Internationally, the Janka Hardness is commonly used to measure the hardness of wood. In this testing method, a pressure device is used to press a steel ball with a diameter of 11.28 millimeters into the wood until the depth of the steel ball reaches half of its height. The load applied at this time is the hardness value of the wood. It should be noted that the unit of Janka Hardness is expressed in "pounds".
List of Hardness of Common Woods
In the wood industry, it is very important to understand the hardness of different woods. The following is a hardness table of common woods to help you quickly understand the hardness of various woods. Please note that the data in the table is for reference only, and the actual hardness may vary depending on the source and treatment of the wood.
Wood with high hardness has a more stable texture. It is suitable for open - work carving techniques. The furniture made from it is not only beautiful but also durable and can be passed down for a long time. However, the price of such wood is often relatively high. In contrast, furniture made from wood with lower hardness is more affordable.
In addition, understanding the characteristics of different cutting surfaces is also crucial for the application of wood. The tangential surface is obtained by sawing and planing at an angle of approximately 45 degrees perpendicular to the tree trunk. It is most suitable for observing the wood grain effect and the texture structure. The cross - section is obtained by sawing and planing at a 90 - degree angle perpendicular to the tree trunk. It can clearly show the growth rings of the wood, the tightness of the texture structure, the crystals, and the oiliness. The longitudinal section, also known as the radial section, is obtained by sawing and planing parallel to the tree trunk. It is convenient for observing the wood grain, the color of the wood, and the fineness of the texture. In the processing of hardwood, the longitudinal section is usually carried out on the heartwood to show its best texture and texture characteristics.
In the cross - section of wood, we can observe many light - colored stripes that radiate from the inside out. They are perpendicular to the growth rings and pass through multiple growth rings, sometimes continuously and sometimes intermittently. Among these stripes, some that radiate from the pith are called pith rays; some located in the xylem are called wood rays; and some located in the phloem are called phloem rays. Wood rays play a crucial role in the identification of wood.
On the cross - section of wood, the wood rays appear as thin, radiating lines, which precisely reflect their width and length characteristics. On the radial section, the wood rays show as intermittent ribbon - like or flaky shapes, from which the changes in their length and height can be observed. As for the tangential section, the wood rays turn into short vertical lines or spindle - like shapes, further revealing the differences in their height and width. It is worth mentioning that the wood rays in hardwoods are relatively developed and abundant. They account for about 17% of the total volume of the wood, becoming an indispensable part of hardwoods and one of the important characteristics for identifying hardwoods.
Types of Wood Rays
(1) Uniseriate wood rays are only 1 cell wide on the tangential section. Such uniseriate wood rays are relatively rare in hardwoods and can be observed in the woods of the Salicaceae family, the Hippocastanaceae family, and the Pterocarpus genus.
(2) Biseriate wood rays have a width of 2 cells on the tangential section, and occasionally 3 cells. Rosewood is a typical wood with biseriate wood rays.
(3) Multiseriate wood rays have the wood rays arranged in 3 or more rows on the tangential section, which is a characteristic of most hardwoods. For example, the woods of the Juglans genus and the Acer genus belong to this category.
(4) Aggregate wood rays have wood fibers or vessels mixed in the multiseriate wood rays, forming a special type of wood ray. The woods of the Ormosia genus and the Lithocarpus genus have this type of wood ray.
(5) Oak - type wood rays refer to wood rays that have both uniseriate rays and extremely wide rays, and the two are clearly distinguishable. The woods of the Cyclobalanopsis genus and the Quercus genus are typical oak - type wood ray woods.
In addition, wood rays play an important role in wood. They are composed of many parenchyma cells and are the relatively weak part of the physical and mechanical properties of wood. During the drying process, wood is often prone to cracking along the direction of the wood rays. However, the radial sections of these wood rays often show a beautiful silver - like texture, adding a unique charm to the wood. At the same time, wood rays also have the functions of transverse conduction and storage, ensuring the normal growth and function of the wood.
Next, let''s discuss the wood grain. Wood grain refers to the pattern formed by the main cells that make up the wood arranged in a certain direction. Different arrangement directions produce different grain effects, thus describing the directionality of the wood fibers inside the tree. The grains are mainly divided into straight grain, interlocked grain, spiral grain, wavy grain, and some irregular grains. Among them, the fibers of the straight grain are parallel to the long axis of the tree trunk, showing a smooth and high - strength characteristic; while the interlocked grain grows in the opposite direction of the long axis of the tree trunk, and its surface is prone to burrs and has a lower strength. These different grain characteristics not only affect the aesthetics of the wood but also have an important impact on its physical and mechanical properties and processing performance.
Wavy grain: This kind of grain is arranged in a wavy shape parallel to the long axis of the tree trunk, making the surface of the wood uneven and lacking smoothness. At the same time, due to the relatively low strength of the wavy grain, it is more difficult to process.
The yield rate, which measures the ratio of the volume of sawn timber to the volume of harvested standing stock, is a key indicator for evaluating the utilization efficiency of forest resources. The level of the yield rate directly reflects the utilization of forest resources. Specifically, the wood yield rate refers to the percentage of the volume of available wood to the volume of debarked logs after processes such as longitudinal and transverse cutting to obtain qualified square timbers. This ratio is affected by several factors, such as the difference in the diameters of the large and small ends of the log, the degree of curvature, and the number of knots.
In the production process of wood product enterprises, the yield rate is a crucial indicator. It is not only related to the economic benefits of the enterprise but also directly affects the production efficiency. Especially in the context of relatively scarce forest resources in China, improving the wood yield rate is particularly important. At the same time, differences in tree species, peeling processes, and processing equipment will all have a significant impact on the wood yield rate.
There are several factors affecting the wood yield rate. First, the diameter of the log is a key factor. Logs with a large diameter often have a higher yield rate, but the irregularity of the log shape may also lead to a decrease in the yield rate. Research shows that the optimal log diameter range is 18 to 42 cm. Second, the thickness and wear of the saw blade will also affect the yield rate. An overly thick saw blade will increase the width of the saw kerf, thus increasing wood loss and reducing the yield rate. In addition, the reasonable control of the sawing allowance is also an important part of improving the yield rate. During the sawing process, the allowance should be reasonably reserved and kept to a minimum. At the same time, the operation level of employees cannot be ignored. From the design of the sawing plan to the selection of the sawing position, employees need to have solid professional knowledge. However, with the increase in the degree of factory mechanization, operational errors of employees have been greatly reduced. Finally, the condition of the sawing equipment is also an important factor affecting the yield rate. The equipment should be regularly updated and inspected to maintain an excellent state to ensure the accuracy of sawing.