Research in historical timber assessment is hindered by the limited availability of samples, yet understanding the fire resistance of historic wood is crucial for preservation efforts. There is an opinion that historic wood behaves similarly to contemporary wood in terms of fire resistance. The aim of this paper is to observe the rate of charring of historical pine wood during the experiment, the color changes in the sample that occurred due to thermal loading, and the changes in the chemical composition of pine wood. Test samples made from historic pine wood were loaded with a 50 kW∙m⁻² radiation panel for 60 min. The charring process was faster at the beginning of the charred layer formation. The charring rate at the beginning of the test at a depth of 10 mm from the exposed side reached values from 1.28 mm∙min⁻¹ to 3.16 mm∙min⁻¹. At a depth of 30 mm from the exposed side, the individual charring rates approached a value of 1 mm∙min⁻¹ (0.99 mm∙min⁻¹ to 1.08 mm∙min⁻¹). Observations during medium-scale testing revealed distinct layers forming on the exposed side: a charred layer, charring base, pyrolysis layer, and intact wood. The chemical composition of the wood changed under the influence of the thermal load. The relative contents of extractives and holocellulose decreased with the increasing temperature while the lignin content increased. The highest value of combustion heat was measured in the charred layer of the sample. Correlation analysis demonstrated a negative relationship between the combustion heat and holocellulose, while a positive correlation was found with the lignin content. Chemical changes were also monitored using the FTIR method. These findings provide valuable insights into the behavior of historic pine wood under thermal loading, which is essential for understanding and preserving historical structures.

The effects of thermal treatment on the changes in the chemical composition and higher heating values (HHV) of tropical hardwood meranti were investigated in a study. The samples of light red meranti wood (Shorea spp.) with dimensions of 20 mm × 100 mm × 700 mm were conditioned at 20 °C (control samples) and thermally treated at 160, 180, 200, and 220 °C. The chemical composition and HHV of control samples and thermally treated samples were evaluated. The chemical composition was measured using the procedures of Seifert, Wise, and ASTM. After thermal treatment, a significant reduction in holocellulose and hemicellulose content and an increase in extractives and lignin were observed. Consequently, we observed a rise in HHVs. HHV of thermally treated wood was strongly positively linearly correlated with lignin content (Pearson r = 0.9850, p < 0.001, R² = 0.9702, n = 15). Regression analysis showed that the model HHV = 0.1443(lig) + 16.012 is suitable for predicting the HHV of thermally treated wood, if the lignin content is known.

This study investigated microstructural, optical, and thermal changes in spruce wood (Picea abies) exposed to controlled laboratory heating to identify indicators associated with the transition from progressive thermal degradation to the initial char formation. Cubic specimens measuring 20 × 20 × 20 mm were exposed to selected temperatures between 240 and 300 °C under atmospheric conditions, with a 15 min isothermal exposure period. Microstructural changes were evaluated by scanning electron microscopy (SEM) and quantitative tracheid double cell wall measurements, supported by simultaneous thermal analysis (STA) and color and reflectance analyses. Simultaneous thermal analysis (TG/DTG/DSC) was performed on separate specimens from the same wood material to provide complementary thermal evidence. The most pronounced microstructural changes were observed between 250 and 260 °C, including substantial thinning of tracheid cell walls, degradation of bordered pits, increased brittleness, and localized structural collapse. Quantitative measurements showed reductions in double cell wall thickness exceeding 50% at 260 °C. TG/DTG analysis indicated the onset of intensive thermal degradation at 254.2 ± 1.48 °C, while optical measurements showed pronounced darkening and reduced differentiation of reflectance spectra above approximately 260 °C. The combined evaluation of complementary analytical methods indicates that the 250–260 °C interval represents a condition-dependent microstructural transition associated with accelerated thermal degradation and the early development of a charred structure under the applied experimental conditions. These findings provide complementary experimental evidence for interpreting the early stages of wood charring and may support the interpretation and future refinement of heat transfer and pyrolysis models. They complement, rather than replace, the conventional 300 °C engineering char line criterion used in structural fire design.