The buoyancy of bone is determined by the laws of physics and the biological state of the bone. Taphonomy, the study of how organisms decay and fossilize, examines this issue in forensic and archaeological contexts. A bone’s flotation or sinking is determined by its density relative to the density of the surrounding fluid. The condition of the bone—whether fresh, dried, or decomposed—dramatically changes this density ratio.
The Science of Buoyancy and Density
An object’s ability to float is governed by the principle of buoyancy. This principle states that an object immersed in a fluid is pushed upward by a force equal to the weight of the fluid it displaces. This force relates directly to the object’s density (mass divided by volume). For an object to float, its density must be less than the density of the surrounding fluid.
The density of pure fresh water is approximately 1.0 gram per cubic centimeter (g/cm³). Saltwater is slightly denser, averaging around 1.025 g/cm³. Any bone fragment with a density higher than 1.0 g/cm³ will sink. The overall structure and composition of the bone determine whether it meets this threshold.
Internal Composition of Fresh Bone
Fresh bone is a composite material generally denser than water, causing it to sink immediately upon submersion. The bone matrix has two primary components: a heavy inorganic mineral phase and a lighter organic phase. The mineral component, mainly calcium phosphate (hydroxyapatite crystals), provides hardness and possesses a very high density.
The organic phase consists of collagen protein, water, and bone marrow (a mix of fat and blood-forming tissue). Compact cortical bone, the dense outer layer of long bones, has a very high density, often ranging between 1.6 g/cm³ and 2.1 g/cm³ in its wet state. This high mineral density ensures cortical bone sinks.
Trabecular bone, the spongy bone found inside vertebrae and at the ends of long bones, contains numerous small spaces. These spaces are filled with water and marrow, giving this bone a much lower density. While dried trabecular bone can be as low as 0.72 g/cm³, fresh wet trabecular bone is typically near or slightly above the density of water. However, fresh bone is part of a larger structure that, as a whole, is denser than water.
How the Condition of Bone Affects Flotation
The long-term fate of a submerged bone depends on post-mortem changes that alter its composition and volume. Decomposition is a significant factor, as soft tissues, including marrow and organic components, begin to decay. This loss of material can initially reduce the bone’s overall mass. However, decomposition may also lead to water logging, where porous parts absorb water, increasing density again.
Weathering and drying also change buoyancy, especially for bones exposed to air. When a bone dries out, the water content is lost, making the bone lighter. Since the volume remains constant and the dense mineral components remain, the bulk density of the bone structure changes unpredictably. Generally, a fully dried and defleshed bone is still denser than water and will sink, particularly if it is compact cortical bone.
Burning a bone, known as calcination, drastically affects its density. High temperatures remove all organic material, including collagen, leaving behind only a brittle, highly porous mineral structure. This loss of organic mass and increased porosity significantly reduces the overall density of the fragment. Although the mineral itself is heavy, the air-filled spaces created by burning can sometimes make calcined bone fragments less dense than water, potentially allowing them to float.