The question of why some types of wood float easily while others struggle to stay on the surface, or even sink entirely, is a fascinating intersection of physics and botany. The behavior of wood in water is not random; it is precisely governed by the relationship between the wood’s inherent structure and the water’s properties. Understanding the simple concepts of buoyancy and density provides the complete answer to which woods are the best floaters. The differences in flotation ability are primarily determined by how much solid wood material exists compared to the amount of air space trapped inside the wood’s cellular structure.
The Physics Behind Floating
The flotation of any object is explained by the principle of buoyancy, which is the upward force exerted by a fluid that opposes an object’s weight. An object is able to float if this upward buoyant force is greater than the downward force of gravity acting on the object’s mass. The magnitude of this upward force is equal to the weight of the fluid that the submerged portion of the object displaces.
The determinant of whether wood floats is its density, which is a measure of mass per unit volume. For an object to float in water, its average density must be less than the density of water, which is approximately 1.0 gram per cubic centimeter or 1,000 kilograms per cubic meter. Density is the most important factor because it dictates how much water an object must displace to support its own weight.
Scientists use a specific metric called specific gravity (SG) to directly compare a substance’s density to that of water. Specific gravity is a unitless ratio, where water has an SG of 1.0. Any wood species with a specific gravity less than 1.0 will float, while a species with an SG greater than 1.0 will sink. The closer a wood’s specific gravity is to zero, the better and higher it will float, as it displaces less water to remain afloat.
Factors Determining Wood Density
The physical structure of wood is the primary reason why different species have such varying densities and, therefore, different floating abilities. Wood is not a solid mass but rather a composite material made up of cellulose, lignin, and a large proportion of void spaces or air pockets within dead cells. The actual solid wood substance, cellulose and lignin, is denser than water with a specific gravity of about 1.5.
The vast majority of wood volume, however, consists of empty space, which is what gives wood its low average density. Softwoods, such as pine and cedar, generally have a simpler cellular structure dominated by long cells called tracheids, which feature large lumens, or hollow centers. This simple structure results in a high ratio of air space to solid cell wall material, leading to lower overall density.
Hardwoods, in contrast, possess a more complex cellular arrangement that includes thick-walled fiber cells and vessel elements, sometimes called pores. These denser fiber cells provide structural support and contain less air volume than the tracheids of softwoods. This difference in composition explains why hardwoods often have a higher specific gravity than softwoods.
Another highly variable factor that influences a wood’s effective density is its moisture content. When freshly cut, or “green,” wood can have a moisture content so high that the water filling the cell lumens adds significant mass. This increased weight can temporarily raise the wood’s specific gravity above 1.0, even in species that float when dried. Kiln-drying removes much of this water, making the wood lighter and preserving its buoyancy.
Which Woods Float Best: The Low-Density Champions
The woods that float best are those that possess the highest air-to-wood ratio, resulting in an extremely low specific gravity. Balsa is the champion of low-density wood, which has an average specific gravity as low as 0.12 to 0.17. This wood is composed of cells with extremely thin walls and very large internal cavities, making it exceptionally light and highly buoyant.
Cork, which is technically the bark of the cork oak tree, is another superior floater, registering a specific gravity of approximately 0.20. The structure of cork is nearly 90% air, with cells that are sealed and gas-filled, making it practically impervious to water absorption. This remarkable structure makes cork an ideal material for flotation devices and bottle stoppers.
Common construction softwoods also demonstrate excellent buoyancy due to their relatively low density. Eastern White Pine, for example, typically exhibits a specific gravity around 0.36, while Northern White Cedar often falls in the range of 0.31. These species float well because their primary cellular structure is composed of large, air-filled tracheids.
The Exceptions: Woods That Sink
While most wood species float, a small group of extremely dense hardwoods, sometimes collectively called ironwoods, have a specific gravity greater than 1.0 and will sink. These woods have a cellular structure that is packed with solid material. Lignum Vitae, often cited as the densest wood, has a specific gravity that can range from 1.28 to 1.37.
The high density of these sinking woods is due to their exceptionally thick cell walls and minimal internal air space. In some of these species, like Lignum Vitae, the wood also contains a high concentration of natural oils and resins, which fill the already small cell cavities. This high concentration of non-structural materials further increases the wood’s mass without significantly increasing its volume.
Other examples of woods that readily sink include Ebony, with a specific gravity often exceeding 1.12, and Desert Ironwood, which can reach 1.15. These woods are not just dense because of their cell structure; they are also slow-growing, which results in tighter growth rings and a higher proportion of solid wood substance.