The process by which ice forms during the cooling of water and aqueous solutions has been studied intensively mostly in conditions where the nucleation occurs heterogeneously - on some particulate impurity surface. When the more fundamental homogeneous process has been deliberately investigated1-3 it has nearly always been in conditions in which the crystallization process, once initiated, occurs with great rapidity1,2. Seeking an additional 'degree of freedom' in the investigation of this important phenomenon, we have initiated the crystallization in conditions where the growth is extremely slow and the nucleation events can be controlled. Our observations are described below.
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