observable changes (Shipman et al. 1984):

1. 1) change in bone color;
   

2. 2) change in the microscopic morphology of bone surfaces;
   

3. 3) changes in the cystalline structure of bone; and
   

4. 4) bone shrinkage. 
   

change is dependent on (Brain 1993; Herrmann 1977; McCutcheon 1992; Nicholson 1993; Shipman et al. 1984; Sillen and Hoering 1993; Stiner et al. 1995; Von Endt and Ortner 1984):

1. 1) temperature at which bone is exposed,
   

2. 2) the duration of exposure,
   

3. 3) position of bone in relation to heat source,
   

4. 4) bone composition, and
   

5. 5) bone size . 
   

processes involved during thermal alteration (Bonucci and Graziani 1975; Kizzely 1973; Shipman et al. 1984)

1. 1) water loss,
   

2. 2) carbonate loss, and
   

3. 3) mineral sintering.
   

Relationship between temperature and degree of thermal alteration: Chemical reactions are accelerated two fold for each 10oC rise in temperature at which bone is exposed  (Von Endt and Ortner 1984).

~ 100-600oC (212-1112 F) - alterations in bone color (attributed to alterations in the chemical composition of bone, the oxidation of organics, and thin films of carbon layers deposited on bone mineral (Bonucci and Graziani 1975; Grupe and Hummel 1991; Shipman et al. 1984)).  Studies indicate that the color of bone changes progressively with increased temperature, and that the color of thermally altered bone can provide a rough index of the temperature range that bone reached as a result of exposure to heat.

         >300oC – 400oC (572-752 F) - Shell May delaminate, burn

600oC range (1112 F) - the organic phase is effectively burned away,

600oC – 800oC (1112-1472 F) - Bone becomes calcined

700oC and beyond (>1292 F) - recystallization of hydroxyapatite and the eventual fusion of those crystals.

     Shipman et al. (1984) provide the most systematic and widely cited assessment of heat-induce color alteration of bone. The researchers observed that

heat-induced color alteration can be divided into five stages:

1. stage 1 (20- <285oC, 68- <545 F), neutral white/pale yellow;
   

2. stage 2 (285- <525oC, 545- <977 F), red brown, very dark grey-brown, neutral dark grey, and reddish yellow; Grey is associated with the final stages of organic component combustion. Carbonized bone with a blackened or charred appearance is likely to have reached a temperature in the 250-550oC range.
   

3. stage 3 (525- <645oC, 977- <1193 F), neutral black dominant with some medium blue and reddish-yellow; Dark colors, particularly black, are related to the carbonization of collagen. At temperatures of 600oC and beyond the organic component is completely burned away resulting in calcination and a neutral white, chalky appearance.
   

4. stage 4 (645- <940oC, 1193- <1724 F), neutral white dominant with some light blue-grey and light grey;
   

5. stage 5 (940+oC, 1724+ F), neutral white with minimal medium grey and reddish-yellow. 
   

Microscopic morphology changes to bone surfaces: carbonization, cracking, and eventual recrystallization

its crystalline structure, and bone shrinkage due to thermal alteration observable using Scanning Electron Microscopy (SEM) (Bonucci and Graziani 1975; McCutcheon 1992; Nicholson 1993, 1995; Shipman et al. 1984), and standard light microscopy (Brain 1993; Herrmann 1977; Nicholson 1993; Richter 1986).

micro-morphological changes (Shipman et al. 1984)

1. stage 1 (20- <185o C, 68- <365 F), normal bone texture observed, surface undulating but intact;
   

2. stage 2 (185- <285o C, 365- <545 F), surface increasingly irregular, tiny pore and fissures present, but surface intact;
   

3. stage 3 (285- <440o C, 545- <824 F), bone surface becomes glassy and smooth, patterned cracking appears;
   

4. stage 4 (440- <800o C, 824- <1472 F), bone surface highly particularized;
   

5. stage 5 (800- <940o C, 1472- <1724 F), particles melt and form larger polygonal structures. 
   

hydroxyapatite crystals to increase in size as temperature increases. 

525-645oC (977-1193 F) - The major change in crystal size

> 645oC (>1193 F) - larger hydroxyapatite crystals begin to expand at the expense of the smaller crystals

> 700oC (1292 F) - observable changes in metric values and overall morphology (Buikstra and Swegle 1989; Herrmann 1977; Shipman et al. 1984)

> 800o C (1472 F).  - fusion of hydroxyapatite crystals

David (1990) has shown that bone burned during an Australian bush fire (20-30 seconds flaming combustion, no temperature data) became carbonized and showed superficial cracking and longitudinal collapsing of the mid-shaft. 

Bellomo and Harris 1991 and Sayler et al. 1989 have shown that grassfire will generate minimal thermal alteration of surface bone generally consisting of carbonized color alteration and minor charring. 

Sayler et al. (1989) have shown that while surface bone is moderately affected by prairie fire, bones buried as shallow as 2cm are not likely to be susceptible to significant thermal alteration.

Thermal alteration of subsurface bone as the result campfires can also be demonstrated (Bennett 1999; De Graaff 1961; Stiner et al. 1995).  Campfires can reach temperatures in excess of 800o C, and it has been shown that bone buried as deep as 10cm below campfire can show significant thermal alteration.  The degree of thermal alteration is dependent on the depth of burial beneath the heat source and the heat transfer and retention properties of the sediment.  Other important variables include the pre-burn condition of bone as well as species and skeletal element represented.  It is unlikely that significant thermal alteration of subsurface bone will occur during natural fires.    

References

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Bennett, J.L. 1999. Thermal Alteration of Buried Bone. Journal of Archaeological Science 26:1-8.

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