Cremation is a method of final disposition of a dead body through burning. During modern cremations, the body is placed in a cremation chamber or retort where it is subjected to intense heat and flame, which reduces the body to its basic elements and bone fragments. The remaining bone pieces are then processed into ash and returned to the deceased’s family. Given the high degree of heat and duration of the cremation process, the entire body is eventually consumed by the flames. However, some parts of the body take longer to completely burn than others. The bones take the longest to burn and are often the last part of the body to fully disintegrate during cremation.
What happens during the cremation process?
The modern cremation process takes place within a cremation chamber or retort, which is an enclosed furnace-like machine designed to deliver heat of 1400-1800 degrees Fahrenheit and proper air flow to facilitate burning of the body.
During a cremation, the body is placed in a combustible container or casket, which is then loaded into the cremation chamber. Once the cremation starts, the container and body are subjected to intense heat and flame. First, the container catches fire, and the body within also ignites soon after. Over the course of 1 to 2 hours, the body is gradually reduced from solid form to ashes and bone fragments as the organic soft tissues are consumed.
The high heat dries out the body, causing water to evaporate and fluids to boil. Meanwhile, the heat breakdown of proteins and melting of fats leads to liquefaction of the soft tissues. As the soft tissues burn away, the skeletal system and areas with more connective tissue and collagen will remain the longest. The bones start disintegrating once temperatures reach about 1000°F and become calcified by the heat. However, the bones take the longest to fully break down.
After an initial 1-2 hour cremation, the skeletal remains contain fragments of bone and calcium phosphates. The fragments are then raked out of the main chamber and any metal surgical implants are removed. The remaining bone pieces undergo final processing using a processor that pulverizes them into a powder-like consistency to create the cremated remains or “ashes.”
Why do bones take the longest to burn?
Bones are more resistant to fire than the soft tissue organs. Their composition and structure make them difficult to fully destroy and turn to ash. Here are some reasons why bones take the longest to burn during cremation:
- High mineral content – Bones are made up of about 65% mineral salts and calcium phosphates like calcium hydroxyapatite. These minerals have very high melting points of over 2500°F.
- Organic collagen matrix – Bone contains an organic protein-based matrix made up mainly of Type 1 collagen. This gives bones tensile strength and a fibrous quality.
- Low water content – The low moisture content of bone makes it harder for the heat to break it down through evaporation.
- Compact structure – Dense sections of bone don’t allow heat to penetrate easily.
- Geometry of bones provides support – The shape and geometry of bones like the skull and pelvis create structural supports that maintain their form even when subjected to high temperatures.
The inorganic mineral portion gives bones their rigidity and high melting point. The collagen matrix holds the minerals together in a structural lattice while allowing some flexibility. This combination gives bones toughness and resistance to fracture and burning.
Regions with the highest bone density, like the petrous portion of the temporal bone or base of the skull, take the longest to fully cremate since it’s difficult for heat to penetrate them. Prosthetics, dental fillings, surgical screws or plates will also be slow to breakdown and remain amongst the bone fragments after cremation.
What is the order in which parts of the body burn?
While bones take the longest to burn, the other soft tissue organs and regions of the body burn away in a general sequence:
1. Body fat – Fatty areas burn and liquefy first.
2. Muscle, organs and soft tissue – Muscles, internal organs, skin, veins and arteries burn next. The brain, heart, lungs and liver, in particular, have high moisture content and burn faster than denser organs.
3. Tendons and ligaments – Connective tissue and collagen-rich areas evaporate and burn after organs.
4. Cartilage and joints – Cartilage also has a collagen matrix that takes time to break down. The joints will burn after organs and soft tissues.
5. Bone – The mineral composition of bone makes it the last to fully burn. Smaller, porous bones burn faster than larger, dense ones.
During the cremation process, the body essentially burns from the inside out with the organs and soft tissues burning first while the hardest parts like bone, cartilage and joints burning last. The sequence is due to the physical and chemical properties of each tissue type when subjected to heat. The water content, amount of fat, collagen levels, and mineral composition all affect the burning rate. While skeletons burn over a period of hours, research on burn rates suggests that exposure to temperatures of 1650°F can calcify and burn bones to a white powder within just 40 minutes.
Do the skull and teeth burn?
Yes, the skull bones and teeth eventually burn during cremation. However, they can withstand higher temperatures than other bones before breaking down.
The skull bones, especially the cranial base and mandible, have areas of very dense compact bone that require prolonged exposure to high heat before fully burning. The dense petrous portion of the temporal bone is often the last skull bone to burn.
While tooth enamel is highly mineralized and can remain intact during initial phases of burning, with sufficient heat and time, the teeth will eventually calcify and fully burn. However, small tooth fragments or fillings may remain behind with the bone residue following cremation. The skull typically burns in 30-45 minutes at heat above 1600°F.
Factors affecting burn rates
Several external factors can affect the burn rates of different body parts during a cremation:
– Temperature of cremation chamber – Modern retorts can reach over 1800°F. Higher temperatures above 1600°F calcify bones faster. Cooler temperatures below 1400°F burn tissue slower.
– Air flow – Air circulation removes combustion byproducts and allows better oxygenation for burning. Increased air flow speeds up burning.
– Body size and weight – A larger, overweight body burns slower than a petite body since there’s more mass and fuel to consume.
– Body positioning – Body parts directly exposed to flames burn faster. Fetal positioning promotes air flow and burns tissue quicker.
– Casket/container – A combustible container burns and exposes the body faster than a sturdy metal casket.
– Foreign objects – Items like batteries, silicone implants or radioactive seeds burn slower and may need to be removed afterwards.
– Bone disease – Bones are weaker in conditions like osteoporosis or decalcification and burn faster.
Under standard cremation conditions, the expected time for complete bone burning is 1.5-3 hours for an average adult. Bigger bodies may take closer to 3 hours for full skeleton consumption. The most critical factor is ensuring the cremation chamber reaches a peak temperature above 1400°F and can sustain heat exposure over time.
Conclusion
During the modern cremation process, the skeletal system is the last part of the body to fully burn and turn to ash due to the heat resistance of mineralized bones. While the exact order varies between individuals, the bones burn after the organs and soft tissues have evaporated away. Dense regions of compact bone, such as in the cranium and joints, take the longest to calcify and disintegrate under high temperatures, so they remain as the final fragments before the cremated remains are processed. Understanding the stages of burning during cremation helps explain why bone is the last to go.