Hollow glass microspheres
Hollow Glass Microspheres?Syntactic foams

It was in the USA, in the 1960s, that hollow glass microspheres were first produced on an industrial scale, using a process inspired by the observation of the following phenomenon.

During quality control of retroreflective panels made from solid glass microbeads, it became apparent that in some places, the incident light, instead of being reflected, was scattered in all directions. Analysis of these defects revealed that impurities in the glass formulation acted as expanding agents, resulting in the formation of hollow spheres that scattered light rather than reflecting it.