A fired clay surface, thought to have been a hearth, was discovered during archaeological excavation at the Royal Mint site in London. Archaeomagnetic dating suggests that the feature was last fired towards the end of the 13th centry AD although, owing to crossovers in the archaeomagnetic calibration curve used, two date ranges in the Roman period must also be considered. The archaeomagnetic measurements also suggested that the feature had been disturbed slightly since this date.