Alar Läänelaid is a docent of landscape ecology at the Institute of Ecology and Earth Sciences of the University of Tartu.
We’re all used to bar codes in supermarkets. In a way, the patterns of annual rings of trees turn out to be bar codes too, as they contain information about the condition the trees have grown in.
The only difference is that the bar codes of trees have a healthy portion of individuality, so distilling data from them is a bit harder than creating beeps with a bar code reader at a supermarket checkout. It’s amazing that the patterns of annual rings also contribute to art history.
What can the bar codes of trees show us? The width of an annual ring is influenced by climate. That is the key scientists have learned to utilize. As the saying goes, years are no brothers, and that’s why a unique pattern of wider and narrower annual rings forms in the trunk of a tree. When we have a long enough succession of annual ring widths, composed by the patterns of many trees, we can use it as a blueprint to detect an unknown individual succession.
Original work or a copy?
Now it’s time to link the biological part to the art. In past centuries, wooden panels were used for painting. Thin planks of oak were glued together edgewise. One can find many paintings without signatures in art galleries, with their closer origin shrouded in mystery. Paintings by famous artists were often imitated to multiply a piece of work.
Here art history can use some help from dendrochronology – the science of annual rings. The pattern taken from the edge of a painting panel determines if the time decoded in the rings fits the estimated date of creating the work. If not, the work comes from a different period and could be either a copy or a knock-off.
Amongst many other things, the Geography Department of the University of Tartu is involved with dendrochronology. It is an area that requires active international co-operation. Pieces of art were sold and they could end up in different countries.
But even more exciting is the fact that today, researchers of annual rings can determine the growth region of wood used in works of art. It’s a known fact that already many centuries ago, timber became scarce in Western Europe.
Wood from the Baltic countries
In the 16th and 17th centuries, wood was imported from areas with lots of forests that were mainly located in Eastern Europe. The southern Baltic towns of Danzig and Riga were important timber export harbours. But the timber itself was rafted to these centres from remote hinterlands, such as Belarus and Ukraine.
Scientists at the University of Tartu are analysing paintings of Dutch artists, preserved in the Art Museum of Estonia. Examining the annual rings makes it possible to determine the growing years of trees used for making the painting panel.
The source region of trees is determined by comparing the tree rings in the painting boards to various comparative sample tree rings.
Data about the amount of sapwood (light-coloured annual rings under the bark) in different growing regions can also be used. As the light-coloured rings are trimmed away in the course of panel-making, estimating their approximate number helps to specify the painting’s age.
A recent international co-operation involved investigation of four paintings with the same theme, „Christ expelling money-lenders from the temple“, attributed to the Bosch/Bruegel school. One of these paintings is in Tallinn, another in Glasgow, the third one in Copenhagen and the fourth belongs to a private collection.
Dendrochronologist Aoife Daly from Dublin and myself measured the widths of annual rings in the edges of the painting boards. It allowed us to detect the approximate times when three paintings out of the four were produced. To achieve this, we exchanged the data and compared the results. It turned out that the copies in Tallinn and Copenhagen are from the same time, both painted after 1562, whilst the painting panel from the private collection appeared to be about thirty years older.
Who’s the author?
The results showed that none of the paintings could have been produced by Bosch himself.
Many kinds of studies in the fields of art history and material science make dendrochronologial research studies more complete. The age of paintings influences their value significantly as well.
Sometimes unexpected details can come from studying painting panels. For example, the difference in the dates of two planks used for the panel of a still life by Clara Peeters posed a problem: An initial study of the annual rings showed that the oak used for one plank was chopped down 150 years before the oak of the other plank. The mystery was solved by Peter Klein of Hamburg.
He applied his professional glance to the graphs of widths of the annual rings and figured out that both planks were the result of sawing one wide oak plank lengthwise into two halves. So the dates were bound to differ as one succession of annual rings started where the other ended. The painting by Peeters received an accurate date that was in accordance with the creative period of the artist.
When it comes to the Baltic countries, dating Flemish masters’ painting panels made of oak has a certain emotional resonance. The wood that was used for making painting panels in the Netherlands was often imported from the Baltic countries. In a way, as pieces of art painted on the panels, the oaks have arrived back home.