Bone

Description

Bone is mineralized hard tissue and forms the skeleton. The main morphologically distinguishable kinds of bones include hollow bones (e.g. the long bones of the extremities), flat bones (e.g. the pelvis or shoulder blade) and short bones (e.g. hand or tarsal bones).

The outer layer (compacta) of hollow bones is up to several millimetres thick and very resistant to bending stresses. In smaller and flat bones, this layer is thinner and is called corticalis. These bones contain sponge-like tissue, the cancellous bone, which provides stability and reduces the weight compared to compact bone.

Bone is a composite tissue with a mineral content of about 70% and an organic content of about 20%. The mineral content provides compressive strength and corresponds essentially to hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), a calcium phosphate. However, since it contains carbonate components and numerous substitutions of minerals and trace elements, from a mineralogical point of view, it corresponds more to a carbonate apatite (dahllite) or a biological apatite. For example, the trace element strontium used for strontium isotope analysis replaces the main element calcium. The carbonate groups (CO₃^2-) used for carbon and oxygen isotope analysis and ¹⁴C dating can replace the OH^– or PO₄^2- groups.

The organic fraction provides elasticity and torsional strength of the bone. It consists of 90% of the structural protein collagen of type I. ¹⁴C-dating and light stable isotope analysis of carbon and nitrogen are performed on collagen.

Bone is living tissue and constantly remodeled. It stores main and trace elements absorbed from food and drinking water. Data of archaeometric analyses of bone samples therefore represent average values composed over several years.

Sample properties

The kinds of bones to be sampled and sample sizes depend on the research questions and the methods used. Isotope analyses and dating are preferably performed on compact or cortical bone. Sampling cancellous bone should be avoided, because the trabeculae are very thin and have a large surface, which makes this material very susceptible to diagenetic changes during burial.

Typical kinds of samples and sample sizes for analyses on bone commonly performed at CEZA are:

¹⁴C dating as well as stable carbon and nitrogen isotope analysis

• A piece of bone, preferably 4-5 cm long, at least 1g. Please contact the laboratory if the sample is considerably smaller.
• Analyses for larger sample series should be performed on the same skeletal elements. For stable isotope analyses, we prefer rib fragments. Sampling of other bones is also possible, especially if preservation and availability require it.
• Collagen extraction is performed on the compact or cortical bone
• Bones with a very high proportion of cancellous bone and very thin cortical bone (carpal and tarsal bones, vertebral bodies, joint ends of long bones, etc.) should be avoided.

Strontium isotope analysis

• Due to diagenetic alteration during burial, strontium extractable from bones of archaeological contexts often does not reflect the isotopic composition of the biogenic strontium, i.e. strontium that originated from food and drink ingested during lifetime. Bone samples are therefore more likely to provide information on the labile strontium in the soil than about places of long-term residence of a person in adulthood.
• Bones can — in addition to other sample materials — provide information on the isotopic composition of the local biologically available strontium. In the case of bones from archaeological contexts, the strontium stored in them, often represents at least in part strontium that is labile in the soil. In contrast, modern or non-soil deposited material represents the strontium ingested during the last years of life.
• In most cases, compact bone is used as sample material. Approx. 10-12 mg are sufficient for one analysis (including pre-cleaning). For ease of handling, we recommend a sample size of at least 1 g, or a piece of at least 1-2 cm edge length.