An experiment is described in which velocity and pressure changes during the transition from slow burning to detonation in cast explosives are measured. A simple one-dimensional physical model of explosive burning under confinement is assumed and used, in connection with experimental evidence, as the basis for a calculation of the time and distance necessary to start a shock in a deflagrating solid explosive. The calculated distance of incipient shock formation is 11–15 cm. This result supports the hypothesis that transition from slow burning to detonation is due to a shock which arises spontaneously in a confined burning medium. The nature of the shock-forming pressure pulse, p=p(t), is discussed.