These tiny hollow glass micro spheres are manufactured by 3M and are essentially a fine powder. This product is commonly used in conservation as a bulking agent or thickener in thixotropic mixtures such as resins and castings.
Using micro balloons allows for a low resin-to-microsphere volume concentration to form cohesive fills in objects, and is easily reversible with a minimal amount of an appropriate solvent. Ghosting problems associated with plaster or other fillers commonly used on porous materials are avoided, as are potential problems for water sensitive gesso, wood and painted surfaces. This fill material also produced promising results for the filling of marbles and faience, and can be easily sanded and painted.
3M Grade K25 Glass Bubbles have a density of 0.25 g/cc and an isostatic crush strength of 750 psi.