## 1 Introduction  
The accuracy of the log dimensions, the material dimensional stability, stiffness, strength, resistance to rot, density, or even the appearance of the material are some of the requirements that the customer may place on timber. One of the most common reasons customers avoid using wood, e.g., in timber structures, is lower dimensional stability. Visible changes in shape, leading to unacceptable difficulties in wood processing, are the most significant adverse effects of timber construction. Since spiral grain in wood causes issues in timber structures, there are reasons why wood is often not preferred for use in structures and is replaced by other materials, such as concrete with steel. Moreover, the negative impact of the spiral grain on the quality of timber and its products can be observed, as the fibers copy the curvature but not the saw cut. Such a log has lower strength properties than a log with no spiral grain. When processing logs, the size of the spiral grain must be considered. This fact is essential in the case of a warp twist because it negatively affects the tensile properties of wood. Logs with extreme spiral grain are unacceptable as the material is further processed. Depending on the purpose of use, the spiral grain plays an essential role in wood utilization; therefore, it was also included in the classification criteria [1]. The changes in the mechanical properties (Hickory, Ash, Elm, Beech, Maple) due to the spiral grain were also investigated in [2]; it was found that the grain deviation angle (from 0\({}^{\circ}\) to 15\({}^{\circ}\)) from the rectilinear pattern has a significant impact on the values of mechanical properties. For all species (Hickory, Ash, Elm, Beech, Maple), they investigated an absolute decrease in value exceeding 50% of the initial value; this decrease was even over 60% for the two examined wood species. These findings have direct implications for the construction industry, highlighting the need for careful consideration of spiral grain in timber selection and utilization. Similar results were discovered in [3]. The authors investigated the effect of the global slope of grain on the bending strength of scaffold boards. Results showed that the magnitude of the slope of grain has a very marked effect on its bending strength (boards were 24% weaker in comparison with boards made of equivalent timber quality). The impact of spiral grain formation on timber quality is not just a theoretical concept. It is a practical concern that is reflected in the standards, which clearly state the permissible curvature values when sorting timber [4]. For instance, according to the American ASTM standards, left-handed logs and subsequent products may not be suitable for certain applications, such as ceiling beams, rafters, purlins, etc. However, material with left-handed fibers can still be used in timber structures for load-bearing walls, highlighting the nuanced understanding and application of this knowledge in the industry. The formation of spiral grains is controlled genetically. However, environmental factors also have a noticeable influence on spiral grain--genetic predisposition, older age, position, type of cell formation, rotation of the earth, or the gravitational effects of the moon [5; 6; 7; 8]. Most of the data from previous studies indicate that tortuosity formation originates in the cambium and may be associated with cell division in this region. This process can be regulated according to studies of the plant hormones [9]. Three methods of cell division are thought to cause spiral grain. The angle of rotation of the spiral grain depends on the transverse divisions of the vertically oriented cambial cells. The cumulative effect of transverse divisions accompanied by poor growth has received much attention for over a century [10].