The Pb–Ca is an age hardening alloy that allows for an increase in the hardness compared to pure lead. The hardening is obtained after different successive ageing transformations. In addition, this hardening is followed by an overageing which induces a softening. The ageing and overageing transformation mechanisms are now well identified in lead–calcium alloys. In this paper, we propose to represent the domain of stability of each transformation via time–temperature-transformation diagrams for a calcium concentration from 600 to 1280ppm and in a range of temperatures from −20 to 180°C. These diagrams are constructed with the data obtained by in situ ageing with metallographic observations, hardness and electrical resistance measurements. The specificities of lead–calcium such as its fast ageing at ambient temperature and its overageing over time required the design of specific devices to be able to identify the characteristics of these alloys.