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A glass with the composition 60 SiO2, 37 B2O3, and 3 Na2O was casted and subsequently cooled to room temperature. During subsequent heat treatment at temperatures in the range from 520 to 680°C, the glass shows phase separation and SiO2‐rich droplets in a B2O3‐rich matrix are formed. The droplet size strongly depends on the temperature and duration of heat treatment. In the dilatometric curves of the phase‐separated glasses, two glass transition temperatures are observed. At high annealing temperatures, the transition temperatures vary according to the decomposition cupola. At lower temperatures, the phase formation is restricted by the kinetics. Particle sizes and growth rates are not governed by Ostwald ripening and deviate from the expected rate law d ~ t1/3. The exponent in the rate law is much smaller than one‐third, and hence, the coarsening of the droplet structured is hindered. In analogy to crystal growth models reported in the literature, this growth hindrance is explained by the formation of a highly viscous shell around growing particles.

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