Contributed by PETER IAN KUNIHOLM The Malcolm and Carolyn Wiener Laboratory for Aegean and Near Eastern Dendrochronology Department of the History of Art and Archaeology Cornell University
Many European paintings are painted on solid wooden panels or boards, typically oak for Netherlandish paintings. The wood is usually split radially so that, in ideal circumstances, a sequence of annual growth rings from pith to sapwood is present. These sequences are then matched, one against another, by the dendrochronologist and compared with growth sequences whose dates are known from living trees. Absolute dates can thus be assigned to specific annual rings. Sometimes the geographic origin of a board can be determined as well.
The Material: Oak is not only the most common support for European panel paintings, but is also the most easily dated because of the large number (now in the thousands) of oak panels and oak timbers that have been studied. Paintings also appear, but less commonly, on panels cut from beech, lime, fir, pine, spruce, poplar, and other species. These, too, can sometimes be dated, with the exception of poplar which tends to have enormous rings. For example, the so-called Mellon Madonnas in the National Gallery of Art in Washington, D.C., are painted on poplar panels, each ring of which is about an inch wide. Thus the total ring count for each panel is about two dozen rings, all the same size, and the paintings cannot be dendrochronologically dated. Since many Italian paintings are painted on poplar panels, den-206 drochronology is therefore an inappropriate analytical technique for art works south of the Alps. North of the Alps, however, and in the Low Countries in particular, dendrochronology has proven to be an extraordinarily valuable tool.
The Concept: The mechanism that makes tree-ring dating possible is that trees growing in the same climatic region (Europe) or subregions (Eastern Europe vs. Western Europe) respond in a recognizably similar fashion to the same general climatic stimuli. These stimuli may be temperature or rainfall or some combination of the two. In trees from the Eastern Baltic to the English Channel and even across into Great Britain and Ireland, adverse growing conditions will produce narrow annual rings, while favorable growing conditions will produce wide rings (Bail-lie, 1983). The sequences of such changes over time are unique. Moreover, subtle variations of ring growth, specific to a particular region, sometimes allow the dendrochronologist to determine the precise geographical origin of a panel (Eckstein, WaZny, Bauch, and Klein, 1986). For example, it has been shown that a large number of oak panels now found in a wide variety of Western art collections were cut from trees that grew in the Eastern Baltic (Eckstein, et al., 1986). The boards were shipped west from Gdansk (Danzig), bought on the Amsterdam and Rotterdam docks, and subsequently brought into the studios of Western European artists whose names are household words today. Panels painted on by the same artist or workshop have even been identified sometimes as coming from the same tree (Eckstein and Bauch, 1974; Klein, 1994). One has a mental image of, say, Rembrandt making his way down to the dockyard or to the shop of a middleman, buying a wagonload of oak boards imported from Poland, and bringing them back to his studio. While he paints on panel A, panels B, C, and D from the same shipment lean against the wall of his atelier. Which panel was painted first and which second, however, is beyond the scope of dendrochronology.1
The artist's taste or practice must also be considered. If a given artist has all of his known oeuvre on oak, and if a painting attributable to him were to appear on a poplar or lime panel, the investigator might well ask why there has been a new choice of material for the support.
The Technique: A transverse surface must be prepared across the end-grain of the board, which is usually not painted and is normally hidden by the frame. This surfacing is most easily done with a sharp razor blade. The annual rings are counted and measured with a microscope to 1/100 mm. Although early accounts of the dendrochronological analysis of panels show attempts at using a simple loupe or hand-measuring lens with 1/10 mm resolution as in Figure L.1, these have proven to be
'Twenty paintings by Lucas Cranach the Elder have now been reported as coming from the same beech tree (Klein, 1994, and personal communication). It is reasonable to assume that they must have been painted at approximately the same time.
relatively unsatisfactory, and the use of a proper microscope is the only acceptable method. For example, the only painted panel I have ever tried to date, a Byzantine icon on juniper boards (Figure L.1), is a classic demonstration of how not to measure panels. An appropriate microscope was not available in the museum, and one of the two boards had as many as 238 rings, some so small that the resolution of the hand lens was not enough to permit me to take sufficiently sensitive measurements. I think one panel had a last ring from 1539, but I cannot tell the date of the second panel until I go back and remeasure the icon with a real microscope.
These microscopic measurements are recorded and plotted on an x, y graph. This graph is then compared and matched with graphs from other panels and from living trees. Figure L.2 from top to bottom shows a cross section of a ninth-century oak board, the graph of its measurements, and a comparable graph from another dated board from the same period. Note how the alternately large and small rings can be matched from one tree to another. These patterns, or "signatures," are unique in time. Statistical methods have been developed to test the probability of any proposed match. A recent detailed exposition of the technique and its applications is by Schweingruber (1988).
In order for the dendrochronological method to be applied successfully, several preconditions are necessary. The sample should preferably have at least one hundred rings to prevent an apparently good but accidental and incorrect fit. The wood must have been cut radially (Figure L.3a). A panel cut in this manner not only provides a more stable surface for the painter but is less likely to warp and shrink than is wood cut tan-
gentially to the ring growth (Figure L.3b). The latter type of panel will usually not have enough rings to produce a sequence long enough for tree-ring dating. Obviously, the wood should be free of knots and other growth aberrations. Painters, at least those partial to oak panels, almost always are extremely particular in their selection of straight-grained, stable wood, and will avoid such blemishes in their choice of wood for the support.
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