An experiment was conducted to determine how the slaking characteristics of quicklime produced from the calcination of selected limestones relates to calcination time. The experiment also permitted the determination of the optimum calcination time by determining when each limestone was under or over burnt (as indicated by minimum water reactivity values). Four limestones, three from Scotland Dornie, Parkmore, Trearne and one from England- Ham Hill, were calcined at 900°C for a range of times between 2 and 5 h. The limestones used include both granoblastic metamorphic and bioclastic-micritic limestones with variable silicate contents. The resulting quicklimes were tested for reactivity by recording temperature rise and rate of temperature rise during slaking. Sieve analysis of the residue after slaking was performed on some samples. The Dornie limestone was the most reactive with an optimum calcination time of 4 h, Ham Hill was the second most reactive with an optimum calcination time of 3.5 h, Trearne followed at 4 h and the least reactive was the Parkmore sample with an optimum calcination time of 4.5 h. Dornie, Ham Hill and Trearne limestones underwent complete calcination with a weight loss greater than 40%. The experiment showed that the determination of the optimum calcination time through the examination of water reactivity was possible.