Abstract The results of a series of experiments aimed at characterizing the elastic and failure parameters of lava specimens from Mount Etna, Vulcano and Stromboli volcanos is presented. The double torsion—constant configuration—load relaxation method has been employed to obtain the behavior of the stress intensity factor versus mode I subcritical crack velocity. The experimental technique has been optimized in several respects. First, a very stiff machine, expressly designed and built, has been used. Second, all samples have been carefully machined to low tolerance. Third, the experimental conditions have been carefully controlled, with temperature variations within 1°C and humidity within 10%. Fourth, high resolution electronic measuring systems have been used together with a pulse stacking procedure. Fifth, the length of the prefracture has been optically checked. This optimization allowed us to measure the slope of the subcritical fracture propagation curve in the III region, which extends from velocities of 10 −7 m/s to catastrophic, with a standard deviation around 20%, suggesting that the results can be effectively used for modeling purposes. As a by-product, we could measure the elastic parameters with an accuracy of a few percent.