SELECTION OF WOOD SPECIES FOR OUTDOOR SCULPTURE

Jorge Palacios

Thinking in terms of durability and long term conservation most types of wood can be considered suitable for setup in indoor locations, as long as conditions of stable humidity and temperature are met, as is the case in contemporary art museums.

Sculptural process of Jorge Palacios, wood selectionThis would not the case for outdoor setups, and it is particularly useful for me to analyze the information available to set and put in context these approaches, since amongst the hundreds of species found in wood trade and the more than 40.000 from which you can obtain wood, just a few hold a natural predisposition and might be perfectly suitable for outdoor use.

This does not mean that these few species are incorruptible, since nature itself takes care of reintegrating them in the biological cycle. If we add the knowledge about how the degradation of these particular species takes place I believe some minimal preventive measures can be taken to render their structural longevity far superior to the rest. Applying this is of enormous interest to me in the field of the creation of outdoor sculptures.

In my opinion, as a sculptor with a deep knowledge of the material, and putting it quite simple, wood cracks or breaks because it moves more than what it can withstand, because the forces due to hygroscopic exchanges with humidity in atmosphere are greater than the maximum tension the elasticity of this particular wood can endure. This constant movement generates tensions, and added up to different factors that weaken the wood end up generating checks and cracks, or widening existing ones derived from the growth of the tree or the seasoning process.

For a sculptor working for the outside like me it is extremely important to understand what happens in order to solve certain situations. That is why I consider it fundamental to comprehend that, among the factors capable of weakening the structure of the wood there are biological agents, i.e. chromogenic and rot-producing fungi, mildew, insects  – xylophages coleopterans and termites – and abiotic agents like decay due to light, fire, atmospheric humidity and temperature changes, on top of chemical agents.

Likewise the inherent anisotropic characteristics of wood make these tensions I was mentioning uneven along one piece, and only if it was independent and could move freely would it be able to cope with the forces its own movement generates, without endangering its structural integrity.

An anecdote illustrating the notable forces and tension wood is capable of refers to the Egyptians, who, as astonishing it may seem, used wooden wedges to split stones; when they were moistened, they generated such forces that the stone would break along the line of wedges disposed.

Cutting marks on the wood block for "Drop" sculpture by Jorge PalaciosWhen carrying out an open air sculpture parting from a solid block one of the basic principles I consider is that I am not working on one independent piece, but rather a collection of anisotropic parts of wood that generate different degrees of tensions due to their characteristic irregular movements. The success in setting up this block for sculptural purposes resides, according to my experience, in being able to minimize these tensions through two concepts: choosing a species with the highest dimensional stability and the design of the block (sawing strategy, seasoning and distributing the structures of the wood according to the setup of the block).

Thus, when choosing a species to sculpt my work, one of the top priorities I take into account is the different movements in axial, tangential and radial directions as well as the expansion and contraction coefficients. Not only should these values be as low, but also as similar as possible to each other.

To estimate and confirm what this means in practice and what implications these movements of the wood would have on a sculpture I have done an analysis on what dimensional changes different blocks of different woods would experience; 1m3 pieces undergoing an increase in internal humidity of 8%.

In this way I try to calculate how wood facing an extremely adverse situation would swell, starting from a 12 % internal humidity value, and considering that in standard atmospheric conditions in Europe the wood would hardly exceed a value of 20%. Therefore, this hypothetical situation would take place when the wood experienced an increase of 8% in its humidity content.

In this study, I have compared wood that presents highest movements (European beech) with that of one of the most stable species (standard teak), and with selected teak wood; we have obtained the corresponding values from tests performed at the Technological Research Centre Tecnalia Research & Innovation.

In the study, I have seen the usual dimensional changes in European beech wood*1, with a unitary tangential contraction coefficient of 0.50 and a unitary radial contraction coefficient of 0.30. In case of a humidity increase of 8% it would suffer an increase of 40 mm in tangential direction and 24 mm in radial direction. In comparison, we show the dimensional changes for standard teak wood*2; given a unitary tangential contraction coefficient of 0.27 and a unitary radial contraction coefficient of 0.14 it would suffer an increase of 21.6 mm in tangential direction and 11.2 mm in radial direction. Finally we show the changes that the teak wood I have chosen*3 would show; it has a unitary tangential contraction coefficient of 0.16 and a unitary radial contraction coefficient of 0.08, it would suffer an increase of 12.8 mm in tangential direction and 6.4 mm in radial direction; all these values are quite extraordinary and really uncommon and I must confess they made me really happy, since they were confirmed by the lab tests.

As a result of this study it appears that teak wood is a suitable candidate to carry out my sculptural work, due to its natural predisposition, the fact that it is highly resistant to abiotic and biotic agents – since it is so hard and chemically aggressive it is disliked by most xylophags and bacteria – and to the fact it presents exceptionally low expansion and contraction coefficients.

  • *1. Various authors. Especies de maderas. Madrid: Asociación de Investigación Técnica de las Industrias de la Madera, AITIM, 1997. p.354 – 356.
  • *2. Various authors. Especies de maderas. Madrid: Asociación de Investigación Técnica de las Industrias de la Madera, AITIM, 1997. p.650 – 652.
  • *3. Laboratory tests done by Tecnalia Research & Innovation according to UNE 56533:1977 on teak wood samples provided by Jorge Palacios.
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