Proceedings e report - Firenze University Press

WOOD SCIENCE FOR CONSERVATION OF CULTURAL HERITAGE
Five from ten samples (beams) showed the divergence between wood density and latewood percentage
at the outer part (close to the bark) – like in Fig. 6. The others presented stable density trends and no
changes of the relationship to the latewood percentage.
4. Discussion
Five out of ten samples (beams) showed a decreased wood density and consequently a reduced
bending strength at the outermost part (close to the bark). The age of the samples (alos the origin)
were not related to the loss of density. There was no obvious attack of beetles or fungi. Obataya [2]
described an increase in bending and compression strength for the first 100 to 200 years. Afterwards
the strength is declining. Hoffmann et al. [10] also reported a decline in density and strength with time
at water logged wooden findings. Furthermore Bednar and Fengel [11] reported about decreased wood
density and strength at sub fossil oak. Schniewind [7] reviewed about a reduction of density up to 20%
(compared to recent material). Drdácký [12] found slightly lower density at the surface of about 200
year old Spruce beams than for the whole cross section.
The wooden material of these analyses was about 150 and 300 years old. Because of the long time
axes of the reviewed literature the comparison has to be done with caution.
For softwood, latewood percentage is a good predictor of wood density [5, 6, 8, 13, 14]. The
relationship between latewood percentage and wood density (mean ring density) was R² = 0.58 for
European Larch (more than 300 trees) and R² = 0.58 for Norway spruce (8 trees – data not shown).
The wood-chemical constitution is changing with time [2, 3]. Also cristallinity of cellulose get altered
[2, 15]. The wood anatomy and especially the amount of latewood can not be altered due to ageing
processes. Therefore, latewood percentage is a good predictor of age-related changes of wood quality.
50% of the samples showed a reduced wood density at the outermost part of the radius. This loss of
density was not dependent on the age of the beams. No explanation of this phenomenon can be given
at the moment.
5. Conclusions
Studying wood density and mechanical strength across the radius (from pith to bark) showed that at
50 % of the samples the density and strength is lowered at the outer part (close to the bark). Latewood
percentage was proved to be a good predictor of changes due to ageing effects.
To understand ageing processes better, more samples – and especially older samples – have to be
analyzed. As the method of x-ray densitometry is a destructive method (but low invasive), it’s hard to
get samples.
Aknowledgement
We are grateful to two Bachelor-Students (Julia Stifter and Andreas Daim), who did a big part of the
lab testing.
References
1. Binda, L., Drdácký, M., Kasal, B. (2007): In-situ evaluation & non-destructive testing of historic
wood and masonry structures. Institute of Theoretical and Applied Mechanics of the Academy of
Science of the Czech Republic. Praha, Czech Republic.
2. Obataya, E. (2007): Characteristics of aged wood and Japanese traditional coating technology for
wood protection. Conserver Aujourd`hui: Les „Vieillissements“ du Bois, Musee de la musique à
la Cité de la musique, 2007. Musee de la musique, Paris, France
3. Fengel, D. (1991): Aging and fossilization of wood and its components. Wood Science and
Technology 25(3) :153-177
4. Fengel, D.; Wegener, G. (1984): Wood – Chemistry, Ultrastructure, Reactions. Walter de Gruyter
, Berlin
5. Wimmer, R. (1994): Structural, chemical and mechanical trends within coniferous trees. In
Spiecker, H. Kahle, H.P. (Editors): Modelling of tree-ring development – cell structure and
45