Biological, chemical and technological characteristics of waterlogged archaeological piles (Quercus petraea (Matt.)Liebl.) of a medieval bridge foundation in Bavaria

Abstract

The biological, chemical and technological study of waterlogged archaeological piles of oak wood (Quercus petraea (Matt.)Liebl.) of a medieval bridge in Bavaria was primarily undertaken in the context of finding an adequate drying method to assure stable long-term conditions while presenting the archaeological finds in a local history museum. There was a unique chance to study the oak wood preservation of five generations of a bridge foundation from one and the same place. According to the dendrochronological dating of the felling dates of the oaks, the first identified generation is already medieval with an earliest date of 1447, followed by the second generation at about 1572, with the third one at about 1658, the fourth one at about 1727 and the youngest one at about 1787. The experienced selection of the trees in the forest and the subsequent careful manufacture of the piles without any sapwood and obviously their immediate application allowed for studying the alteration of the wood characteristics with increasing years of underground pile exposure. The light microscopical study did not reveal remarkable structural alterations of lignin and cellulose. Microbial activities of erosion bacteria and soft rot could only be detected within the uppermost rounded part of some piles and in their periphery. However, it was informative to see how green and wet manufactured oak wood responds as a sensitive ion-exchange system underground. The iron ions penetrated particularly into the outer heartwood and reacted with the special accessory compounds, such as tannins, essentially within the parenchyma cells. However, the radial distinct gradient had no notable difference between the fifth generation (1787) and the older ones. The uptake of Si-, Ca- and Mg-ions significantly represents an influx with a radial penetration even to the pith. Surprisingly, the monomeric K-ions, which are present in the green woody tissue with a mean value of 330 ppm moved radially out of the wood, down to about 10% compared to the beginning of the underground exposure. Moreover, the shift of the original pH value of about 3.6 of the heartwood to finally a mean of pH 4.6 clearly underlines a correlated efflux effect, which might slightly chemically alter the woody tissue. The changes of the chemical and technological characteristics such as basic density, compression strength, sorption behavior, shrinkage, and ash content actually indicate that over the centuries some change in the chemical characteristics of the heartwood must have occurred.