This paper describes the use of electron spin resonance spectroscopy to estimate the degree of heating of quartzite cobbles from hearths on the floor of the archaeological remains of an eighteenth‐century Dutch colonial slave lodge. A novel technique based on the comparison of line intensities for the E’and O‐2centres in quartz distinguished successfully between cobbles which had been heated to estimated temperatures ranging from 300 °C to 450° C and controls from an adjacent stream bed. This inexpensive and simple technique could be applied to a wide range of archaeological problems involving the thermal history of objects consisting of or containing