Review Thermally Modified Wood Exposed to Different Weathering Conditions: A Review Delfina Godinho 1,2, * , Solange de Oliveira Araújo 1 , Teresa Quilhó 1 , Teresa Diamantino 2 and Jorge Gominho 1   Citation: Godinho, D.; Araújo, S.d.O.; Quilhó, T.; Diamantino, T.; Gominho, J. Thermally Modified Wood Exposed to Different Weathering Conditions: A Review. Forests 2021, 12, 1400. https://doi.org/10.3390/f12101400 Academic Editors: Lina Nunes, Dennis Jones and Bruno Esteves Received: 15 September 2021 Accepted: 12 October 2021 Published: 14 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal; araujo@isa.ulisboa.pt (S.d.O.A.); terisantos@isa.ulisboa.pt (T.Q.); jgominho@isa.ulisboa.pt (J.G.) 2 Laboratório Nacional de Energia e Geologia, I.P. (LNEG), Estrada do Paço do Lumiar, 22, 1649-038 Lisboa, Portugal; teresa.diamantino@lneg.pt * Correspondence: delfina.godinho@lneg.pt Abstract: Outdoor wood applications are exposed to several different biotic and abiotic factors, and for that reason, they require protection to increase their service life. Several technologies of wood protection are already commercialized. One of these technologies is thermal modification, which refers to the structural, mechanical, and chemical transformations occurring in the lignocellulosic material when gradually heated up to specific temperature ranges. In the past few years, several researchers have undertaken weathering resistance evaluations on different wood species. Some cases have considered natural exposure in different countries with different climatic conditions, while others focused on artificial exposure under UV and xenon radiation tests. Most works evaluated the weathering effects on the chemical, mechanical and physical, and anatomical shifts compared to the original characteristics of the material. This review has established a considerable lack of studies in the bibliography focusing on abiotic factors, such as the industrial and maritime environment, or even isolated climatic factors such as salt spray (simulating maritime environments) or pollutant gases (simulating industrial environments). This lack of information can be an opportunity for future work. It could help to understand if thermally modified wood is or is not sensitive to pollutant gases or salinity, or to a combination of both. By knowing the degradation mechanisms caused by these factors, it will be possible to study other forms of protection. Keywords: degradation; abiotic and biotic factors; thermal modification; weathering; wood 1. Introduction 1.1. Context Wooden constructions are usually characterized by different components and treat- ments that together offer the best possible properties of load-bearing capacity, thermal, acoustic, moisture insulation, fire resistance, and long service life. Increasing the proportion of wood in constructions can reduce the quantities needed of other construction materials, such as concrete, steel, and bricks. These construction materials do not come from renew- able raw materials. They require a great deal of energy for their production and entail higher carbon dioxide emissions [1]. There are a wide variety of applications of wooden structures, and nobody questions the value of the majority of these products [2,3] for building materials, including indoor and outdoor products, structural frames, window and door frames, floors, and façade systems [4–6]. However, wood is a complex biological tissue composed of different cell types [7]. This cellular diversity, with a diverse molecular structure, largely determines the physical and mechanical properties and profoundly influences the performance of wood as a construction material and its demand as such [8]. Therefore, knowledge of the wood structure and species identification is essential [1,9]. Forests 2021, 12, 1400. https://doi.org/10.3390/f12101400 https://www.mdpi.com/journal/forests