Citation: Mili´ c, G.; Todorovi´ c, N.; Veizovi´ c, M.; Popadi´ c, R. Heating Rate during Thermal Modification in Steam Atmosphere: Influence on the Properties of Maple and Ash Wood. Forests 2023, 14, 189. https://doi.org/10.3390/f14020189 Academic Editors: Miklós Bak and Róbert Németh Received: 7 December 2022 Revised: 30 December 2022 Accepted: 3 January 2023 Published: 18 January 2023 Copyright: © 2023 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/). Article Heating Rate during Thermal Modification in Steam Atmosphere: Influence on the Properties of Maple and Ash Wood Goran Mili´ c* , Nebojša Todorovi´ c, Marko Veizovi´ c and Ranko Popadi´ c Department of Wood Science and Technology, Faculty of Forestry, University of Belgrade, 11030 Belgrade, Serbia * Correspondence: goran.milic@sfb.bg.ac.rs Abstract: This study aimed to compare two thermal modification (TM) schedules—with short and long heating phases—and their influence on the properties of maple (Acer pseudoplatanus L.) and ash (Fraxinus excelsior L.) wood. Two TM runs were conducted in industrial conditions (open system, steam atmosphere; substantially longer method compared to the processes usually described in the literature), with the same peak phase (200 ◦ C, 3 h), but with different heating rates—slow (1.1 ◦ C/h) and fast (2.5 ◦ C/h). The results revealed that both TMs significantly reduced hygroscopicity and swelling of wood, but the influence of slow heating rate—through prolonged exposure of wood to relatively high temperatures—on dimensional stability was more pronounced. The modulus of elasticity, compressive strength and Brinell hardness remained mostly unchanged after TM (except for fast-modified maple), while the modulus of rupture was strongly reduced by TM in both species. It is assumed—at least in the case of maple wood—that a combination of initial moisture content above 8% and fast heating rate during TM can cause more intensive degradation of wood polymers. Relatively small differences in colour between slow- and fast-modified wood were found. The results confirmed the hypothesis that the heating phase is an important part of the TM schedule, and it can directly affect (together with peak temperature and time) certain wood properties. Keywords: thermal modification; heating rate; maple; ash wood; wood properties 1. Introduction The industrial production of thermally modified (TM) wood began in the 1990s in Eu- rope, and annual production gradually increased to currently more than 500,000 m 3 [1]. Thermal modification changes the properties of wood, mainly due hemi- cellulose degradation—it interferes with the load-sharing capabilities of the cell wall to decrease the strength and toughness of the wood, reduces the number of sorption sites for water and removes easily accessible nutrients for decay fungi [2]. The properties and characteristics of TM wood vary greatly, depending on the raw materials used [3], pro- duction methods (open/closed system; different treatment atmosphere) and conditions applied (treatment temperature and duration). Explaining the behaviour of TM wood as a time–temperature function was a topic of different studies [4–7], but due to numerous variables, a comparison of data reported in these studies is a difficult task. Most of the authors agreed that thermal degradation of the material modified under the same condi- tions is strongly dependent on the wood species and their chemical compositions [2,8]. On the other side, the same wood species behaves differently during modifications in different heat transfer media (steam, nitrogen, vacuum, oil, etc.) [1]. The variability of TM wood properties is even more pronounced on an industrial level due to uncontrolled variables [9] and dual coupling effects within boards and the stack [10]. Although there are some methods that can be used for quality control of TM wood, such as mass loss measurements [4,11], colour assessment [12,13] or spectroscopy techniques [14,15], one of the main difficulties remaining is to produce product with constant and controlled quality (durability, dimensional stability, colour). Forests 2023, 14, 189. https://doi.org/10.3390/f14020189 https://www.mdpi.com/journal/forests