Bones exposed to fire undergo significant transformations, a topic of interest in various fields, including forensic science and archaeology. While the common understanding might suggest bones simply burn away like other organic materials, the reality is more complex. Bones do not combust entirely in the same manner as wood or paper. Instead, they experience irreversible changes in their composition and structure when subjected to high temperatures.
Understanding Bone Structure
Bones are intricate composite materials, consisting of both organic and inorganic components. The organic portion, about 30-35% of bone’s dry weight, is primarily collagen, a protein providing flexibility and elasticity.
The inorganic component, making up 60-65% of dry weight, is largely crystalline calcium phosphate, specifically hydroxyapatite. Hydroxyapatite gives bone its rigidity and hardness, allowing it to provide structural support and protect organs. This combination of flexible organic and rigid inorganic materials provides bone with both strength and resilience.
The Process of Bone Alteration in Fire
When bones are subjected to heat, their organic and inorganic components react differently. As temperatures rise, particularly above 200-300°C, the organic collagen matrix degrades. This degradation involves charring and combustion of the collagen, releasing smoke and volatile gases.
Simultaneously, the inorganic mineral matrix, primarily hydroxyapatite, undergoes calcination. This process removes water and breaks down any remaining organic material within the mineral structure. While hydroxyapatite does not combust, calcination changes its crystalline structure, often increasing crystallinity and crystal growth at higher temperatures. Temperatures above 600°C are required for complete combustion of organic carbon, leading to a calcined state.
Different temperature ranges produce distinct effects on bone. At lower temperatures, charring and discoloration occur, progressing from yellow to brown and then black as organic material carbonizes. As temperatures exceed 600°C, the bone transitions to a white or bluish-gray appearance, indicating full calcination. Extreme heat can also cause shrinkage and increased porosity due to the loss of organic content and water.
What Remains After a Fire
After significant exposure to fire, what remains of bone is predominantly its calcined inorganic mineral framework. This residual material is characterized by its fragility and often a white or grey coloration, depending on the maximum temperature reached and the availability of oxygen during the burning process. The loss of organic material and water makes the calcined bone significantly lighter than its original state. While the overall shape of the bone might be largely preserved, the material itself becomes extremely brittle and prone to fragmentation. This altered mineral structure is distinct from typical ash; it is a transformed mineral residue, chemically durable but physically fragile.