British Museum collections, £12.99
Identification of the ivory of the Lewis chessmen
Scientific examination of the Lewis chessmen is ongoing, but recent investigations at the British Museum have been focused on identifying how they were shaped and decorated, and what they are made of.
Using a variable pressure scanning electron microscope (SEM), scientists at the Museum can see traces of wear and modifications caused by burial and damage.
Some of the chessmen are made from whale tooth but most are made from walrus ivory, the long upper canine teeth of the Atlantic walrus. Teeth and tusks have the same physical structures, which consist of the innermost pulp cavity, dentine, cementum and outermost enamel.
Walrus ivory structure consists of outer hard yellowish-white enamel, cementum and two distinct inner layers of calcareous dentine. The image below shows this unique structure: the primary dentine has fine concentric rings whereas the innermost core of secondary dentine, which is spongy in appearance but is as hard as the primary, has a more globular texture, sometimes described as marbled. The original carvers have maximised the use of the smoother primary dentine for the carving.
Because of its colour, old walrus ivory is reputed to be more valuable and sought after for carving than fresh material. The same structure is present in both fresh and old walrus ivory; both can be carved in the same way. SEM examination shows that many of the Lewis chessmen are made of old walrus ivory. The ivory has been damaged by marine gastropods, indicating the walrus was long dead when the tusks were collected.
Some of the chessmen are made from whale tooth. Whales have conically shaped teeth with a small amount of enamel at the tips. The rest of the tooth is covered by a substance called cementum. The dentine is deposited in parallel layers which shows up as concentric rings.
The structure of walrus ivory (and whale tooth) differs markedly from that of elephant ivory. Polished cross-sections of elephant ivory dentine display uniquely characteristic Schreger lines, often referred to as intersecting arcs or engine-turnings.