1.2.1 Composition and Structure of Bone and Related Materials
Bone is a biological material that is produced by vertebrate animals. In living bodies, bone functions as both an organ and as a rigid skeleton that supports and contains soft tissues. Fresh bone is composed of inorganic calcium phosphates precipitated in an organic collagen matrix. More specifically, bones are generally composed of 20ā30% collagen (protein) and 60ā70% calcium phosphates (bone mineral), with the remaining <10% comprising a combination of other components such as complex sugars, lipids, carbonates, Mg, Na, trace elements and metal ions (White & Hannus 1983; Posner et al. 1984; Pate & Hutton 1988; Linse 1992; McCutcheon 1992; Currey 2002). The mineral component of bone is commonly referred to as: (i) hydroxylapatite or hydroxyapatite (Ca5(PO4)3(OH)); (ii) bioapatite (a poorly crystalized calcium phosphate resembling hydroxylapatite); or (iii) carbonate hydroxylapatite (Ca5(PO4CO3)3(OH)) also known as dahllite (Pate & Hutton 1988; Linse 1992; McCutcheon 1992; White & Hannus, 1983; Stiner et al., 1995; Karkanas et al. 2000; Hedges, 2002; Berna et al. 2004; Trueman et al. 2004; Smith et al. 2007). In reality, bone mineral is difficult to characterize. Mineralogical analyses reveal that pure hydroxylapatite is never actually found in bone or teeth, thus Weiner (2010) argues that this term should be reserved for geogenic, noncarbonated forms of apatite. Furthermore, the mineral dahllite is no longer recognized by the International Mineralogical Association. Therefore, despite the general inconsistencies found in the literature regarding bone mineral, the terms bioapatite, carbonate hydroxylapatite, or carbonate apatite are most appropriate.
The same combination of collagen and bioapatite occurs in mammal, bird, reptile and fish bones. The only exceptions are fish of the elasmobranch type (sharks, skates and rays), whose skeletons consist of cartilage containing a different type of collagen (type II) and no bioapatite (Szpak 2011).
Besides bone, other hard biological tissues that can appear in the archaeological record are antler, teeth and keratin structures like horn and hair (of the aforementioned tissues, hair is the only nonskeletal one). Antler is a bony extension of the skulls of deer that has the same composition as bone (bioapatite, collagen, noncollagenous proteins and water ā Currey 2002). Teeth are composed of three different hard biological materials: enamel, dentin and cementum. These materials have the same general composition as bone, but differ in the relative proportions of mineral to other components; enamel, dentin and cementum contain >95%, 75%, and 45% carbonate hydroxylapatite, respectively (Provenza & Seiber 1986; Weiner & Wagner 1998; FrancillionāViellot et al. 1990; Weiner 2010). Horn exists on animals from the Bovidae family (cattle, sheep, goat, etc.). It differs from bone in that it contains keratin (alphaākeratin, a fibrous protein also found in hair, nails, wool and claws) with minor amounts of bioapatite (Hashiguchi & Hashimoto 1995; Salamon 1999; Hashiguchi et al. 2001; OāConnor et al. 2015) or no crystalline phase at all (Tombolato et al. 2010). The hair fibre is made of hard keratin, water, lipids, pigment and trace elements (Wilson & Tobin 2010). Horn, hair and other keratinous tissues rarely survive in the archaeological record unless burial conditions impede biological activity (Wilson et al. 2007; Wilson & Tobin 2010; OāConnor et al. 2015).
In addition to having different compositions, hard biological tissues differ in their macroscopic structure. Bones can be divided into two different structures according to their porosity. Compact or cortical bone has low or null porosity. Spongy bone, also known as trabecular or cancellous bone, has high porosity. The boundary between both types is diffuse since compact and spongy represent a continuum (Weiner & Wagner 1998; FrancillionāViellot et al. 1990; Currey 2002; Weiner 2010). Spongy bone is frequently, though not exclusively, found in bone epiphysis or inside vertebrae, whereas dense compact bone is found in the shaft of long bones (bone diaphy...