We discussed how to extend the life of wood as a construction material and maintain its aesthetic properties through appropriate impregnation in a recent article. This time, the team of scientists together with Ing. Gašparík from the Faculty of Forestry and Wood Sciences at the CULS focused on the extent to which the colour fastness is affected by the high temperatures to which wood is subject during its processing.
The oak wood used for this research has always been one of the most sought after on the market. It is very popular in the furniture industry for its evenly coloured core, hardness, strength and durability.
The colour of wood is one of the main features of its quality. It is a factor that fundamentally affects the customer at his first contact with the product. And although this property is mainly the result of the structure and composition of the wood itself, it is also greatly influenced by production processes, such as heat treatment.
An experiment to evaluate these thermal effects was performed on two species of oak: a 75-year-old summer oak (Quercus robur L.) and a sub-fossil oak found in the floodplain of the Sura River in the Russian Autonomous Mari Republic. The samples of summer oak were divided into samples left naturally untreated and samples heat-treated. Sub-fossil oak was classified into groups A, B, C according to age and served only for comparison with the final shade of summer oak samples.
The process itself took place in cooperation with the Volga State University of Technology in the Republic of Mary, which provided a test room, measuring instruments and thermo chambers. In these "furnaces", the samples were exposed to temperatures of 160, 180, 210 and 240 degrees of Celsius for five hours and then allowed to cool slowly.
The colour of the wood changes after heat treatment, as the temperature triggers the degradation of the lignin, cellulose and hemicellulose components. In general, a permanent change in hue occurs in wood when exposed to temperatures in the range of 180-250 degrees of Celsius. After evaluation of the heat-treated samples, it turned out that the maximum colour change (darkening) occurred at the same time as the maximum temperature of 240 ° C. The overall colour change had a very rapid onset in the temperature range 160-180 ° C. At temperatures of 180–210 ° C, the process of colour change was tempered and graduated again at temperatures of 210–240 ° C. In this temperature range, the shade of the samples was almost identical to that of the sub-fossil oak, the darkening of which is the result of the fossilization process.
"The results of the experiment can be useful to eliminate the undesired transformation of the colour of the wood during its processing, and thus to preserve the natural visual appearance of the resulting products. Furthermore, it is possible to subjectively determine the treatment temperature according to the colour shade of thermally treated wood, and thus, for example, the suitability of using products from such treated wood, p“Ing. Gašparík suggests possible applications of the study.
* sub-fossil wood – wood in the process of petrification, as a result of conservation for hundreds to thousands of years in rivers or swamps under mud deposits without air access
Ing. Miroslav Gašparík, PhD.
He is a graduate of the Technical University in the field of Wood Engineering in Zvolen, where he then obtained a doctorate for his work "Plasticization of wood by microwave heating for bending purposes". Since 2013, he has been working as an assistant professor at the Department of Basic Wood Processing, Faculty of Forestry and Wood Sciences, CULS. The subject of his research is plasticization and shaping of wood and wood materials, as well as material handling and safety and health protection at work in the field of education.
Prepared by: Jakub Málek