In this work, the crystal volume fraction, α(t), of a barium disilicate (BS2) glass-ceramic was carefully investigated by optical microscopy (OM) and differential scanning calorimetry (DSC). X-ray diffraction experiment revealed that the reflected peaks of the glass-ceramic, which was prepared by heat treatment at 1000°C for 12 h, were indexed as the low-temperature orthorhombic sanbornite mineral phase. This result was confirmed by the refinement of the crystal structure parameters. Bulk samples were then heat-treated in the range of 760°C to 830°C. In each case, the α(t) values obtained by DSC were higher than those determined by OM due to surface crystallization and the formation of new nuclei during the heating/cooling steps in the DSC experiments. OM and DSC techniques were also used to estimate the number of preexisting nuclei, Nq, in a set of samples heat-treated at 790°C directly in the DSC furnace. At this temperature, it was found that the Nq obtained directly by OM measurements were in reasonable agreement with those calculated from the combination of overall crystallization with crystal growth kinetics.