Fused silica long term stability: case studies

The time stability of fused silica is investigated, reporting new data on two flats that were purchased in 1981. It is found that their present shape is no more plane, exhibiting instead a concavity that is compatible with a very slow laminar flow under the action of gravity. The data are examined altogether with those of prior observations, confirming the occurrence of a relaxation process whose time constant is estimated in the order of 10 years; during such a time period, viscosity approximately increases by a factor of three. Starting from the experimental data so far collected, the computations accounting for the estimates above are given in detail.