The self-healing of a crack in a glass-boron composite has been observed by X-ray nanotomography. It shows the occurrence of a healing effect within the bulk of the composite, despite of a limited oxygen access in the crack. This 3D tomographic observation offers new insights in the mechanism of healing, complementary to in situ high-temperature environmental scanning electron microscopy. In addition, nano-X-ray fluorescence imaging, electron microprobe and solid-state NMR gave evidence that the molten B2O3, produced by the oxidation of boron particles at 700 degrees C, reacts with the glass matrix to form borosilicate compounds that also contribute to heal the crack. The high viscosity of B2O3 at 700 degrees C leads to the formation of bridges between the walls of the crack, which limit oxygen diffusion. Thus, the B particle oxidation is not completed after a single healing cycle, meaning that several healing cycles can be obtained in a composite.