The recent eruptions of Eyjafjallajökull and Grimsvötn volcanoes in Iceland demonstrate the importance of a better understanding of processes leading to the formation of volcanic ash, specifically of fine volcanic ash that poses a threat to air traffic. Continuous deformation and brittle‐type experiments were carried out to better constrain these processes. The studies on short‐time continuous deformation behavior of basaltic melt showed viscoelastic properties deviating from hydrodynamic Newtonian models by more than 5 orders of magnitude. High‐temperature deformation experiments on basaltic rock samples revealed an increase of elastic strengths as approaching the melting regime, also pointing to a very complex behavior at the solid‐ductile boundary. Understanding magma fragmentation from the “liquid” side is a challenge, but meanwhile we propose a pragmatic solution: a thermodynamic model based on fracture mechanics. This model is in agreement with experiments and observations that show that fine volcanic ash is produced by brittle‐type fragmentation of magma. A critical material property was defined, characterizing the conditions for brittle fragmentation: the fracture surface energy density, which represents the critical fragmentation energy. Short‐term fracture experiments using silicate glass have been performed to investigate the formation of ash‐sized particles by brittle failure and to extract this critical physical property, which was found to range between 40 and 130 J/m2. This value is in good agreement to fragmentation energies determined from experiments using remelted volcanic rocks. Now there is a tool to define critical conditions for the production of volcanic ash of a specific magma type.