It is well known that radiation (both particle and photon) can cause substantial density changes in certain solids. In cases where the radiation damage is non-uniformly distributed, these changes are manifested as gradients of residual stress. With ion bombardment, where penetrations under accelerating voltages of several hundred thousand volts are typically on the micrometre scale, one may reasonably describe the mechanical damage in terms of a lateral “surface stress.” Now if the irradiated material is brittle, the possibility exists of such stresses causing incipient surface flaws (so-called Griffith flaws, present in abundance on all typical brittle surfaces)1to grow into dangerous large-scale cracks. Measurements on a number of brittle solids2–6reveal a general tendency for the level of radiation-induced surface stress to increase steadily with fluence up to a maximum, beyond which saturation (or even decline) sets in. Of the solids investigated, fused is unique in that the surface stress is tensile, indicative of a structural compaction; silicate glasses might accordingly be expected to show a particularly high susceptibility to radiation-enhanced cracking.