the process of the invention in flat, single-layered sheet form at eight different temperatures ranging from 21??? C. (69.8??? F.) to 149??? C. (300??? F.) at 15% or less relative humidity for three different periods of time, i.e. 8, 24 and 168 hours, respectively, as shown in Table 1 under the column heading Heat Tempering Conditions, 15% or less R.H..