A question that is often put to me is why instead of sculpting directly into a big solid trunk I saw it into planks or strips and compose a block again. The fundamental reason for this is that it would be almost impossible to season (dry) it in a uniform way as a whole, and the wood would have different humidity percentages in the inner and outer parts. The excessive humidity in the interior could lead to rot too.
I still remember my surprise when I learned that with the traditional technology of wood seasoning in the drying chamber the deepest uniform drying possible was only 5 cm; now everything was clear. My constant obsession to find big trunks e.g. big trees knocked down by the wind or removed from public places, proved meaningless, since it would be impossible to season them correctly, and I understood this was the reason why wood was always found as planks in commerce.
Therefore, if our aim is to guarantee the structural integrity of the sculpture to be created from this material the most appropriate thing to do would be to saw the trunk into planks of a maximum thickness of 10 cm, achieving in this way that the innermost part of the piece would not be more than 5 cm away from the surface.
Nowadays there are several methods of seasoning, among others: drying chamber, vacuum seasoning, with microwaves or simply in the open air. Concerning this last method, if we tried to dry a trunk in a natural way in the open air, depending on species and climate, it would generally take about 1 year per each cm on average, rising to 2 years per cm from 10 cm depth on. This means that a 50 cm radius trunk would need about 90 years to reach an acceptable degree of humidity, but even so during this process it would undergo dimensional changes and crack due to the pressure appearing between the rings in the shrinkage and expansion processes.
Among the seasoning procedures I believe the chamber method is best, since you can control the temperature and humidity changes applied to the material with precision.
A fundamental premise of the seasoning is the need to adapt as much as possible the percentage of humidity of the wood as much as possible for the intended service and humidity during use in the location where it is going to be set up, since the bigger the difference between the humidity of the wood and the average annual humidity of the chosen location at the moment of setup, the bigger the risk the sculpture will endure during the acclimatizing time; its in this time that the wood will look to find by itself the hygroscopic equilibrium humidity (HEH).
To highlight the importance of this aspect one might think of the specialized transport of pieces of art between museums, where certain sculptures made of sensitive woods are moved in special climate-controlled chambers and where a very slow acclimatizing process to their new location is respected, in some extreme occasions for extremely delicate works, where the transport containment is open 1% a week before the piece of art is removed from it. In fact, one of the premises of museum conservation is the strict control of relative humidity and temperature of their premises, heavily insisting on the most important factor: the avoidance of radical changes of these parameters.
During this acclimatizing time to the conditions of use for a certain wood species the hygroscopic factor plays an important role, it is the ability of a species to exchange humidity with the atmosphere. One must understand that not all kinds of wood exchange humidity at the same rate; this is dependent on the intrinsical characteristics of the cellular walls of each wood, so according to each species the speed of this interchange will vary substantially. Although these small changes in humidity content may not be significant and affect the state of the piece of art in a critical way, you must consider that wood does not attain its Hygroscopic Equilibrium Humidity instantly, there is a certain delay in relation to the environmental conditions due to the inherent inertia in its permanent adaptation to the environmental conditions; in a non controlled climatic environment this generates an out of phase condition between the required humidity degree and the real degree attained by the wood, since usually climatic conditions vary faster than the time wood needs to adapt its humidity.
Another curious fact is that when the wood attains a humidity degree above 30% its cellular walls are saturated with water, thus even if it absorbed humidity above this value it would not undergo any substantial dimensional changes. On the other hand if humidity falls below 30% the cellular walls start dehydrating, and therefore a contraction proportional to the humidity loss will take place, reducing the global volume of the wood.
To finish I would like to insist on the fact that a correct method of sawing and seasoning of the wood may be just as important as the choice of the species we are going to work with.
As one may have noticed throughout these lines, as a sculptor I could talk for hours about a material I love and understand – wood. Allowing it the responsibility to express and voice the things that move me, and that I can hardly put into words.