  Over the past 40 years measurements of the nucleation rates in a large number
of silicate glasses have indicated a breakdown in the widely used Classical
Nucleation Theory (CNT) for temperatures below that of the peak nucleation
rate. The data show that instead of steadily decreasing with decreasing
temperature, the work of critical cluster formation enters a plateau, and even
starts to increase. While many explanations have been offered to explain this
anomaly, none have provided a satisfactory answer. We present the first
experimental results that demonstrate that the anomaly is not real, but is
instead an artifact arising from an insufficient annealing time at the low
temperatures. The time-dependent nucleation rate was measured in a 5BaO.8SiO2
glass at a temperature 50 K below the peak nucleation rate temperature for a
time many times longer than any previous measurement time for a silicate glass.
The new data give results that are consistent with the predictions of the CNT.
Since the artifact has been widely observed in many silicate glasses, these
results indicate that much of the existing nucleation rate data at low
temperatures are incorrect.
