in Elbląg (Poland). The small average ring width (1.72 mm) indicates that the foundation sill wood came from trees growing in natural forests. The wood fibres have an average length of 1.33 mm and a width of 0.0193 mm. The narrow lumen (0.0055 mm) and high cell wall thickness (0.0069 mm) contribute to the low elasticity of the fibres (0.2814). The chemical composition of the wood has undergone only slight changes, mainly due to a decrease in the content of substances soluble in water (especially cold), ethanol, and 1% NaOH. The cellulose content (38.43%) is close to that found in contemporary wood, while the content of pentosans (22.14%) is slightly lower, and the lignin content (27.35%) is higher. Contents of nitrogen, sulphur, chlorine, and the proportion of ash in the wood (1.8%) were shown to be higher. The wood has a high air-dry density (787 kg/m³), this being the result of penetration by inorganic substances. It also exhibited strong moisture-induced deformation (20.34% in the tangential and 10.18% in the radial direction). Static bending strength was higher (96 MPa), but compression strength parallel to the grain (50 MPa) and modulus of elasticity (11.006 MPa) were lower than in contemporary wood. The anaerobic conditions and a high groundwater level in which the wood had remained had not significantly impaired its technical quality, except for a large increase in wood shrinkage.