Conserving the Lewis Chessmen

Some of the Lewis chessmen have undergone minor conservation treatments over the years. Mobile splits and cracks have been found, which are stabilised by injecting a suitable reversible adhesive into the fractures. If necessary, slight pressure is applied until the adhesive is set.

The chess pieces are made of ivory. Ivory is a mixture of organic and inorganic materials: the inorganic part consists of calcium phosphate in association with carbonate and fluoride (hydroxyapatite); the organic material is ossein (a constituent of bone). Ivory is hygroscopic (it absorbs water from the atmosphere) and is prone to splitting and warping on exposure to moisture and/or heat. The ossein is broken down on prolonged exposure to water, leaving the object brittle and chalky. The hydroxyapatite is readily attacked by acids. The porous and hygroscopic nature of ivory make it susceptible to the effects of fluctuating humidity: as the humidity level rises an ivory artefact absorbs moisture and swells; as the level falls, absorbed moisture is released and the object shrinks. Repeated swelling and shrinking can cause the object to crack or split, like the Lewis Chessmen. Ideally, the relative humidity should be maintained at between 50 and 55% at 19 degrees centigrade. Where this is not possible a stable environment that is neither too moist nor too dry is desirable.