Modern wood research can make an important contribution to archaeology through its many different analysis methods, including techniques to identify specific wood types and to date wood finds. This is particularly relevant in Hallstatt, where a large number of prehistoric objects made of wood have been found in excellent condition, mainly in the mines themselves.
Reference curves in dendrochronology
Profile of the reference curve for Hallstatt
Dating of wood tapers
Dating of special finds using CT scans
Wooden finds can be dated precisely (with one year of resolution) with the help of dendrochronology (tree ring analysis). In temperate climate, every year a living tree forms an envelope of cells around the body of the tree. A cold winter or a rainy summer will have different effects on the life and growth of a tree. A fir will grow better in a warm and rainy summer than in a hot and dry one. The differences between the good and bad years can be seen at the width of the growth rings.
The sequence of the rings formed by a tree throughout its lifetime is a very characteristic feature. Dendrochronological dating is based on an unbroken chain of growth ring sequences, a reference curve or chronology leading back into the past, whose starting point is the present. By assembling many growth ring sequences of individual trees and single samples from historic buildings and archaeological excavations, this curve can be extended into the past. If we want to date a wooden item, we have to compare ist growth ring sequence with the reference curve and determine ist exact position on this curve and thus its age. Statistical parameters give hints of possible synchronous layers that are checked visually. If the analysed wood samples still show bark ('wood wane edge'), it is possible to determine the exact year the tree was felled.
A specific reference curve had to be established for the dating of wood from Hallstatt. The potential area from which round wood was brought to the Hallstatt High Valley extends from an altitude of 900 m to 1600 m above sea level. Various curves already existing in laboratories and dating back to the Bronze Age were either been established for considerably lower altitudes - for instance wood from German or Swiss lakeside settlements - or for much higher altitudes near the timberline.
The first step in establishing the chronology was the sampling of living trees (for example in the Sattelalm area), historic buildings and prehistoric wood. There is, however, a gap of approximately 2550 years between wood preserved in historic buildings (the oldest dating from the 12th century AD) and Bronze Age wood items (16th to 13th century BC). To cope with this, dendrochronology has taken advantage of the fact that certain mountain lakes and bogs have preserved tree trunks fallen into them hundreds or even thousands of years ago.
For the Hallstatt reference curve, tree trunks preserved in the Schwarzer See Lake were recovered and sampled in 1999. The analysis of more than 200 samples from tree trunks resulted in the establishment of a mixed chronology of spruce and larch, dating back continuously to the year 1474 BC. In 2004, more than 300 wood samples were recovered from the Karmoos in an attempt to improve this chronology. The Karmoos is a raised peat bog situated at an altitude of 1390 m, above the High Valley.
It was possible to date this series of growth ring widths using the Schwarzer See chronology, thus establishing from spruce wood a continuous Karmoos chronology back to 1523 BC. We now have a reference chronology for prehistoric wood finds from the Hallstatt salt mining area, and we were able to date the major part of the prehistoric wood from the Christian von Tuschwerk, including not only the oldest wooden staircase preserved in Europe (1344 and 1343 BC), but also wood waste and tapers.
There was also a high interest in dating tapers. Some of these small wooden fragments split from the outer part of the tree trunks, mainly from firs, show up to 80 growth rings. The width of the growth rings is measured on the finely sanded cross section. Only the cross section will clearly reveal the growth rings. To measure the width, the sample is placed on a measuring table and put under the microscope. To make sure that all growth rings, some of which are formed by just a few cells, are detected and measured, every single cell of the wood must be recognizable. On the basis of certain features, for example the existence of resin ducts, it will also be possible to identify the wood species.
The actual dating entails several steps. After the measurement, we must check whether there is matching wood from the same excavation area. If this is the case, it will be possible to establish mean curves from the synchronous ring width sequences, even if such curves cannot yet be associated with calendar dates. The dating of the wood from the Christian von Tuschwerk followed this procedure. More than 100 samples were analysed in our laboratory and synchronised into a mean curve covering 282 years. On the basis of the Dachstein-Hallstatt chronology, the end year could be dated unambiguously to the year 1245 BC.
Obviously, special wooden objects, for instance wooden vessels or pick handles, must not be sampled. In these cases, computer tomography is a possible method for measuring the width of growth rings, and dating tests have been made at the University of Applied Sciences in Wels and in Nara, Japan. The work has yielded unprecedented results: it was possible to date among other items an axe handle from the Christian von Tuschwerk. The beech from which the tool handle was made was felled in 1403 BC.