Burning issues – analysing the burnt bone from Structure Eight

Wednesday, December 27, 2023

Jackson with his animal bone deposit outside Structure Twenty-Seven' north-western wall. (📷 Sigurd Towrie) — Jackson excavating an animal bone deposit outside Structure Twenty-Seven’ north-western wall during the 2023 excavation season. (📷 Sigurd Towrie)

By Jackson Clark

In the fall of 2022, the Ness of Brodgar Trust, in conjunction with the UHI Archaeology Institute, sought out a graduate student willing to pursue one of the hot-button issues facing the Ness of Brodgar post-excavation efforts. Their question: why is there so much burnt animal bone coming from the Ness and what might that mean?

This past April, I began to pursue this Masters by Research (MRes) project under the guidance of Professor Ingrid Mainland. Now, nearly two thirds of the way through my study, we can begin to see some of the results of our efforts.

Figure 1. Burnt cattle femur from Ness of Brodgar burning experiments.

Bone is a composite material made of both organic collagen and inorganic mineral (hydroxyapatite). When bone is burnt, it progressively loses the organic material, leaving a mineral structure that is melted and rearranged by intense temperatures.

In figure 1, you can see the progressive loss of organic material as the upper portion is charred or “carbonized,” indicating the fire has started the collagen degradation.

The deeper buried (and thus hotter) portion of this femur has lost all organic material, leaving only white “calcined” mineral.

During the 2023 excavation season open day, we devised and implemented a burning experiment to examine how decay of bone might impact its appearance after firing.

The results showed that fire covers up all signs of rotting, making the timeline of when burning occurs post-mortem hard to parse.

The hearths constructed for the burning experiment were documented using photogrammetry and the models shown below.

Ness of Brodgar Trench P: Structure Eight.

After the conclusion of the 2023 excavation season, I started to investigate the massive collection of burnt bone.

Given that my project is only one year long, it is only able to focus on a small subset of the 175,000+ faunal remains collected from the Ness over the last 20 years.

This project looks exclusively at Structure Eight‘s faunal remains and closely examines the colour and fragmentation size of burnt bone.

Burnt bone is especially difficult to treat the same way as unburnt bone. Most fragments are too incomplete for the same level of species and skeletal element identification.

The loss of their organic material leaves bones much more brittle than unburnt specimens. However, there are still fragments capable of being identified, especially with the help of UHI Archaeology Institute’s comparative faunal collection.

Femur actively burning off excess fat around distal (knee) joint

A refit cattle long bone that has fragmented along heat-induced fracture lines

More medium mammal rib fragments, this time displaying exterior surfaces

Tibial fragment displaying many heat-induced fractures.

Photo of an intact lateral malleolus (distal fibula). This dense ankle bone has survived fully intact in part due to the compact and sturdy nature of the bone structure.

Figure 2. Cattle 1st Phalanx comparative specimen next to matching burnt fragment.

If a bone has an irregularly shaped surface, it becomes significantly easier to match that to a specimen, as seen in Figure 2.

Early results from Structure Eight indicate much of what has previously been asserted about the species distribution of mammals at the Ness of Brodgar.

The most populous animal is cattle, followed by sheep, with occasional specimens of red deer and even a few instances of pig. Yet the species identification is only half the battle.

Skeletal elements sometimes can be identified, even when species cannot. Ribs, for instance, are rarely identified to species level due to their similarities between similarly sized mammals. However, they have an interior texture that is unlike any other sort of long bone and thus easily identifiable to element if you know what to look for.

Figure 3. Medium mammal rib fragments (ribs are not often identified to a species level).

Figure 4. Butchering marks visible on left side of fractured lamb scapula.

Butchering can rarely be found on burnt bone, especially calcined specimens. Fire alters the external bone surface, which can erode or entirely erase the light butchering marks that Neolithic stone tools can produce.

Despite the mask of heavy burns however, there are butchering marks present on dozens of specimens, indicating that animals at Structure Eight were disarticulated and potentially consumed before the skeletons were burned, as seen in Figure 4.

This project will culminate in a thesis describing both the burnt bone at Structure Eight specifically and diagnose the next steps for cataloguing the tens of thousands of burnt bone specimens at the Ness of Brodgar.