The compressive failure of various glasses under uniaxial shock loading has been the subject of much recent discussion. Evidence of failure occurring behind a boundary that follows a shock front has been accumulated by Bourne and co–workers, and this boundary has been called a failure wave. The variations in material properties across this front include complete loss of tensile strength, reduction in shear strength, lowered acoustic impedance and sound speed, and opacity to light. While these observations are generally held to be true, there has been no universally agreed mechanism for the process or processes that give rise to the failure. This work presents the results of plate impact experiments that aim to identify the mechanism by which the failure wave propagates in soda–lime glass and a fully filled lead glass. Internal interfaces within the glasses show different effects in the two materials that shed light upon the mechanisms that operate. The failure wave propagates only after a delay in sodalime glass, and the failed strength of the material is increased at the inner interface compared with that at the impact face. While the failed strength of the second tile is greater, there is no delay in initiating fracture in the filled glass.