Bubbles make cause it to fracture when under pressure from eating and biting
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If it had airspaces in it, it would be susceptible to changes in pressure, if the patient went scuba diving the filling could implode/explode on ascent/descent. Other activities could involve a change in pressure but this is an obvious one.
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Compaction and removal of excess water before firing are essential to reduce porosity and lower firing shrinkage.