Burned bones hold stories. From them, anthropologists can gather clues about how an ancient culture dealt with its dead or cooked its meat; forensic scientists might glean identifying information about victims of a house fire or a terrorist attack. Yet it has been notoriously difficult to extract useful information from burned bones because fire dramatically changes their chemical nature and appearance.
In a new study, researchers report that Fourier transform infrared spectroscopy (FT-IR) can not only differentiate bone burned in a fire from unburned bone but also distinguish it from fossilized bone, the composition of which might resemble that of burned bone (Anal. Chem. 2018, DOI: 10.1021/acs.analchem.8b02868). “To make a thorough and reliable analysis of skeletal remains, it would be very, very useful to know whether bone had been burned or not,” says David Gonçalves, a biological anthropologist at Portugal’s Archaeosciences Laboratory of the Directorate General for Cultural Heritage. “For biological anthropologists, this is probably the most difficult human material to work with.”
A key component of bone is a form of calcium phosphate called bioapatite. When burned, it becomes a more crystalline hydroxyapatite, with hydroxyl groups replacing some of the carbon and phosphates in the mineral, explains Tim Thompson, a biological anthropologist at Teesside University who was not involved in the work.
Previous studies have shown that FT-IR can determine if a bone specimen was burned based on the relative amounts of hydroxyl and phosphate appearing in the spectrum. But Gonçalves wanted to compare the different methods and determine whether FT-IR could distinguish such burned bones from fossilized bone samples, which also become more crystalline over time due to minerals replacing the bone tissue. Gonçalves and his colleagues tried their FT-IR-based approach on modern human bone samples—both unburned samples and ones they burned experimentally at various temperatures—as well as cremated human remains from Bronze and Iron Age burial sites in Spain and Italy. They also examined fossilized bones of ancient reptiles.
They found that the right combination of spectral analyses reliably identified bone burned at temperatures above 700 °C—indicative of direct exposure to fire—in both modern and ancient bone samples. At lower burn temperatures, organic matter and water still present in the bone interfere with the appearance of hydroxyl in the spectrum, Gonçalves says. At higher temperatures, these components get burned away, so the hydroxyl peak from the hydroxyapatite emerges more clearly.
Fossilized bone rarely shows the hydroxyl signal, the researchers found. The technique is the first to be able to separate burned from fossilized bone, Thompson says. “If it is reliable and robust, it will actually be quite useful.”
Knowing the burn temperature of a bone sample provides information about the bone’s context, Thompson explains. At an archaeological site, for example, an animal burned at high temperature might have been hunted and cooked, whereas a fossilized one might have died a natural death.
Thompson notes that the study is rather preliminary: Whether the approach can distinguish burned from fossilized bones found in different archaeological contexts—in soil, in a cave, or in water, for example— remains to be determined.