Kimmeridge Bay, Dorset has long been the focus of industry, reaching back two thousand years. The bay’s cliffs are rich in bands of clay and shale. Including a 1 m band of bituminous oil shale or ‘Blackstone’. Used in road surfacing, it is a natural mix of hydrocarbons. The Iron age is the first known use of Kimmeridge shale to make polished products such as armlets. Subsequently, due its popularity production continued unaffected into the Roman period. Yet, its production still got forgotten in history. Originally, the waste cores found on beaches, known locally as Kimmeridge coal money, was a mystery. In 1955, J Bernard Calkin is thought to have donated the samples to the BNSS.

Kimmeridge Coal Money specimen at the BNSS

Discovery

The earliest written record is from Rev. Hutchins in March 1768. He states the locals use the round discs (1 to 3 inches in diameter and ¼ inch thick), for fuel. However, the discs original purpose was disagreed upon. Predominately, it was either leftover in armlet production or ancient money. 50 years on, Mr W. A. Miles became intrigued by them. Miles observed the discs were frequently found with ancient pottery, consequently he concluded that it was Phoenician. Therefore, suggesting the discs acted as imitation money, used to thank their deity Hercules for good trades. Rev. William Barnes published a repudiation on ‘coal money’, maintaining that the discs were a byproduct of the armlet industry. Owing to similar discs being discovered alongside armlets at a Roman cemetery, Fordington. By 1844, Mr John Sydenham observed signs of ‘coal-money’ in partially made armlets, which further suggested they were made with a lathe.

Production

A lathe is a machine which has rotating discs to rotate material. It was turned by a treadle, a lever worked by the foot, like the throttle in a car. Initially the shale is cut into 20 mm thick circles. In addition, two circular indentations are chiselled into both faces of the disc. Pegs, (the chuck) are put into the depressions, enabling the disc to be held securely. A V-shaped circular groove is cut into both sides by flint tools, and later iron, as the lathe is turned. Two sizes of armlets were produced from the same disc. The central disc is then removed.