Heat Induced Changes to Bones
Final Paper
By Danielle M. Santos
Tuesday, December 11, 2012
At Humboldt State University
For Forensic Anthropology: ANTH332
Instructor Mary Glenn
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Heat Induced Changes to Bones
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Abstract: This study looks at the changes occurring to bones when exposed to fire and
heat. Findings are largely a result of bones in a controlled research environment. The goal
of this document is to explain heat induced changes to bone material for use in forensic
anthropological research methods. Methods used to produce this paper are a result of
archival research and analysis of peer reviewed articles. Findings include changes at
microscopic and macroscopic levels of bone material. The changes are based upon a
multiple of factors occurring during the time of exposure.
Introduction: Often times the only bones left at an archaeological site are those that have
been burned or calcinied; especially in moist temperate and tropical environments where bones
often decompose completely. (Whyte) Human and animal bones are often burned as a result of
cremation, cooking and deaths involving fire. For humans the act of practicing cannibalism and
cremation are a worldwide phenomenon and are often linked to ritual and torture. (Whyte) In
regards to forensics cases, events resulting in death are the main focus for burned skeletal
remains. Forensic cases involving burned remains typically follow events that include aircraft
accidents, bombing, explosions, earthquakes, homicides, suicides, and accidental deaths. The
event of using fire to destroy human remains in attempts to hide evidence of identification or
recovery should also be considered. (Ubelaker)
Applications: Many cases in forensic anthropology involve a need for interpretation of
fire and heat affected bone remains, but scientific studies are sparse on the matter. Recently more
studies are being produced on thermal alterations as forensic teams routinely need a systematic
form of analysis for such cases. Issues occurring in regards to these types of cases include:
“recovery, recognition, trauma interpretation, bone recognition, weight interpretation,
thermal correlations with coloration, shrinkage and structural changes, distinguishing
bones burned in the flesh from bones burned without flesh, technological analysis and
DNA extraction techniques and success rates.”(Ubelaker)
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The degree of importance in researching thermal alterations is to improve the anthropological
techniques used to analyze thermal changes to bone. Complicating recovery two major factors in
particular are referred to as: fragmentation and context. Thoroughly collecting remains can be
extremely difficult for forensic teams to determine when burned bone and teeth are mixed
amongst building materials and other items at a crime scene or archaeology site. (Ubelaker)
Misidentifications of bone remains are likely to occur for several reasons and have been a
nuisance to archaeologists, zoologists and forensic anthropologists alike. It is difficult to
distinguish between animal and human bone after they have gone through processes of
calcination, whereas human bones are often confused with mammalian bones. One must be able
to distinguish cremated bones from other kinds of burning and then determine through research
whether it’s safe to assume that humans were the only mammals historically and culturally
cremated in the area. (Whyte) Unidentifiable animal bones should be closely scrutinized if they
meet all the following criteria: 1) they are calcinied, 2) diaphyseal portions exhibit some
combination or transverse cracks, longitudinal splits of warping, 3) trabecular (internal spongy
bone) exhibits reticular cracks and/or dendritic fissures, 4) fragments do not exhibit evidence of
fresh bone perimortem fractures. (Whyte) It is advised to follow these criteria specifically in
places that are commonly known for animal sacrifice or disposal by burning.
Methods: The degree of destruction that appears on bone is unfortunately majorly
studied by using a crematory or muffle furnace. Two of the studies I looked at included literature
of crematory controlled studies, but also included studies on outdoor burnings, campfires, boiled
and roasted techniques. Studies included use of human and animal bones.
Procedures of analysis have not been standardized for burned bone materials and
methodological procedures are employed based on approaches available. A full and
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comprehensive study understanding heat induced changes to bone is necessary for
anthropological practitioners to analyze evidence and properly implement relative techniques.
The sample size, frequency and duration of heat or fire to bone directly correlate to the
relationship of findings. According to Stiner, this relationship holds true for bones throughout
Middle, Upper and late Upper Paleolithic cave assemblages. (Stiner)
Extrinsic Factors: Extrinsic factors are ones “based on the environment surrounding the
bone, such as the pH level of the soil and different organisms, might lead to deterioration.”
(Baxter) These factors are extremely important in predicting changes that occur. Spatial
association or the context of material surrounding a body for example in a house fire, will
influence the differential preservation of the bone.(Stiner) It is also interesting to note that
signature evidence of burned bone like crystallization are now found to overlap with weathering
and fossilization crystallization evidence. (Stiner) Studies conducted through infra-red
spectrometry and x-ray diffraction techniques produce contradictive comparisons on
differentiation of burned bone with bones that have been highly weathered of fossilized, which
offers new insight to analysis of weathered and fossilized bones. (Stiner) Infra-red spectrometry
was used to measure the mineral crystallinity which was a signature for burned bones and is now
method for weathering and fossilized bones as well.
Bone Weathering: Bone weathering is a result of exposure to elements such as wind,
sun and/or freeze thaw cycles. (Stiner) Under these conditions the smooth cortex of bone soon
disintegrates leading to cracking and splitting, and then eventually flaking away from the outside
inwards. (Stiner) Weathering damage of course varies to location by local conditions. Studies
done show that bones weathered for 2 and 9 years from two different locations show some
reduction of collagen content. (Stiner) The data collected suggests that microscopic
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transformation caused by bone weathering occurs rapidly with for the first year of exposure then
stabilizes. Change in bone crystallinity of burned bone also overlaps with bone weathering in
codes 0 and 3 of color code (pg.5).
Conditions: There are four phenomenon’s pertinent to reading burned bone damage:
1)Visible changes to bone color, 2) Changes to bone mineral matrix, 3)Alterations in the
mechanical properties of bone that promote fragmentation, and 4) The extent to which soil
insulates burned bones from fire on the ground surface. (Stiner) In order to thoroughly explore
bone damage it is important to distinguish the condition of the bone at time of exposure and
whether or not it is a fleshed bone(bone covered in soft tissue), defleshed bone (green bone or
fresh bone), boiled bones, baked bones, and anhydrous (dry) bone. (Whyte) The condition of the
bone prior to burning is an important place to begin analysis and to recognize that bone tissue
and soft tissue are intrinsically connected as a unified system. Experiments prove that it is
difficult to distinguish physical differences between burned or cremated in fleshed and fresh
bones.
Microstructural changes: Intrinsic factors “take place within the bone such as "the
spontaneous rearrangement of the crystalline matrix and the action of internal water on the
proteins of bone".” (Baxter) In cases where bones become baked fat content reduces due to fire
or coals. (Whyte) Boiling soups or stews releases collagen from the bones.(Whyte) Both, fresh
and fleshed bones cremated exhibit identical combinations of cracks and fractures. (Whyte)
Fresh bone is often by weight composed of 60-70% of dahilte crystals, also known at
carbonate apatite. At ambient temperatures the diagenesis of the crystals are altered and are
where large crystals grow at the expense of the smaller ones. (Stiner) This process was not
stated, but referred to earlier in this paper, where similarities in crystalline structures were found
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in fossilized and weathered bones. The process occurs when non-burned bones are exposed to
temperatures of 650°C and up the solid state of recrystallization forms. (Stiner) Through
scanning electron microscopy the dentine of teeth show structural changes at 600°C and at
800°C enamel rods began showing altered structure. (Ubelaker) There are significant differences
between burned and unburned bone in the crystalline and texture that are revealed when involved
with microscopic of X-ray diffraction analysis. (Whyte)
Coloration: The chemical reaction of fire is based on four requirements: combustible
material, adequate ignition temperature, sufficient oxygen, and sustainable environment to
maintain conditions; explaining the routine use of a crematory. (Walker) Color changes reflect
the chemical process occurring during cremation or induced heat. (Walker) The color change is
specific to temperature, duration of burning, and availability of oxygen as mentioned above.
Observations of visible stages of burned fresh bones are classified by color on an ordinal
scale of 0-6. These stages begin at code 0 with an unburned bone of ivory or light tan color. The
color of bones exposed to heat in a crematory begin to occur at temperatures as low as 200°300°C; at this temperature the color tends to change from the unburned color of ivory or light tan
into the code 2 followed closely by code 3. The intermediate burning stage code 2 centers on
carbonization transforming the bone color from unburned to dark brown of black. The dark color
is a result of organic components becoming carbonized. 100% Carbonization is distinguished by
code 3 and is visibly pure black in color. At higher temperatures the bone begins to turn from
black to grey as carbonation completes. Carbonate values decrease as damage intensifies as
viewed by infra-red spectrometry. (Stiner) Once carbonates disappear, bones at an increased
temperature of 800°C become “calcined” and the color changes again to blue-gray or white.
Codes 4 and 5 represent partial calcination and are often grey to white in color. (Stiner) The most
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advanced stage is code 6 and is where the bone has reached 100% calcination, the visible color is
pure white. (Stiner) Code 6 shows through infra-red spectrometry that the original ldahilte lattice
recrystallizes therefore losing in carbonate to form, the disputed result, hydroxyapatite. (Stiner)
The “calcined” bone is due to the high temperature causing the carbon to bond with oxygen to
form CO2 and the bone salts begin to fuse. Each of these processes requires a specific type of
energy and depends on the temperature, duration and amount of oxygen present. (Walker) Color
has been determined a reliable indicator of evidence that bone has been burned, but does not
distinguish to what degree the bone has been burned. (Whyte)
The change in color to bones is affected by high temperature exposer based on the
amount of oxygen available, duration, temperature of fire and of the bone. (Ubelaker) Bone may
not become calcined depending on the duration of time and temperature the bones were exposed
to heat or fire. (Whyte) Staining often results in immediate environment around bone, affected by
variations of soil composition. (Ubelaker) Attention has been brought to soft tissue and its effects
on bone color and heat induced changes. (Ubelaker) A recent discovery has led researchers to
believe that when soft tissue is present at time of burning that the protected bone will exhibit a
sequence of ‘calined, charred, border and heat line zones’ that are specific to flesh covered bones
that are exposed to heat. (Ubelaker) Bone color serves as a good indicator as to if collagen is
present or not. (Walker)
Cremation: As mentioned above bones may become exposed to heat induced changes
through cremation, cooking or deaths involving fire. The act of cremation although practiced
currently, is an ancient technique that alongside cannibalism appears all over the globe. These
ritualistic practices are archaeologically common and historically often a result of human
sacrifice or torture. Human experimental cremation often produces studies that concentrate on
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fracture patterns in long bones, it is noted that fracture pattern are influenced by bone anatomy.
(Whyte) The main microscopic stages of cremation or bone degradation include: dehydration,
decomposition, inversion and fusion. (Walker) Soft tissue does not need to be removed, but does
supply a significant amount of protection to the bone. The bone although is a hard tissue is
composed of moisture, bone marrow and blood therefore making a heat induced transformation a
complex phenomenon. (Thompson) The first stage: dehydration is where “…hydroxyl bonds
break and loosely-bound water (physisorbed) and bonded water (chemisorbed) are lost.” The
second stage Decomposition: “…is when the organic components of the bone are removed by
pyrolysis.” The third stage of inversion is the loss of carbonate. The last stage is Fusion: and
“…is characterized by the melting and coalescence of the crystal matrix.”
Macrostuctural changes: During diagenesis fresh bone loses its strength that it had as
living bone tissue, hence significant rearrangement in crystal lattices affecting the pressure
resistance when bone is exposed to heat induced changes. (Stiner) Microscopy and Scanning
electron microscopy (SEM) have been used to scrutinize fracture patterns, also proving useful in
the transition of the inorganic phase in Fusion stage of bone degradation. (Thompson) Heat
factures are always associated with color change. (Whyte) The susceptibility of a bone to
fracture can be determined observed in three ways: 1) direct product of heat alone (without
added pressure), 2) vigorous agitation of fragments sorted by burn color, and 3) trampling
premeasured whole bones buried beneath a cooled fire bed. (Stiner) Notes have been made that
the larger the mammals are the more cracks show up per unit area than does with bones of
smaller mammals, relative that the two endure similar conditions.
Dry bones exhibit longitudinal splitting superficial checking of external surfaces with less
warping. Fleshed bones displayed warping, transverse fractures appearing in a curvilinear pattern
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and irregular longitudinal fractures. (Ubelaker) “The clearest form of dimensional change to
bone is warping.” (Thompson) Warping is most apparent in fleshed bone, this implies heat
induced muscle fiber contraction to pull and twist the bone in an unnatural shape. Air exposed to
the medullary cavity also causes dimensional changes. (Thompson) Areas of dense bone should
experience less reshaping. If the bone is calcined through cremation they tend to exhibit warping,
transverse curvilinear fracturing and deep checking or reticulations. (Whyte) Burning renders
bones more susceptible to fracture and fragmentation. The greatest decline in macroscopically
changes occurs between color codes 0 and 3. (Stiner)
Trauma: Trauma can survive a burn event and be recognized, but its appearance may be
affected by fragmentation related to the burning. (Ubelaker) Studies have also noted that
evidence of sharp force blunt trauma survives burn events, but needs further analysis on fractures
and surface morphology. It was suggested that further analysis would produce findings that sharp
blunt force trauma is similar to fractures that occur from heat induction. (Ubelaker)
Fragmentation can influence degradation of bone affecting the organic material such as collagen
or albumin used for identification. Identification can be useful in assessing the MNI and
extremely pertinent in Forensic Cases.
DNA: DNA is extremely useful in forensic cases, it is important in determining sex of the
individual which will increase the ability to identify burned remains. (Walker) Dental pulp can
hold DNA to up to 300°C successfully; temperatures above 300° were not successful in DNA
extraction. (Ubelaker) Although, studies have been conducted on fire victims exhibiting extreme
charring reported successful in extracting DNA, research through experiments on bone samples
heated to temperatures between 800°- 1200°C indicated that human mitochondrial DNA did not
survive, but histological structures preserved. (Ubelaker) Collagen persists in bones up to
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temperatures as high at 600°C. (Walker) This phenomenon suggests that factors such as
temperature, duration, amount of oxygen present and other intrinsic and extrinsic factors
mentioned above have played a role in the process of degradation. In the house victims fire case
it is possible and common that the fire did not reach the temperature of 800°-1200,° as a
crematory can reach. It is also possible that since it was a house fire that the well ventilation
exposed to fire to too much oxygen also lowering its temperature.
Relationships: The organic component of bone can survive up to temperatures of 400°C;
and at 600° C bone mineral recrystallizes. (Ubelaker) Studies show that shrinkage is initiated at
temperatures of 700° C and then augmented at 800°C; this process is greatly affected by the
duration of time and temperature the bone is exposed to heat. (Ubelaker) Wood fires in natural
environments show that changes in crystals can occur and that live coals can cause calcination,
changes in surface texture and in color. (Ubelaker) Fully calcinied bones are rare in these cases,
but many are fully carbonized. (Stiner)
Conclusion: There is a clear cut relationship amongst bone discoloration, microscopic
and macroscopic morphology, crystalline structure and shrinkage caused by heat induced
changes. (Stiner) As talked about, events involving need for forensic analysis are in increasing
demand. Although, methods have produced results in past studies extrinsic and intrinsic factors
vary. The variation amongst factors skew results, in understanding that unless alike conditions
exist between case and study, results and analysis will differ. The condition of the bone is
pertinent to reading and understanding bone damage.
Studies included emphasis of how intrinsic factors change within the bone during
exposure to heat. A highly studied area includes the macroscopic visible color change that results
from microscopic transformations the of bone matrix.
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The extrinsic factor of a crematory was also discussed to explain relationships
specifically related studies being produced methodically using a crematory. Findings in using a
crematory show changes in the macrostructural changes that include warping: of bone and
fracture patterns, and of course coloration. It is noted that trauma may also be successfully
assessed, given the condition of the bone to permit observation. Lastly, DNA extraction and
relationships between factors is discussed in a closing observation to give a wrap-up analysis. It
is obvious that heat induction on bones is a sought after understanding, but the main conclusion
is that more research is needed in being able to provide effective methods and analysis of this
type of remains.
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References Cited
Thompson, T. (2005). Heat-induced dimensional changes in bone and their consequences for forensic
anthropology. Journal of Forensic Sciences, 50(5), 1008-1007.
Ubelaker, D. (2009). The forensic evaluation of burned skeletal remains: A synthesis. Forensic Science
International, 183, 1-5.
Bohnert, M. , Rost, T. , Faller-Marquardt, M. , Ropohl, D. , & Pollak, S. (1997). Fractures of the base of
the skull in charred bodies--post-mortem heat injuries or signs of mechanical traumatisation?. Forensic
Science International, 87(1), 55-62.
M.C, S. , S.L, K. , Weiner, S. , & Bar-Yosef, O. (1995). Differential burning, recrystallization, and
fragmentation of archaeological bone. Journal of Archaeological Science, 22(2), 223.
Bohnert, M. , Rost, T. , & Pollak, S. (1998). The degree of destruction of human bodies in relation to the
duration of the fire. Forensic Science International, 95(1), 11-21.
Devlin, J. , & Herrmann, N. (2008). Bone color as an interpretive tool of the depositional history of
archaeological cremains-6. The Analysis of Burned Human Remains, 109,xThompson, T. , & Chudek, J. (2007). A novel approach to the visualisation of heat-induced structural
change in bone. Science & Justice, 47(2), 99-104.
Thomas R. Whyte(2001). Distinguishing Remains of Human Cremations from Burned Animal Bones.
Journal of Field Archaeology , Vol. 28, No. 3/4 (Autumn - Winter, 2001), 437-448
Baxter, Kyle, "EXTRINSIC FACTORS THAT EFFECT THE PRESERVATION OF BONE" (2004). Nebraska
Anthropologist. Paper 62.
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