4.5.1 Wood
4.5.1.1 General Characteristics of Wood
Wood is a material of natural origin, obtained by felling trees. Further processing of wood, cutting logs for lumber and sawing it into chocks and friezes, leads to obtaining semi-finished products necessary for the production of furniture. Due to the natural heterogeneity of wood, the presence of defects, sensitivity to changes in temperature and humidity of the environment, a tendency to warping and shrinking, the varied chemical composition and the diverse vulnerability for gluing and finishing with paint and lacquer, the selection of different species of wood when designing furniture should be particularly well thought out and based on thorough knowledge of the material. Because it is not possible to discuss in this work all the important properties of wood or criteria, which should guide the designer when selecting a specific species of wood, the author recommends that the reader become acquainted with other publications on the subject of wood, its structure and properties (Galewski and Korzeniowski 1958; Kollmann and Cote 1968; Kokocinski 2002; Krzysik 1978; Pozgaj et al. 1995), as well as works on gluing wood and finishing its surface (Pecina and Paprzycki 1997; Proszyk 1995; Tyszka 1987; Zenkteler 1996). However, other important aspects related to safety of using wood and its elastic properties needed when analysing the strength of furniture constructions have been presented below.
As a construction material, wood has always been and is the primary, and most popular material for furniture manufacturing. The conviction has been widely developed that it is an eco-friendly material, durable, safe and friendly for the health of the user. Unfortunately, not all species of wood have the appropriate references and not all of them should be applied in the same way. Therefore, when designing furniture, it is necessary to very carefully select particular species of wood, paying particular attention to species deriving from trees from tropical zones.
In the world, there are close to 30,000 species of woody plants, of which 5000 species can potentially be used in construction and ornaments (Kokocinski 2002; Krzysik 1978). On the global market of wood, however, there are only 250 species
registered, which are considered useful for industrial applications, including about 150 traditionally used in the manufacture of furniture.
The wood of various tree species recommended for use in furniture has significantly varying characteristics of mechanical, physical and chemical properties (Galewski and Korzeniowski 1958; Kokocinski 2002; Krzysik 1978).
Side components of wood, so-called unstructured, saturating in varying intensity of the wood tissue or filling the porous spaces, are called physiologically active, having a significant impact on the health of workers processing the wood and users of the finished products made of wood. Physiologically active compounds, found in many species of wood, can be a cause of metabolic and functional disorders, allergic and asthmatic diseases, poisoning and irritation in the digestive tract, as well as carcinogenic effects (Kokocinski and Romankow 2004). Therefore, choosing the right species of wood at the design stage of furniture should take into account not only the colour, drawing or relevant mechanical properties of the wood, but also its smell and potential harmful effect on the health of the user. The right choice of wood species constitutes a key element of the project, in which finishes using painting and lacquer coats are planned.
The main threat of wood on the skin can be caused by the juices or milky secretion of certain species of shrubs and trees, e. g. from the apocynaceae (Apocynaceae Juss.), euphorbiaceae (Euphorbiaceae Juss.), moraceae (Moraceae Link) and anacardiaceae (Anacardiaceae R. Br.), whereas the wood dust of araroba (Andira araroba) can cause rashes (Hausen 1981). Skin irritations in the form of burns due to contact with the same wood also occur. Wood species causing similar reactions are obeche (Triplochiton scleroxylon), larch (Larix decidua), swiss pine (Pinus cembra L.) and occasionally teak (Tectona L. f.) (Hausen 1981).
Table 4.2 provides information on selected major wood species indicated in the literature as toxic, irritant or sensitising (Hausen 1981).
