A new concept for lost cores for gravity castings is presented. In contrast to the conventional principle of thermally breaking down binder strength, demolding of the new cores is based on a mechanical collapse of parts of the filler material. For this purpose, the usual sand filler is substituted for hollow microspheres with defined isostatic compression strength. After casting, the core is subjected to high hydrostatic pressure, leading to destruction of the hollow filler and a resulting loss of core structural strength and volume, allowing it to be washed out easily. In the present study, phosphate-, gypsum-, and cement-bonded cores with additions of hollow glass microspheres and cenospheres are produced and their bending strength and surface quality are determined. The core behavior during the casting process is evaluated using permanent mold gravity and low pressure aluminum casting experiments. Phosphate bonded materials containing 62 vol% of glass microspheres exhibit the best combination of surface quality and collapse behavior while reaching strength levels comparable to conventional sand cores. The study contributes to a deeper understanding of the collapse mechanisms, suggesting a major role of 1) open porosity of the matrix phase, 2) sufficient strength of the filler-matrix interface, and 3) filler particle shell integrity. Due to a lack of the latter, cenospheres are proved unsuitable for the new demolding approach.