The mechanical properties of artificially aged bone: Probing the nature of the collagen–mineral bond

The past two decades have seen enormous advances in our understanding of the diagenetic changes that bones undergo in the archaeological record and the potential for survival of biochemical, isotopic and genetic trace evidence in excavated human and animal bones. What remain relatively poorly understood are the very early changes that take place following skeletonisation because these changes are overprinted by the slower but more dramatic modifications arising from microbial degradation and other diagenetic effects. These very early changes are of interest because of their potential impact on the longer-term survival of biomolecular evidence such as DNA. The mechanical properties of bones, in particular tensile strength, are interesting because they are sensitive to changes in the collagen fraction of bone tissues and the integrity of the protein–mineral bond. In the present study, data is presented on standard samples (N = 220) of modern bovine metapodial bone artificially aged in water at 60 °C for up to ~ 200 days. Changes in tensile strength were evaluated using the indirect diametral compression test (Brazilian test). In the control samples tensile strength was 74.14 (SD 12.9) MPa parallel to the long axis of the bone and 57.52 (SD 6.7) MPa tangential to the mid-shaft. Tensile strength shows rapid reduction with artificial ageing, losing ~ 27% after 196 days compared to the controls. Interestingly, there was a rapid deterioration (~ 16%) in mechanical properties for the initial four days, but this recovers to ~ 96% at 8 days and then declines more slowly. The reasons behind this behaviour are unclear but, if real, may represent changes in the “straight-jacketing” effects of HAP crystallites within and around collagen fibrils. High resolution SEM images of the fracture surfaces show evidence for partial demineralisation of collagen bundles and re-precipitation of crystallites on individual collagen fibrils.