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Article

Advanced Evaluation of Fire Resistance in Spruce Wood (Picea abies spp.) Treated with Innovative Surface Coatings

by
Redžo Hasanagić
1,*,
Selma Mujanić
1,
Eli Keržič
2,
Leila Fathi
3,*,
Mohsen Bahmani
3,
Mohammad Dahmardeh Ghalehno
4,
Boštjan Lesar
2 and
Miha Humar
2
1
Department of Wood Science and Technology, Faculty of Technical Engineering, University of Bihać, 77000 Bihać, Bosnia and Herzegovina
2
Department of Wood Science, Biotechnical Faculty, University of Ljubljana, 1501 Ljubljana, Slovenia
3
Department of Wood and Furniture Industry Engineering, Faculty of Natural Resources and Earth Sciences, Shahrekord University, Shahrekord 64164478, Iran
4
Department of Wood and Paper Sciences and Technology, University of Zabol, Zabol 9861335856, Iran
*
Authors to whom correspondence should be addressed.
Fire 2025, 8(4), 120; https://doi.org/10.3390/fire8040120
Submission received: 3 March 2025 / Revised: 18 March 2025 / Accepted: 19 March 2025 / Published: 21 March 2025
Browse Figures
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Abstract

:
This study investigates innovative surface coatings’ effectiveness in enhancing spruce wood’s fire resistance (Picea abies spp.). Spruce wood samples were treated with various agents, including oils, waxes, boric acid, commercial coatings, and fire-retardant agents. The evaluation was conducted using the small flame method (EN ISO 11925-2:2020), surface roughness analysis, hyperspectral imaging (HSI), and contact angle measurements. The results demonstrated significant improvements in fire resistance for samples treated with specific coatings, particularly the Burn Block spray and Caparol coating, which effectively prevented flame spread. The analysis revealed that the Burn Block spray reduced the average flame height to 6.57 cm, while the Caparol coating achieved a similar effect with an average flame height of 6.95 cm. In contrast, untreated samples exhibited a flame height of 9.34 cm, with boric acid-treated samples reaching up to 12.18 cm. Char depth measurements and the surface roughness analysis revealed a clear correlation between the type of treatment and the thermal stability of the wood. Hyperspectral imaging enabled a detailed visualisation of surface degradation, while contact angle measurements highlighted the impact of hydrophobicity on flammability. This research provides in-depth insights into the fire-retardant mechanisms of spruce wood and offers practical guidelines for developing safer and more sustainable wood materials for the construction industry.

1. Introduction

Wood elements have become an integral part of building structures’ internal and external components [1,2]. However, the improper use of wood can lead to issues, as wood exhibits significant variability in its physical properties, mainly dependent on moisture content and load duration [3,4,5,6,7]. The well-known theories of wood flammability [8,9] and its limited fire resistance [10,11,12] necessitate the application of appropriate surface treatments for wooden elements [13]. The purpose of a wood surface treatment can vary. As protective elements, a particular group of coatings are fungicidal coatings [14]. The fire performance is associated with several terms, namely the following: flame retardants (flame retardants are chemicals added to materials to prevent or slow the spread of fire); fire protection (fire protection involves measures and practices designed to prevent, control, or mitigate the effects of fire to protect people, property, and the environment) and fire resistance (fire resistance is the ability of a material or structure to withstand fire and maintain its structural integrity and function for a specified period of time).
The devastating impact of the London Fire highlighted the critical need for enhanced fire protection in wooden constructions. Wood, being highly combustible, requires effective fire-resistant treatments and building techniques to slow the spread of flames and maintain structural integrity. The adoption of comprehensive building codes and standards for wooden structures is essential to ensure they can withstand fire incidents better [15]. Additionally, incorporating advanced fire detection and suppression systems can provide crucial time for evacuation and help manage fires more effectively in wooden buildings.
Wood fire protection began with clay and mud plaster coverings in ancient times. In the 18th and 19th centuries, intumescent paints and salt solutions offered improved methods. The 20th century saw the advent of fire-retardant chemical treatments that reduce wood flammability. Today, protecting wooden structures involves advanced chemical treatments, fire-resistant designs, and strict building codes to enhance safety and durability.
Fire retardant substances are typically available in liquid, gel, foam, and powder forms, tailored to materials that differ in physical properties and chemical compositions. Most fire retardants act synergistically to enhance fire protection [16]. Since the components in fire retardants react with fire in different ways depending on the material, the selection of fire retardants must respect the type of material used [17]. Many studies focus on wood retardant treatments with detailed explanations of the application methods [18], including both inorganic [19,20,21] and organic [22,23], as well as plant oil-based treatments [24,25,26].
Cellulose molecules are chemically simpler than most polysaccharides and other natural polymers. However, cellulose obtained from different living species exhibits remarkably diverse properties due to its hierarchical self-assembly [27,28]. Similarly, the properties of spruce wood (Picea abies spp.), particularly in terms of its fire resistance, vary depending on the applied treatments and surface coatings. Research into innovative surface treatments that enhance the fire resistance of spruce wood has become a key area in modern wood industry technology, given the growing interest in improving wood’s mechanical and protective properties [29,30,31,32,33].
Understanding the mechanisms behind self-assembly, the interactions between surface coatings and wood, and the impact of these coatings on fire resistance has become a central focus of research in this field. By applying innovative surface coatings, it is possible to create materials that enhance fire protection and increase the longevity of wood through an efficient fire barrier. These coating materials, such as those based on graphite, hydrated compounds, or silicates, show significant potential for improving the fire resistance of wooden materials [34,35,36,37,38,39].
Although innovative coating materials have already demonstrated their capabilities in fire protection, challenges, such as the environmental impact, costs, and product efficiency, still present barriers to their widespread application [40,41,42,43,44,45]. Given these challenges, research on new and enhanced treatments for spruce wood aimed at improving its fire resistance continues to be a focus in the industry, along with the ongoing development of new materials and methods for enhancing fire protection [46,47,48].
It is important to note that this study focused on conventional coatings, while advanced treatments, such as nano-material-based coatings, were not included. Given their potential to enhance fire resistance due to their high surface area and thermal stability, future research will aim to explore the effects of nano-coatings on spruce wood. This paper discusses the advanced evaluation of fire resistance in spruce wood treated with innovative surface coatings, adding a new dimension of fire protection to this multifunctional material. This research aims to demonstrate how various surface treatments can improve the fire resistance properties of spruce wood, creating adequate fire protection that is crucial for the safety of wood applications in the construction industry.

2. Materials and Methods

2.1. Wood Samples

To investigate the fire resistance of spruce wood (Picea abies) treated with innovative surface coatings, a total of 130 standardised wood samples measuring 250 mm × 90 mm × 20 mm were prepared (Table 1). The samples were sourced from mature spruce trees grown in the Una-Sana Canton, in the northwest region of Bosnia and Herzegovina. This area, known for its diverse forest ecosystems, provided a consistent and reliable source of wood with uniform density and moisture content. The geographic coordinates of the sampling area are approximately 44.771° N latitude and 15.923° E longitude.
To ensure consistency in this study, the wood was conditioned to a moisture content of 12% before treatment, following standard conditioning protocols used in wood technology research. This moisture content was maintained throughout the treatment and testing phases to accurately evaluate the fire resistance and other properties of the treated wood samples.
The wood samples were systematically divided into several groups based on the type of processing and applied coatings to enable a precise analysis of their properties (Table 1). Each group of samples had 10 replicates. The first group was processed solely by sawing. In contrast, the second group was further treated with orthogonal cutting to achieve a smoother surface. The third group of 10 samples underwent thermal modification using the Silvapro technique at the Biotechnical Faculty in Ljubljana, employing a temperature of 220 °C for 3 h [49,50].
The remaining 100 samples were divided into 10 groups, each treated with a different coating. This research focused on commercial coatings available in the market, specifically for testing the fire resistance of the materials.

2.2. Applied Treatments

Various surface treatments were applied to the spruce wood samples, including natural and synthetic oils [51,52,53], waxes [54], chemical agents [55], commercial coatings [56,57], and fire-retardant sprays [58]. The amount of coating applied per square meter to the wood samples varied, with the SPBB sample showing 239.72 g/m2, the SPBK sample 256.42 g/m2, the SPPCS sample 264.53 g/m2, the SPUB sample 295.39 g/m2, the SPV sample 303.66 g/m2, the SPPC sample 255.79 g/m2, the SPUW sample 261.47 g/m2, the SPVS sample 1041.13 g/m2, the SPUE sample 278.14 g/m2, and the SPPD sample 236.57 g/m2. Each treatment was selected for its specific protective properties and applied in accordance with the manufacturer’s guidelines to ensure consistent results. The following surface treatments were applied to the wood samples as described in Table 2.
Each treatment was applied following the manufacturer’s guidelines. Before applying the coatings, the samples were planned on all four sides to ensure a smooth and even surface, providing optimal conditions for the treatments. This four-sided planning process was carried out to remove any imperfections and to enhance the uniformity of the wood surface, ensuring consistent absorption of the applied coatings. For oils and waxes, samples were brushed with two coats, allowing 24 h for drying between applications. Boric acid was applied as an aqueous solution, ensuring deep penetration into the wood fibres. Commercial coatings and fire-retardant sprays were applied uniformly using spray equipment, followed by controlled drying. Special attention was given to achieving consistent coating thickness across all samples to ensure reliable comparative analysis.
Due to equipment limitations, advanced thermal and chemical characterisation techniques, such as Thermogravimetric Analysis (TGA) and Fourier Transform Infrared Spectroscopy (FTIR), were not performed in this study. These analyses are planned for future investigations to provide deeper insights into the thermal degradation mechanisms and the chemical changes in treated wood. Additionally, the long-term durability of the treatments under environmental conditions was not assessed, which is crucial for understanding the sustained performance of these coatings. Future research will incorporate accelerated weathering tests to assess the longevity and effectiveness of the fire-retardant coatings over time, providing a more comprehensive evaluation of their real-world applicability.

2.3. Testing Methods

2.3.1. Small Flame

The small flame test method was developed to assess the fire reaction and fire resistance of materials. This method defines a test to determine the combustibility of products when directly exposed to a small flame without radiant heat. The samples are positioned vertically during testing (Figure 1). The fire resistance test under small flame exposure was conducted according to the standard EN ISO 11925-2:2020 [59], which primarily relies on a visual evaluation of the samples’ behaviour.
Ten samples from each group were tested. The samples were prepared by marking two reference lines on their surface: the first line positioned 40 mm from the lower edge and the second 150 mm above the first line. The burner was mounted on a holder at a 45° angle (in accordance with the standard), with the flame height set to 2 cm.
The burner flame was directed at the lower reference line of the sample, where the sample was exposed to the flame for 30 s. Subsequently, the burner was removed, and the sample was observed for 30 s. Upon completion of the test, the height of the carbonised (charred) surface, the charring depth, and the height of the swollen surface due to the fire-retardant coating were measured.
In addition to observing the detachment of ignited particles, further analysis was conducted to assess the persistence of combustion after the flame was removed for 30 s and the potential for the flame to reach a height of 150 mm.

2.3.2. Surface Roughness Measurement

The Olympus LEXT OLS5000 (Tokyo, Japan) laser microscope was used to analyse the samples’ surface roughness precisely. This device enables high-precision measurements in 3D format. Using this device, the Sa [µm] (Arithmetic Mean Height) parameter was explicitly employed. This parameter represents the average absolute height of points on the surface relative to the mean plane and is analogous to the Ra parameter used in 2D roughness measurements.
Measurements were performed for all surface treatments. For each surface treatment, two samples were selected, on which three measurements were performed.

2.3.3. Contact Angle Measurement

In order to determine the influence of roughness on the interaction of water with treated wood samples, the contact angles on the unburnt parts of the samples were measured in parallel to the roughness measurements. The water contact angle was determined with an Attension Theta optical tensiometer (Biolin Scientific, Gothenburg, Sweden) using the sessile drop method, under controlled conditions at room temperature (20 ± 2 °C) and relative humidity (50 ± 5%). The following parameters were employed during the measurement process:
  • Droplet size: 5 μL;
  • Contact angles were recorded with a camera for 60 s.
Data were recorded using OneAttension, a digital system connected to the device, allowing for real-time tracking of the water uptake dynamics. Control samples, without any coating, were included in the measurement to facilitate comparison between the absorption properties of treated and untreated wood surfaces.
This methodology allows for the assessment of the effectiveness of protective coatings in improving surface hydrophobicity. The results obtained provide valuable insights for optimising wood protection processes and selecting coatings for specific application conditions.

2.3.4. Hyperspectral Imaging (HSI)

Hyperspectral imaging (HSI) combines imaging and spectroscopy to enable detailed material analysis. Unlike traditional RGB cameras that capture three basic colours (red, green, and blue), HSI sensors record a broad spectrum of light wavelengths. This method facilitates material recognition and classification based on their spectral characteristics.
Hyperspectral analysis was performed with a high-resolution ClydeHSI Hyperion A3 Scanner (Clyde Hyperspectral Imaging (ClydeHSI), Glasgow, Scotland, United Kingdom) using visible and near-infrared (VNIR; 400–1000 nm, Δλ  =  3 nm) or short-wavelength infrared (SWIR; 900–2500 nm, Δλ  =  10 nm) hyperspectral cameras. Subsequently, data analysis was performed using Principal Components Analysis (PCA) to determine the spread of fire.
Hyperspectral images are not traditional photographs but spectrograms that display data for each pixel across multiple wavelengths. This enables the visualisation of materials within specific spectral ranges, helping differentiate material composition. In this study, spectrograms were used to identify material differences in areas exposed to flame. The system captures data across 350 spectral channels, analysing each point on the sample through 350 distinct light wavelengths.
A key method in hyperspectral analysis is Principal Component Analysis (PCA), which reduces data dimensionality by extracting the most important information from a large number of spectral channels. PCA identifies dominant variations within the spectral data, highlighting material differences and revealing specific patterns. After PCA analysis and selection of key spectral components, images were visualised using false-colour techniques to enhance interpretation. Specific colours were assigned to spectral channels, revealing various wood characteristics, such as distinguishing dry and wet zones or highlighting structural irregularities and defects. This combination of wavelength identification, PCA analysis, and false colouring enabled precise classification and analysis of wood samples, providing insights that are not visible to the naked eye or with traditional imaging methods.

3. Results and Discussion

3.1. Fire Resistance

The small flame test applied in this study enabled a precise assessment of the flammability of spruce wood treated with various protective agents. The key parameters included the occurrence of a flame after 30 s of exposure, the duration of the small flame (standardised at 30 s), and the average flame height for each sample.
The results reveal significant differences in flammability depending on the applied treatment. Natural saw-cut spruce samples exhibited an average flame height of 9.34 cm with the consistent occurrence of a flame after 30 s. In contrast, planed samples showed a reduced average flame height (7.42 cm) and a decreased frequency of flame occurrence. This difference suggests that surface treatment significantly influences the fire resistance of wood. Samples treated with oils displayed variable results. Ekotep oil increased the average flame height to 11.44 cm, likely due to its composition, which may contribute to increased flammability by forming a combustible layer on the wood surface. Conversely, Wepos oil had a protective effect with a lower average flame height of 8.88 cm, possibly attributed to its synthetic components that enhance the formation of a protective barrier. Similarly, samples treated with Belinka oil had an average flame height of 9.40 cm. Treatments with boric acid showed limited effectiveness, resulting in the highest average flame height among treated samples (12.18 cm), indicating low resistance under direct flame exposure. Boric acid’s limited impact could be due to its water-soluble nature, leading to insufficient retention on the wood surface.
Samples treated with waxes exhibited different reactions. Montan wax allowed an average flame height of 8.02 cm with an occasional flame occurrence. In comparison, Schacht wax resulted in an average height of 7.29 cm, but with pronounced melting and the formation of a sticky residue. Coatings, such as Chromos Svjetlost and Caparol, demonstrated improved resistance, with Caparol showing no flame occurrences in any samples and an average flame height of 6.95 cm. Chromos Svjetlost had a slightly higher average flame height (6.98 cm) with an occasional flame occurrence. The superior performance of these coatings can be attributed to the formation of a protective film on the wood surface, preventing direct contact with flames and reducing heat transfer.
Thermally treated samples exhibited increased flammability with an average flame height of 10.97 cm, indicating the need for additional protective measures after thermal treatment. Thermal treatment reduces moisture content and degrades hemicellulose, increasing combustibility. The Decolux and Burn Block spray coatings provided the best protection. The Burn Block spray completely prevented flame occurrences in all samples, with the lowest average flame height of 6.57 cm, while Decolux had an average of 6.82 cm with an occasional flame occurrence. Burn Block’s superior performance is likely due to its ability to form a stable char layer that insulates the wood and prevents further combustion. All results are presented in Table 3.
The results confirm the significant impact of the treatment type on wood fire resistance. Treatments, such as the Burn Block spray and Caparol coating, proved most effective, while treatments with boric acid and thermal modification offered no protection and somehow worsened the fire resistance of the wood surface. While this study demonstrated improvements in fire resistance with conventional coatings, the literature indicates that nano-material-based treatments could offer superior protection due to their enhanced thermal stability and the formation of protective barriers at the molecular level [60,61,62]. Incorporating these advanced coatings in future studies would provide a more comprehensive evaluation of fire-retardant performance. These findings contribute to a better understanding of the role of different protective agents in enhancing wood fire resistance, offering guidelines for their optimal practical application.

3.2. Analysis of Char Depth in Spruce Wood (Picea spp.) During Small Flame Test

Measuring the char depth in spruce wood is crucial for assessing material fire resistance and understanding thermal degradation processes. After the small flame test, the char depth of the samples treated with various protective agents and the untreated samples was analysed (Figure 2). The applied methods included analyses of the charred layers, enabling the precise quantification of structural changes in the wood.
The results revealed significant differences between treated and untreated samples (Figure 3). Untreated samples exhibited the greatest char depth (SPRP, up to 1741.473 µm), while samples treated with the Chromos Svjetlost coating (SPPCS) had the lowest char depth (191.319 µm). These findings confirm the protective coatings’ effectiveness in reducing the thermal degradation rate [63,64].
Oil-treated samples displayed varying levels of resistance. Ekotep oil (SPUE) showed a higher average char depth (1097.103 µm) compared to Wepos and Belinka oils (~376 µm), which can be attributed to differences in their chemical compositions and penetration characteristics [65]. The efficiency of the protective agents depends on their chemical properties, penetration capability, and ability to form thermally stable layers [66]. Wood properties, such as its moisture content and density, significantly influence char depth [67]. These findings contribute to a better understanding of wood combustion behaviour and the effectiveness of protective treatments, offering guidelines for optimising fire protection strategies in the construction industry.

3.3. Surface Roughness

The surface roughness measurements of wood samples treated with different methods indicate significant microstructural changes that directly affect their technical and mechanical properties. A comparison between the untreated and treated samples shows a notable decrease in the mean arithmetic surface roughness (Sa) in the samples treated with fire-retardant coatings. For example, the SPUW sample, treated with a specific coating, had an average Sa value of 5.749 µm, indicating a smoother surface compared to the SPRP sample, which had an Sa value of 12.912 µm. This difference suggests that treatments involving fire-retardant coatings may contribute to greater homogeneity and fewer microscopic irregularities on the surface of the wood, which could be important for improving fire resistance and reducing wear. These findings align with recent studies, such as those by Mitrenga et al. [68], which investigated fire-retardant coatings for wood and reported a significant reduction in surface roughness after treatment, leading to enhanced fire resistance. Similarly, the study [69] demonstrated that fire-retardant coatings not only improved the fire resistance of the wood but also resulted in a smoother surface with fewer irregularities, much like the results observed in our study.
The values presented in Table 4 illustrate the significant differences in Sa between the untreated and treated samples, with the treated samples showing consistently lower roughness values, thus reinforcing the correlation between fire-retardant treatments and surface smoothness. The ANOVA analysis, applied for statistical testing of the differences between samples, showed high statistical significance with a P-value less than 0.001, confirming the existence of significant differences between the treated and untreated samples in terms of surface roughness. The F-values indicate a strong correlation between the treatments and changes in roughness, suggesting that the application of fire-retardant coatings has a significant impact on the microstructure of the surface layers of the wood.
Figure 4 Visualisation of Surface Roughness at 20× Magnification. Visual analysis, as shown in Figure 4, highlights the significant differences between the treated and untreated samples. The surface roughness varies depending on the type of treatment, with coated and thermally treated surfaces standing out due to their structural homogeneity.
The analysis demonstrates how treatments significantly affect the morphology of the surface layers, thereby altering the physical–mechanical properties of the wood, including fire resistance and wear. These results provide important insights into the optimisation of the surface treatments of wood materials with the aim of improving their technical properties in the construction industry.

3.4. Contact Angle

The correlation between the angle of wetting and the combustibility of spruce wood treated with various materials provides key insights into the impact of chemical and natural treatments on fire resistance. The angle of wetting, which describes the interaction of liquids with solid surfaces, directly affects the wood’s hydrophilic and hydrophobic properties, thereby altering its thermal characteristics and flammability. The wetting time (CA) analysis for the spruce wood samples indicates a correlation between moisture absorption capacity and flammability, which is crucial for assessing the fire resistance of the treated materials (Table 5). This correlation confirms the findings of previous studies conducted [68,70,71].
Samples treated with oils (SPUE, SPUW, and SPUB) show an increased moisture absorption capacity, while treatments like wax (SPV and SPVS) reduce absorption. The sample treated with boric acid (SPBK) and coatings (SPPCS, SPPD, and SPUB) shows increased wetting values, suggesting better fire resistance due to these treatments’ ability to reduce moisture absorption and enhance flame protection. Thermal treatment (TTS) decreases moisture absorption, which may increase flammability by reducing its resistance to moisture and thermal stability. These results align with the conclusions from similar studies [67].
Linking the wetting time data with the flammability analysis indicates that wood with a higher capacity for moisture absorption, such as samples treated with oil and boron, shows reduced flammability, while thermally treated spruce exhibits increased flammability due to faster combustion. These results confirm the findings of other studies [68] that have explored similar parameters in recent years. These findings contribute to understanding the role of treatments in reducing wood flammability and their application in industries requiring high fire resistance.

3.5. Hyperspectral Imaging

The presented Figure 5 shows the results of the hyperspectral data for four types of spruce wood samples: thermally treated spruce (TTS), spruce treated with wax (SPV), planed spruce (SPBU), and spruce treated with Burn Block (SPBB). Each image visualises the surface changes resulting from the combustibility of the samples.
Figure 5a illustrates the distribution of the spectral data. Thermally treated spruce (TTS) shows a homogeneous distribution with dominant shades of purple and blue, indicating reduced absorption in certain spectral regions, likely due to the decreased moisture content and thermal degradation of lignin. Spruce treated with wax (SPV) shows contrasting zones between darker and lighter areas, indicating wax deposition that reduces the flame spread rate. Planed spruce (SPBU) highlights distinct structural growth rings with clear damage zones, while spruce treated with Burn Block (SPBB) shows significantly reduced reflection in spectral regions that are sensitive to carbonisation, with minimal surface damage.
Figure 5b shows the contrasts between intact and damaged regions. In the TTS sample, dark areas indicate deeper carbonised sections, while lighter surfaces remain relatively undamaged. The SPV sample shows dark areas concentrated around the knots and cracks, while the SPBU sample displays an even distribution of darker tones across the surface. The SPBB sample is dominated by bright regions, confirming the effectiveness of the carbonisation protection.
Figure 5c highlights the subtle surface changes associated with chemical transformations during flame exposure. In the TTS sample, the variations between green and red indicate changes in the chemical composition of lignin and cellulose. The SPV sample shows yellow and purple shades, suggesting partial wax melting. The SPBU sample exhibits an uneven colour distribution with pronounced changes around the knots, while the SPBB sample is dominated by green shades, indicating a stable surface structure.
The HSI method enabled detailed monitoring of the surface changes in spruce wood after flame exposure. The samples treated with protective agents, such as wax (SPV) and Burn Block (SPBB), showed better resistance to carbonisation compared to the untreated (SPBU) and thermally treated samples (TTSs). The SPBB sample demonstrated the best flame resistance, while the TTS and SPBU samples showed more significant surface degradation.
Figure 5a–c clearly illustrate how different treatments affect the behaviour of spruce wood under flame exposure, confirming the importance of protective coatings in enhancing wood fire resistance.

4. Conclusions

This study confirmed the significant impact of surface treatments on improving the fire resistance of spruce wood. The analysis showed that the Burn Block spray and Caparol coating were the most effective treatments, reducing the average flame height to 6.57 cm and 6.95 cm, respectively, compared to 9.34 cm for untreated samples and 12.18 cm for boric acid-treated samples. Commercial fire-retardant coatings, particularly the Burn Block spray and Caparol coating, showed the highest efficiency by preventing flame spread and reducing char depth. The surface roughness analysis revealed homogenisation of the treated surfaces, contributing to enhanced fire resistance. Hyperspectral imaging precisely mapped the damaged areas and identified chemical changes caused by flame exposure. Contact angle measurements highlighted the importance of hydrophobicity in reducing moisture absorption and improving fire resistance. This study excludes nano-material-based coatings, does not encompass advanced thermal and chemical analyses (e.g., TGA and FTIR), and does not include long-term durability testing. Furthermore, the environmental impact of the applied coatings was not assessed. Future research should address these aspects to provide a more holistic understanding of the fire performance and sustainability of treated spruce wood. The findings suggest that applying suitable surface coatings can significantly enhance the safety of wooden structures, contributing to the development of sustainable construction materials with improved fire-retardant properties. Future research should focus on developing coating combinations to achieve maximum protection while minimising environmental impacts and production costs.

Author Contributions

Conceptualisation, M.H. and R.H.; methodology, R.H.; software, S.M.; validation, S.M., R.H., E.K. and B.L.; formal analysis, R.H.; investigation, S.M., R.H. and E.K.; resources, R.H. and M.H.; data curation, R.H.; writing—original draft preparation, R.H., M.B., L.F. and M.H.; writing—review and editing, B.L., E.K. and M.D.G.; visualisation, R.H., S.M. and E.K.; supervision, R.H. and M.H.; project administration, B.L.; funding acquisition, R.H., M.B. and M.H. All authors have read and agreed to the published version of the manuscript.

Funding

The authors appreciate the financial support of the Slovenian Research Agency (ARIS) within the research project J7-50231 (GROWTH: Growth potential and wood properties of selected tree species of various provenances: modification options, prospects, and challenges in response to climate change), research programme P4-0015 (Wood and lignocellulosic composites), and the Infrastructure Centre (IC LES PST 0481-09).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further in-quiries can be directed to the corresponding authors.

Acknowledgments

The authors would like to express their gratitude to the Biotechnical Faculty of the University of Ljubljana, Department of Wood Science and Technology, for providing access to equipment used in this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Fire 08 00120 g001
Figure 1. Small flame test method.
Figure 1. Small flame test method.
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Figure 2. Char depth analysis of spruce wood samples: (a) natural spruce samples after char depth analysis: Sample 2, cut at sawmill; (b) 3D visualisation of char depth analysis for sample treated with Burn Block (1, 2, 3 represent the sequential order of the measurements taken during the experiment).
Figure 2. Char depth analysis of spruce wood samples: (a) natural spruce samples after char depth analysis: Sample 2, cut at sawmill; (b) 3D visualisation of char depth analysis for sample treated with Burn Block (1, 2, 3 represent the sequential order of the measurements taken during the experiment).
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Figure 3. Visualisation of char depth analysis for untreated spruce sample.
Figure 3. Visualisation of char depth analysis for untreated spruce sample.
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Figure 4. Surface roughness visualisation of wood samples at 20× magnification.
Figure 4. Surface roughness visualisation of wood samples at 20× magnification.
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Figure 5. Hyperspectral images of spruce wood samples after flame exposure: (a) spectral distribution, (b) contrasts between intact and damaged regions, and (c) chemical surface changes.
Figure 5. Hyperspectral images of spruce wood samples after flame exposure: (a) spectral distribution, (b) contrasts between intact and damaged regions, and (c) chemical surface changes.
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Table 1. Specifications of spruce wood samples for testing.
Table 1. Specifications of spruce wood samples for testing.
NMaterialAbbreviationSpecification
1Natural Spruce—SawingSPRPUntreated, natural spruce wood.
2Natural Spruce—Planed SamplesSPBUSmooth surface due to planning, natural state.
3Burnblock SpraySPBBFire-retardant spray, reduces flammability and flame spread.
4Boric AcidSPBKAntiseptic, insecticidal properties, fire retardant.
5Chromos SvjetlostSPPCSProtective coating, weather-resistant, prevents moisture absorption.
6Belinka OilSPUBPlant-based oil, enhances durability, water-repellent.
7Wax SloveniaSPVProtective wax layer, enhances water resistance, adds shine.
8Thermally Treated SpruceTTSEnhanced dimensional stability, reduced moisture absorption.
9Caparol CoatingSPPCProtective coating, UV-resistant, weatherproof.
10Wepos OilSPUWProtective oil, moisture-resistant, maintains wood texture.
11Schacht WaxSPVSForms a protective layer, increases shine, repels water.
12Ekotep OilSPUEProtective oil, reduces moisture absorption, enhances wood colour.
13Decolux CoatingSPPDProtective coating, enhances surface durability, prevents moisture damage.
Table 2. Properties and composition of materials used for treating spruce wood.
Table 2. Properties and composition of materials used for treating spruce wood.
Material TypeAppearanceDensity (g/cm3)ComponentAmount (%)
Ekotep (Oil)Transparent liquid0.91Natural oils, fatty acids<100
Wepos (Oil)Transparent liquid0.92Natural and synthetic oils, fatty acids<100
Belinka (Oil)Transparent liquid0.91Natural oils, resins, fatty acids<100
Montan WaxWhite, solid0.90Montan wax, paraffin<100
Schacht WaxWhite, solid0.92Paraffin wax, microcrystalline wax<100
Boric Acid (Chemical Agent)White powder1.43Boric acid100
Chromos Svjetlost (Coating)Glossy, liquid1.03Synthetic resins, pigments, solvents<50
Decolux (Coating)Glossy, liquid1.05Synthetic resins, pigments, additives<50
Caparol (Coating)Glossy, liquid1.10Synthetic resins, pigments, additives<50
Burnblock (Fire-Retardant Spray)Transparent liquid1.08Natural substances, non-toxic compounds<100
Table 3. Summary of small flame test results for spruce samples treated with different protective agents.
Table 3. Summary of small flame test results for spruce samples treated with different protective agents.
Material DescriptionFlame Occurrence After 30 sSmall Flame Duration (s)Avg Flame Height (cm)St Deviation.
SPRPYES309.342.19
SPBUYES/NO307.421.49
SPBBNO306.570.30
SPBKYES3012.183.70
SPPCSYES/NO306.980.87
SPUBYES309.402.09
SPVYES308.021.57
TTSYES3010.973.62
SPPCNO306.950.55
SPUWYES308.881.05
SPVSYES/NO307.292.86
SPUEYES3011.442.64
SPPDYES/NO306.821.35
Table 4. Surface roughness (Sa) measurements of wood samples at 20× magnification.
Table 4. Surface roughness (Sa) measurements of wood samples at 20× magnification.
SamplesMeasurement 1 Sa (µm)Measurement 2 Sa (µm)Measurement 3 Sa (µm)Measurement 4 Sa (µm)Measurement 5 Sa (µm)Measurement 6 Sa (µm)AvgStdDev
SPRP16.7059.27611.32216.76612.29911.10612.9123.119
SPBU5.3995.0814.2122.9565.5044.6084.6270.952
SPBB14.23514.4537.3186.4065.7915.3468.9254.250
SPBK16.55911.65912.5029.8638.9758.66211.3702.954
SPPCS7.49110.0629.0949.19810.2828.9099.1730.993
SPUB9.82610.2943.3195.1536.4566.5246.9292.694
SPV17.8831.5886.2512.9179.76210.5489.8255.579
TTS5.7787.9717.0137.2387.3733.7576.5221.535
SPPC7.5535.9156.9165.4566.8275.6646.3890.830
SPUW6.1316.6875.5215.6643.8936.5955.7491.024
SPVS2.615.221.9853.08713.3252.4934.7874.332
SPUE3.0973.0775.5823.637.3614.2794.5041.683
SPPD5.163.9464.2354.0523.5585.1334.3470.658
Table 5. Contact angles (CA) of spruce wood samples treated with various materials.
Table 5. Contact angles (CA) of spruce wood samples treated with various materials.
Wetting Time (min)SPRPSPBUSPBBSPBKSPPCSSPUBSPV1TTSSPPCSPUWSPVSSPUESPPD
1121.489.588.5138.3131.8114.2135.2129114.6121.4109118.183.1
1098.663.458.9139.6128.8116.7135.3124.6111.5120.7109.1117.588.9
30120.652.644.3137.4128.5112.4134.8124.6115.1120109.4116.272.1
55943938.4122.1121.7113.587.2124111120.784.4115.384.4
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Hasanagić, R.; Mujanić, S.; Keržič, E.; Fathi, L.; Bahmani, M.; Dahmardeh Ghalehno, M.; Lesar, B.; Humar, M. Advanced Evaluation of Fire Resistance in Spruce Wood (Picea abies spp.) Treated with Innovative Surface Coatings. Fire 2025, 8, 120. https://doi.org/10.3390/fire8040120

AMA Style

Hasanagić R, Mujanić S, Keržič E, Fathi L, Bahmani M, Dahmardeh Ghalehno M, Lesar B, Humar M. Advanced Evaluation of Fire Resistance in Spruce Wood (Picea abies spp.) Treated with Innovative Surface Coatings. Fire. 2025; 8(4):120. https://doi.org/10.3390/fire8040120

Chicago/Turabian Style

Hasanagić, Redžo, Selma Mujanić, Eli Keržič, Leila Fathi, Mohsen Bahmani, Mohammad Dahmardeh Ghalehno, Boštjan Lesar, and Miha Humar. 2025. "Advanced Evaluation of Fire Resistance in Spruce Wood (Picea abies spp.) Treated with Innovative Surface Coatings" Fire 8, no. 4: 120. https://doi.org/10.3390/fire8040120

APA Style

Hasanagić, R., Mujanić, S., Keržič, E., Fathi, L., Bahmani, M., Dahmardeh Ghalehno, M., Lesar, B., & Humar, M. (2025). Advanced Evaluation of Fire Resistance in Spruce Wood (Picea abies spp.) Treated with Innovative Surface Coatings. Fire, 8(4), 120. https://doi.org/10.3390/fire8040120

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