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The deformation of thin glass plates during bombardment with 140‐keV He+ ions was measured with an external capacitor. The contour of the plates was determined interferometrically before irradiation, after irradiation, and after aging for several years. These results showed that the calibration of the capacitor equipment by dead‐weight loading a silica plate was faulty. The deflection of a vitreous silica free cantilever on dead‐weight loading was measured interferometrically, and the results showed that deformation occurred within the clamp. It was confirmed that placing a ground shield about the electrode of the capacitor increases the calculated deflections. Data for the permanent deformation of a stressed plate of vitreous silica are analyzed, and it is concluded that stress relaxation by a bulk viscoelastic deformation cannot be detected by a change in plate contour because the maximum precision for such a determination could not detect apparent viscosities greater than ∼1029 P. The stress relaxation of the irradiated vitreous silica plates was about 10% in three years corresponding to an apparent post‐irradiation viscosity of ∼1020 P. The stress relaxations observed for the other glasses, Pyrex, BK7, and LF2 were much greater, and it is uncertain to what extent they were a viscoelastic effect or an annealing effect. The behavior of a sample of a facsimile radioactive waste storage glass, 76–68 indicated it was not a uniform product; the contour became irregular; hence, these techniques were not appropriate for it.