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If phase separation within a condensed system most commonly takes place via partial crystallization, it can also occur as liquid–liquid unmixing (Chapter 5.2). Ternary systems containing boron oxide illustrate that the phenomenon should certainly not be overlooked in glass‐forming systems. For ternary borosilicates, the boundaries of the composition domains where such an unmixing takes place stably, i.e. above the liquidus, are indicated in Figure 7a. If temperature is represented in a third dimension, these domains define the base areas of immiscibility domes that eventually terminate at upper critical points at their tops. The isotherms of the immiscibility dome in the system Na₂O–B₂O₃–SiO₂, which is the base composition of all borosilicate glasses, are drawn in Figure 7b. Here, in contrast to Figure 7a, the entire dome comprising its upper critical point (755 °C at composition 25‐05‐70 by wt) is located below the liquidus surface. Hence, liquid unmixing cannot take place during the initial melting step, but at lower temperatures during the forming process. This is one of the reasons why, in order to minimize phase‐separation effects, pharmaceutical and low‐expansion borosilicate glasses are designed around a composition of 80 wt % silica. The system Li₂O–B₂O₃–SiO₂ (not shown here) displays a similar topology.

It is only with glasses known under the trade name Vycor Glass that liquid unmixing is exploited on purpose. Here, after forming by conventional technology to the desired shape, phase separation develops upon annealing at an appropriate temperature to yield two interconnected phases, namely an Na₂O‐ and B₂O₃‐rich glass along with another one that contains more than 96 wt % SiO₂. Then the former is leached out by a hot strong mineral acid, leaving behind a nanoporous skeleton of high‐SiO₂ glass. This material may then be used directly as filter, for example, or sintered at temperatures below 1300 °C to fabricate dense and almost pure silica glass articles much more readily than with pure SiO₂.

The numerical calculation of liquid–liquid immiscibility ranges in multicomponent systems (e.g. by using the software and databases mentioned in Section 3.2) is even more challenging than the calculation of solid–liquid equilibria. This is because available experimental data hardly reach beyond what has been sketched in the present section, and such a narrow base of information does not allow to fine‐tune the parameters used in the calculations.