Bog oak: Characteristics and characterization of a log from the Venetian Plain (Italy)

•Bog oak forms after a long persistence of oak wood in conditions of humidity and anoxia.•Density and mechanical strength increase.•Degradation phenomena are present in the apparently intact wood.•The different time of stay in the ground does not seem to influence physical parameters. The bog oak is a particular archeobotanical oak wood, which is formed after hundreds of years in anoxia and wet conditions. Following the persistence of oak wood for long periods in conditions of humidity and anoxia, its colour turns to dark brown/black, and its density and mechanical strength, in particular its hardness, increase. Wet and anoxic environments are, for example, the peat bogs of Northern Europe, and the muddy beds of rivers. In Italy, old oak trunks with a characteristic charred appearance emerge periodically during the cleaning of riverbeds. This paper presents the characterization of a bog oak log found in Zero river in the Veneto countryside. The trunk has been dated back to around 400 CE, using radiocarbon dating technique. The log is bark-free, and the black and cracked external surface has a charred appearance. On the inside, underneath the more weathered layer, the wood appears intact, without signs of apparent degradation, carbon black in colour. Towards the internal parts of the trunk, the colour of the intact wood goes from black to light brown. The physical characterization (basic density, shrinkage and maximum water content) indicates that degradation phenomena are also present in the apparently intact wood. Indeed, the bog oak linear shrinkage is more than double that of recent oak wood. Microscopic analysis clearly highlighted bacterial degradation. From the analysis of the ash content, it seems that the density increase of the apparently intact areas is due to the presence of inorganic material.