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.

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.

Maritime pine (Pinus pinaster) wood has low dimensional stability and durability. Heat treatment was made in an oven using hot air during 2 to 24 h and at 170-200 ºC. A comparison was made against steam heat treatment. The equilibrium moisture content and the dimensional stability (ASE) in radial and tangential directions were evaluated at 35%, 65%, and 85% relative humidity. MOE, bending strength and wettability were also determined. At the same mass loss, improvements of equilibrium moisture content and dimensional stability were higher for oven heat treatment, but the same was true for mechanical strength degradation. A 50% decrease in hemicellulose content led to a similar decrease in bending strength.