The increasingly wider application of lime-based materials for the conservation and restoration of historical buildings has led to a revival of interest in lime putty. One of the most well-known and highly regarded properties of lime putty is its plasticity/workability. This paper describes the results of a rheological investigation conducted on putties having identical water content but aged for different times, with the aim of providing an objective and quantitative evaluation of plasticity. Rheological measurements have demonstrated that, all other conditions being equal, shear stress increases with aging. We propose here to consider some parameters derived from Tattersall and Bingham models that can be used as a quantitative measure of plasticity directly correlated with aging.