PEER-REVIEWED ARTICLE bioresources.com Allegretti et al. (2012). “Thermo-Vacuum modification” BioResources 7(3), 3656-3669. 3656 THERMO-VACUUM MODIFICATION OF SPRUCE (PICEA ABIES KARST.) AND FIR (ABIES ALBA MILL.) WOOD Ottaviano Allegretti, a, * Michele Brunetti, b Ignazia Cuccui, a Silvia Ferrari, a Michela Nocetti, b and Nasko Terziev c The study presents results of the characterization of Norway spruce (Picea abies Karst.) and fir (Abies alba Mill.) wood thermally modified by TERMOVUOTO ® technology at temperatures in the range of 160 to 220°C in vacuum conditions. Sixteen thermo-vacuum treatment tests were carried out using a pilot laboratory unit on 30-mm-thick spruce and fir boards in various combinations of the process parameters, i.e. temperature (T), duration (t), and pressure (p). The treated material was characterized to reveal the changes of the physical- mechanical properties including color and durability. The treated wood showed an improved performance with relation to the dimensional stability and durability. The measured mechanical properties did not show any significant decrease. Analytical models, based on the existing correlations between wood properties and process parameters, were assessed, thus allowing the control of the process. Keywords: Color modification; Durability; Fir; Mass loss; Physical-mechanical properties; Spruce; Thermally modified wood; Thermo-vacuum method Contact information: a: CNR- IVALSA, Timber and Trees Institute; Laboratory of wood drying, San Michele all’Adige ITALY; b: CNR- IVALSA, Timber and Trees Institute; Firenze ITALY; c: Swedish University of Agricultural Sciences, Department of Forest Products, Uppsala, Sweden; *Corresponding author:CNR-IVALSA, via Biasi 75, 38010 San Michele all’Adige (TN), ITALY, allegretti@ivalsa.cnr.it INTRODUCTION Thermal modification is a widely used industrial process applied for altering wood properties with a temperature higher than 160°C. Thermally modified (TM) wood has become common in the market worldwide under an increasing customer demand for new wood products (Militz 2005). TM wood is characterized by mass loss that is an indicator of thermal degradation, decreased hygroscopicity, and improved dimensional stability and durability. The color of TM wood is also changed to darker tones that fade away when the timber is exposed to outdoor conditions if not properly protected with a coating. The modification has diverse effects on wood strength, which, in general, decreases and thus the TM timber cannot be used in construction. The above mentioned features are well documented in the literature; recently a very comprehensive literature review by Esteves and Pereira (2009) on many aspects of wood thermal modification has been published. The difference between the industrial TM processes is the medium in which the wood is treated, e.g. it could be steam, water, nitrogen, or oil. On-going research in the