Abstract: In the representative embodiments of the several methods and apparatus of the invention, a barrier formed of a low-temperature fusible metal alloy having a selected melting point is arranged between a receptor explosive and a typical electrically-initiated detonator enclosed in an explosion-proof housing for blocking the transmission of detonation forces from the detonator to the receptor explosive until the detonator has been subjected to well bore temperatures which are greater than the melting point of the fusible alloy. By selecting a fusible metal alloy which has a melting point less than the known temperatures of the well bore fluids, when the tool is exposed to those elevated temperatures, the barrier will be predictably transformed to its liquid state thereby allowing the liquid alloy to flow to a non-blocking position away from the detonation path of the donor explosive.