Furthermore, the statements below regarding the number of sap-
wood rings to be added are derived from statistical evaluation; each case
must be considered individually. In addition to the dependence of the
number of sapwood rings on the tree’s age, the provenance of oak wood is
also significant. In Europe, the number ofsapwood rings varies from west-
ern regions to eastern regions (Hollstein 1980; Baillie et al. 1985; Eckstein
et al. 1986; Kuniholm and Striker 1987; Lavier and Lambert 1996; Wazny
1990). With the elaboration of the new data (eastern provenance) for oak
panels, new evidence for the sapwood allowance has to be accounted for.
The number of sapwood rings found in trees from northern Poland was
analyzed; all trees in the central 50% had 13–19 sapwood rings; the median
value was 15, the minimum 9, and the maximum 36 (Fig. 7). For wood
originating from Germany or the Netherlands, the median value was 17,
with 50% of all values lying between 13 and 23.
To determine the earliest possible felling date, at least 7 or 9
sapwood rings (depending on whether the wood is of eastern or western
origin) must be added to the latest growth ring found on the panel. Using
the median, the felling date of the oak tree can be estimated with a span
of 2 2 to 1 4 or 1 4 to 1 6. If a panel is made exclusively of heartwood, the
felling date of the tree cannot be determined as precisely because there is
always the possibility that an unknown number of heartwood rings were
removed.
For beech (an all-sapwood species), however, the last growth ring
available for measurement corresponds in many cases to the last ring
formed in the living tree (and thus to the felling year). Usually, when
panels were made of beech, the entire tree was used, except for the bark,
which was removed. The same procedure can be verified for panels made
from conifer wood.
The determination ofthe felling date also provides information as
to the time the wood was seasoned before use in paintings. 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 has been
determined to be approximately two to eight years (Bauch, Eckstein, and
Brauner 1978). The few investigations carried out with signed and dated
panels of the fifteenth century do not yet permit such an accurate estimate
(Klein 1991). Instead, present studies regarding this period indicate a sea-
soning time of ten to fifteen years (Tables 1, 2), a finding that corresponds
to the results of analyses obtained from fifteenth-century panels of the
School ofCologne (Bauch, Eckstein, and Klein 1990). Similar investiga-44 Klein
Oak (Quercus sp.)PithHeartwoodSapwood
BarkFigure 6
Various methods of extracting boards from an
oak tree.
Figure 7
Distribution of the number of sapwood rings
in oak trees from northern Poland.