Dendrochronolgy – Art objects

Welcome to Dendro & Art. Dendrochronolgical analyses on art objects Dendrochronology is a discipline of biological sciences which determines the age of wooden objects. This method is used primarily for dating archaeological and architectural objects, but may also aid in investigating art-historical problems The main goal is to offer at least a terminus post-quem for the creation of a painting by determining the felling date of the tree which provided the wood for the panel. Dendrochronology focuses on the annual periodicity of growth which is controlled by the climate, e.g. temperature and rainfall. In cool and temperate climates, there is a dormant season from autumn to spring and a growth season during the summer. When the vegetative period begins, new cells for the transport of water from the roots to the top of the tree are formed. During the summer, the so called latewood formation starts, and around the middle of September the radial growth of the tree stops and rests up to the next spring. The result is an annual ring or tree ring. Besides the differences in structure, two species of wood differ physiologically: in ring-porous wood the latest growth ring fulfils the major task of water transport and, consequently, a new ring must be formed every year. In diffuse-porous woods and in conifer wood, some growth rings participate in the water conduction. Hence, under adverse climatic conditions, the trees do not need to form a growth ring every year and may be characterized by totally or partially missing rings. On the other hand some growth increments may be formed in one year. These occurrences make the determination of growth rings and dendrochronological work with diffuse-porous species more difficult than with ring-porous species such as oak. The biological regularity of the ring series in trees from temperate zones permits dating of wood by comparing the ring sequences of undated wood with those of wood of known age and position in time. In order to establish comprehensive continuous growth ring curves for periods longer than a tree’s lifetime it is necessary to use an overlapping system of individual curves for the establishment of master chronologies; in Europe trees normally live only 200 to 300 years. Standard curves exist for South and West Germany, for several regions of North Germany and, in part, for several areas in the Netherlands and France, and also for the Baltic area from which the wood for most Flemish and Dutch paintings was obtained. In order to determine the ring widths one can use a magnifying glass with an integrated scale. This method is used if measurements have to be taken on site without laboratory equipment. It is more convenient to take measurements in the laboratory using a machine. The laboratory equipment can be connected to a computer to record the data immediately after measurements are taken for use in the next steps of the analysis. In some cases the measurements may be taken from x-rays. Digital cameras may also be used to take photographs and to measure the rings from the photographs. However, in all cases it is necessary to clean the edges of the panel for a full recognition of the rings. The purpose of cross dating is to find out if and to what degree the two sequences match. In general terms this would mean the placement in time of one ring series relative to the other. If one of the curves is attributed to a definite stretch of time, the positioning of the second curve by maximum coincidence leads to absolute dating. For each kind of wood, a master chronology must be established for different regions. Problems Problems encountered in the course of dendrochronological work involving the biological material as well as the methodology include: • Conifers such as spruce or diffuse-porous broad-leafed trees such as lime may not produce a tree ring every year; the missing data may prevent precise dating. • Sometimes the condition of a sample does not permit the determination of ring widths, as in the case of sapwood which may have suffered attack by insects, bacteria, or fungi or may have collapsed due to excess drying. In these cases the number of rings cannot be accurately determined and no precise dating can occur. • For the cross dating of curves, a minimum number of rings must be present in order to obtain reliable results. Unfortunately, it is not possible to list a precise number as the required minimum. Even curves considered quite ‘long’ sometimes do not provide the necessary characteristic patterns which would help to date the curve. There are many variables involved; sometimes dating is possible with as few as fifty tree rings, but sometimes even one hundred rings may not be enough. It all depends on the quality of the sample. Sapwood estimation and seasoning Identification of the year the tree was felled is the most important information the art historian can learn from the dendrochronologist. If the last ring under the bark has been preserved, it provides the exact date—even the season–when the tree was cut. In preparing oak panels for paintings, panel makers usually cut normally the planks radially. The bark and the light, perishable sapwood were mostly cut away, thereby eliminating evidence of the latest growth rings and making a determination of the exact felling year impossible. Only the latest measured growth ring of the panel can be determined to the exact year. In the fifteenth and sixteenth centuries, the presence of sapwood rings is rare, but in the seventeenth century, a lot of sapwood rings may be present. On the other hand, the records regarding the number of sapwood rings to be added are derived from statistical evaluation and must be considered in each particular case. In addition to the dependence of the number of sapwood rings on the age of the tree, the provenance of the oak is significant. In Europe the number of sapwood rings varies from western regions with numbers between 7 to 50, to eastern regions with 9 to 36. The number of sapwood rings found in trees from the Baltic regions was analysed with the result of a median value of 15. 90 % of all trees had 11 to 22 sapwood rings; the minimum was 9; the maximum was 36 (fig. 1). For wood originating from Germany or the Netherlands, a median value of 17 with 90 % of all values lying between 12 and 26 was determined with a minimum of 7. In order to determine the earliest possible felling date at least 9 or 7 sapwood rings (eastern or western origin) must be added to the latest growth ring found on the panel. The determination of the felling date also provides information regarding the length of time the wood was seasoned before use. For oak panels of the sixteenth and seventeenth centuries, in most cases the interval between the felling of the tree and the creation of the painting was approximately 2-8 years.