Wood heat treatment has increased significantly in the last few years and is still growing as an industrial process to improve some wood properties. The first studies on heat treatment investigated mainly equilibrium moisture, dimensional stability, durability and mechanical properties. Mass loss, wettability, wood color, and chemical transformations have been subsequently extensively studied, while recent works focus on quality control, modeling, and study the reasons for the improvements. This review explains the recent interest on the heat treatment of wood and synthesizes the major publications on this subject on wood properties, chemical changes, wood uses, and quality control.

Eucalypt wood (Eucalyptus globulus) was heated in an oven for 2–24 h at 170–200°C and in an autoclave with superheated and saturated steam for 2–12 h at 190–210°C. The chemical composition of untreated wood and thermally treated wood with different mass losses in the range of 1.1–11.9% was studied by summative analysis, and the composition of dichloromethane, ethanol and water extracts was determined by gas chromatography mass spectometry (GC-MS). The hemicelluloses degraded first, mainly regarding the arabinose and xylose moieties. Lignin degraded at a slower rate and cellulose was only slightly affected under severe treatment conditions. The extractive content increased first with heat treatment and decreased later on. Almost all of the original extractives disappeared and new compounds were formed, such as anhydrosugars, mannosan, galactosan, levoglucosan and two C5 anhydrosugars. The most prominent lignin derived compounds were syringaldehyde, syringic acid and sinapaldehyde. The main difference between autoclave and oven treated samples was the appearance of more oxidized extractives for the oven treatment.

Heat treatment of Pinus pinaster and Eucalyptus globulus woods, two important species in Portugal, was made in the absence of air by steaming, inside an autoclave, for 2 to 12h at 190 ºC to 210ºC. Mass losses increased with treatment time and temperature reaching 7.3% for pine and 14.5% for eucalypt wood. The wood behaviour with moisture was improved. The equilibrium moisture content decreased by 46% for pine and 61% for eucalypt, the dimensional stability increased (maximum anti-shrinking efficiency in the radial direction of 57% and 90% for pine and eucalypt respectively) and the surface wettability was lowered. In relation to mechanical properties, the modulus of elasticity was little affected (maximum decrease of 5% for pine and 15% for eucalypt) but the bending strength was reduced (by 40% at 8% mass loss for pine and 50% at 9% mass loss for eucalypt wood).The variation of properties was related to treatment intensity and mass loss but significant improvements could already be obtained for a 3-4% mass loss without impairing the mechanical resistance. The response of eucalypt was higher than that of pinewood. Heat treatment of eucalypt wood shows an interesting potential to improve the wood quality for solid timber products. IntroductionThe first studies on heat treatment to improve dimensional stability of wood have been carried out by Stamm et al. (1946) Heat treatment reduces the equilibrium moisture content of wood (Jämsä and Viitaniemi 2001; Nakano and Miyazaki 2003; Gosselink et al. 2004;Wang and Cooper 2005; Metsä-Kortelainen et al. 2005) and improves its dimensional stability (Kollmann and Schneider 1963; Viitaniemi et al. 1997; Epmeier et al., 2001; Yildiz 2002; Bekhta and Niemz 2003; González-Peña et al. 2004; Wang and Cooper 2005) and rot resistance (Kim et al. 1998; Kamdem et al. 2002; Hakkou et al. 2006), thereby allowing treated wood to be used in less favourable 3 conditions and to compete with tropical wood of higher cost. Heat treatment also darkens the wood (Mitsui et al. 2001; Bekhta and Niemz 2003) which is an advantage for light coloured woods which are usually less appealing to the consumer. The major disadvantage is the reduction of mechanical resistance (Kim et al. 1998, Kubojima et al. 2000 Bengtsson et al. 2002; Reiterer and Sinn 2002; Epmeier et al. 2004; Unsal and Ayrilmis 2005; Johansson and Morén 2006) that can be an impediment for some uses. Although wettability decreases (Pétrissans et al. 2003; Hakkou et al. 2005a Hakkou et al. .2005b) the gluing process can be adapted for treated wood (Militz 2002)..In the last few years research on the heat treatment of wood has been active namely regarding the understanding of the chemical changes in treated wood (Kotilainen et al. 2000; Zaman et al. 2000; ; Sivonen et al. 2002; Wikberg and Maunu 2004; Nuopponen et al 2004a Nuopponen et al . 2004b Bhuiyan et al. 2005 Pinus pinaster and Eucalyptus globulus are two of the forest species with most planted area in Portugal. Pine wood is used for all kinds of carpe...

A hardwood, Eucalyptus globulus Labill., and a softwood Pinus pinaster Aiton., were heat treated at temperatures between 170 and 210ºC in an oven and in an autoclave. The samples were pre-extracted with dichloromethane, ethanol and water and ground prior to Fourier Transform Infrared (FTIR) spectroscopic analysis.The heat treatment caused significant changes in the chemical composition and structure of wood, in lignin and polysaccharides. Hemicelluloses were the first to degrade as proved by the initial decrease of the 1730 cm -1 peak due to the breaking of acetyl groups in xylan. Hardwood lignin changed more than softwood lignin, with a shift of maximum absorption from 1505 cm -1 to approximately 1512 cm -1 due to decrease of methoxyl groups, loss of syringyl units or breaking of aliphatic side-chains. The macromolecular structure becomes more condensed and there is a clear increase of non-conjugated (1740 cm -1 ) in relation to conjugated groups (1650 cm -1 ). However, the changes induced by the thermal treatment are difficult to monitor by FTIR spectroscopy due to the different chemical reactions occurring simultaneously.

Wood heat treatment has increased significantly in the last few years and is still growing as an industrial process to improve some wood properties. The first studies on heat treatment investigated mainly equilibrium moisture, dimensional stability, durability and mechanical properties. Mass loss, wettability, wood color, and chemical transformations have been subsequently extensively studied, while recent works focus on quality control, modeling, and study the reasons for the improvements. This review explains the recent interest on the heat treatment of wood and synthesizes the major publications on this subject on wood properties, chemical changes, wood uses, and quality control.