Taurus cedar (Cedrus libani A. Richard) is a species with the widest distribution area in Türkiye. In our country (Türkiye), it grows specifically in the Taurus Mountains in the Mediterranean region. Cedar, encompassing four species that constitute the Cedrus genus of the Pinaceae family, is an evergreen tree with coniferous leaves. It is argued that this species should now be called the Taurus cedar, as the person who introduced C. libani, which is known as the Lebanon cedar, later stated, “If I saw this species in the Taurus Mountains, I would call it the Taurus cedar.” Over time, its numbers have dwindled because of the use of its valuable wood. It no longer exists in Lebanon, from where it takes its name. Cedar, a coniferous tree species, has unique features such as being heat and humidity resistant, not producing bacteria, not harboring insects, and containing 1,500 empty air sacs per cm³ of its wood (high porosity). For this reason, cedar is a natural insulating material (Yeşilkaya 1994).

Cedar wood was very valuable even in ancient times and was traded between countries. In a study it was reported that the oldest information about cedar wood import described the transportation of cedar wood from Gubla of Finike to Egypt in 2750 BCE, that mummy coffins for Egyptian pharaohs and high officials were always made of cedar, and that the white resin of cedar was used in the mummification of the dead. The cedar tree was particularly sought after by the Egyptians for luxurious and religious buildings because of its pleasant smell, its rot resistance, and its beautiful color, and that the Phoenicians used cedar wood for load-bearing beams in temples and palaces; in the construction of houses, galleys, and ships; and in doors, columns, household goods, furniture, chests, and wood carvings. Although the smell of cedar wood disappears after 4 to 5 years, when the surface of the wood is grated, the scent is released again (Çepel 1959). According to a study, it has been revealed that Taurus cedar wood is a tree species with a low volume shrinkage percentage according to its full dry density value (Efe 2021). Cedrus libani timber is shown in Figure 1.

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A sauna is a type of bath that creates convective heating, with heat and humidity kept at certain levels. The optimal temperature in saunas is 80 to 90°C, and the relative humidity of the air varies between 15 and 30 percent. Cedar wood is one of the most used woods for saunas because it has a rich and pleasant aroma and is easily processed. Cedar is the best choice for its richness, aroma, and elegance, being available in shades ranging from dark cherry to light red (Poolline 2025). Cedar is used frequently as it is a soft wood that flexes and resists cracking over thousands of heating and cooling cycles. Its natural oils are also antibacterial and antifungal. The environment inside a sauna is hot and humid and harbors germs and toxins excreted from the body. In this environment, cedar resists microbial growth and requires less maintenance; it is also suitable for use in outdoor saunas since it is very durable.

Extractive substances contained in wood are factors affecting the percentage of shrinking and swelling of wood. In a study conducted on this subject, extractive substances caused the density of wood to increase but did not cause the percentage of shrinkage and swelling to increase (Bal et al. 2012). Most of the extractive substances in colored heartwoods (red woods, blue woods) are flavonoids and their derivatives (Pizzi 2006). According to a study that was conducted on the chemical components of cedar tree species, it was determined that at 1 percent NaOH solubility, lignin and holocellulose were higher in sapwood compared with heartwood. The percentage of extractive substances was higher in heartwood (extractive substances were obtained by ethanol–benzene extraction; Cardona and Sultan 2016).

The effects of the open air, defined as humidity, light, ultraviolet (UV) radiation, mechanical forces, and temperature, are important risk factors for wood materials. As a result of these effects, some changes occur in the color, chemical structure, and physical structure of the wood material. These negative changes (aging) that occur are very important in terms of shortening the life of the tree and increasing the cost of replacing decomposed materials (Fidan et al. 2018).

Cedar wood contains many extractive substances that provide biological durability. Therefore, it already has a self-defense system against wood pests that may occur in hot and humid environments. This study focuses on the durability and amounts of these components in hot and humid environments such as saunas.

Materials and Methods

Cedar wood timbers were supplied from the Mediterranean region, from lumber mills in Isparta city in Türkiye. The obtained wood was cut to dimensions of 25 by 25 by 30 cm for chemical analysis with the help of circular and band saws, and the samples were classified, named, and marked according to the various conditions to be applied each hour. Wood dust taken from samples for Fourier transform infrared (FTIR) analysis was classified, packaged, and used in required amounts (about 10 mg).

Results

Determination of extractive substance amount

The graph of the change in the amount of extractive substance with time is shown in Figure 2. The amounts of extractive substance were determined from five variations in groups of three. The control group consisted of samples prepared only for comparisons, with a mean of 6.57 percent and a standard deviation of 1.53 and underwent no processing to interpret other variations. In the first variation, which was the 8-hour conditioning group, the extractive substances were removed and decreased because of the temperature and washing exposed to the conditioning device, reflecting an average value of 2.35 percent. The decrease in the amount of extractive substance continued in the samples that underwent 16-hour variations, with an average rate of 1.73 percent and a standard deviation of 0.71. The decrease in the amount of extractive substances continued in the 32-, 64-, and 128-hour variation groups, and when the samples were examined after 128 hours the extractive substance rate was 1.43 percent. In this variation, the smallest deviation value of 0.13 was encountered in the standard deviation and extractive substance ratio. In this evaluation, the highest standard deviation was found in the 8-hour variation. It is thought that the decrease in the substances contained in the wood that can be removed by washing over time causes the standard deviation and the amount of substance removed to decrease. The fact that the standard deviation sometimes increased in the intermediate variations could have been due to differences resulting from the random method of sample selection.

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Mechanical property determination

The MOR change graph is shown in Figure 3 according to variations. Mechanical tests were performed on samples 25 by 25 by 30 cm in size that completed the conditioning process and were exposed to oven and chamber conditions for 24 hours. As a result of these tests, there was a general decrease in the bending strength (MOR) values of the cedar wood. However, some extreme results resulting from the random selection method affected the averages, so some fluctuations can be seen in the graph. These values will become more meaningful with the discussion of the amount of crystal structure formation obtained from the FTIR analysis results.

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The MOE change graph is shown in Figure 4 according to variations. MOE results calculated for the same variations show differences as a result of the physical, chemical, and mechanical changes that occur in the structure of the samples exposed to the hot and humid environment and generally show a decrease compared with those in the control groups. Again, according to the graph, where some outliers are seen, since the main target is to examine the effect of the change in the wood’s crystal structure under a hot and humid environment on its mechanical properties, more understandable conclusions were reached by comparing these data with the FTIR crystal structure formation data. These evaluations will be conveyed when evaluating the FTIR analysis results.

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FTIR analysis

Mass loss formations according to variations in the FTIR experiment (I3423/I1506, I1740/I1506) are given in Figures 5 and 6. With continuous washing at a temperature of 80°C and in the variations in the duration of exposure, the extractive substances and similar chemical structures that are likely to be removed from the wood are indeed lost. This situation continues as the variation periods progress. However, with increased variations, the decrease in the removable substances and the increase in crystal structure formation in the wood cause a decrease in mass loss. Another reason for this decrease could be the fact that water was used, which is a solvent that cannot penetrate deeply into the wood easily.

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Crystal structure formations according to variations in FTIR analysis (I1421/I896, I1371/I2917) are shown in Figures 7 and 8. While performing the FTIR analysis, some peaks were specifically used to determine structures that were formed or transformed in the wood. By calculating the ratio of these peaks, the crystal structure changes occurring in the wood can be determined. These changes as a result of the hot and humid environment remained positive throughout the variations. This is an indication that amorphous structures were transformed into crystal structures. The crystal structure formation is directly proportional to the MOR and MOE values, which represent the cedar wood’s mechanical properties. If the crystal structure formation change is examined from one variation to another and an increase or decrease is observed, the same increase or decrease is usually also seen in the mechanical properties. Because one of the aims of our study was to determine the effect of crystal structure changes in the wood caused by temperature and washing on its mechanical properties, the samples were brought to a constant weight and kept for 24 hours under normal conditions after the conditioning process before the other processes were carried out.

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Status of crystal O–H groups according to variations in FTIR analysis (I2917/I3423) is shown in Figure 9. In the literature, the I2917 peak value represents the status of C–H bonds and the I3423 value represents the status and structure of the O–H bonds and hydroxyl groups in wood. The ratio of these values shows the status and amount of free O–H groups that occur as the variations in the wood progress. It was determined that the hot and humid environment had caused an increase in the amount of free O–H groups in the cedar wood, and this effect peaked at the 128-hour variation. Changes in the number of C–H bonds are expected to be directly proportional to changes in the number of free O–H groups.

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pH changes

Cedar pH ratio changes according to variations shown in Figure 10. The temperature and washing to which the cedar samples in the variations were exposed for certain periods of time caused some changes in the pH rates. According to the pH ratio variation averages, the value starts from 5.48 in the control group, and as the time increases, the variation averages also increase. In light of these data, the changes in the cedar wood caused by the sauna conditions also affect the pH rates, and this effect is detected as an increase. This increase reaches a maximum value of 6.22 and moves away from acidity, approaching neutrality.

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Discussion

In the conditioning process used in this study, water evaporated with increased temperatures, and the resulting water vapor condensed, contacting and washing the cedar samples. As a result of the conditioning process, some changes occurred in the wood’s chemical and mechanical properties. These included the deterioration of the external appearance of the wood, a decrease in its odor, and changes in its mechanical resistance. If saunas are used according to their instructions and are maintained when necessary, certain problems may be avoided. The maintenance times for our sauna can be determined with longer-term studies. For the cedar to regain its scent and regain its former beautiful appearance, its surfaces can be lightly sanded and broken parts can be replaced. As a result of extraction, some of the cedar samples displayed a lot of extractive material and lost their color.

In the FTIR analysis of the wood, some peaks were specifically used to determine the structures formed and altered in the wood. We showed crystal structural changes in the wood, and these changes in the cedar wood due to the hot and humid environment remained positive throughout the variations. This is a sign that amorphous structures changed into crystalline structures. Similarly, according to a study using FTIR analysis to investigate the effect of different aging durations on cedar wood samples, this depended on the part of the wood (internal or external) exposed to the weathering effects and was influenced by both an extended duration of aging and the effects of natural deterioration agents (Bouramdane et al. 2022).

On the basis of FTIR measurements, it was concluded that some modification had taken place during solvent diffusion, and solvent-induced chain modification occurred in the main constituents of lignocellulosic substances: wood, bark, cone, and needle (Şahin and Çam 2022). According to another study (Estaves et al. 2013), it was observed that heat treatment caused significant changes in the chemical composition and structure of wood by changing the structure of polysaccharides and lignin. This situation could be observed with FTIR analysis. It was determined that hemicelluloses were the first components to degrade because of distillation. It was observed that lignin in hardwood changed more than lignin in softwood because of the rupture of aliphatic side chains (Estaves et al. 2013). According to the study conducted on thermal modification and alkyl ketene dimer effects on the surface protection of deodar cedar (Cedrus deodara roxb.) wood, FTIR results and color measurements showed that it could not provide permanent protection to the underlying wood structure because of its own tendency to degrade mainly in color over time under the action of UV rays and atmospheric agents (Lovaglio et al. 2022). Similarly, color changes were also observed in C. libani samples.

According to a study in which the resistance of mulberry tree (Morus spp.) wood extractives to the degrading factors of the external environment, especially UV light, to which they may be exposed in aboveground usage areas, was determined, the amount of moisture in the samples increased as the aging test period was extended, the extracted samples had more moisture than the unextracted samples, and the samples kept in an area close to the sea had more moisture than the samples kept in an area far from the sea (Ermeydan et al. 2023). It was shown that there was deterioration in the lignin of the samples from the first stage of aging. It can be said that the wood of C. libani ages more resistantly than broadleaved trees.

Our aim was to determine the changes in the properties of cedar wood in a hot and humid environment, and interesting results were obtained. From these, we conclude that wood requires maintenance or renewal over time and is negatively affected physically, mechanically, and chemically by sauna conditions. This study is intended to serve as a guide for relevant institutions, organizations, and enthusiasts in this field and to form a basis for future studies.