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Filled glass–ceramic composites, like low‐temperature co‐fired ceramics (LTCC), must densify at temperatures <900°C. The densification mechanism of LTCC is often described by liquid‐phase sintering. The results of this paper clearly show that densification of ceramic‐filled glass–composites with a glass content above 60 wt% can be attributed to viscous sintering, which is decisively controlled by the viscosity of the glass during the heat treatment. This is demonstrated by the experimental determination of the viscosity of a MgO–Al2O3–B2O3–SiO2 glass dependent on temperature, by investigation of the wetting behavior of the glass on the ceramic filler mullite, and of the microstructural development. It was found that the glass does not wet the filler material in a temperature range up to 1000°C. Therefore, liquid‐phase sintering can be excluded. Independent of any wetting effect and therefore in the absence of capillary forces, densification starts at a temperature of 750°C, which corresponds to a viscosity of 109.5 dPa·s. This densification can be attributed to viscous flow of the glass matrix composite.

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