Obtained values of the turnover pressure, p_m, as a function of grain radius, r, as listed in Table 1. It should be noted that we used transition widths in the range ∆ = 0.1... 1.3 for all data points analysed using Method A so that the relatively large range of the derived pm values is not an error range but denotes the range of the formal results. Most of the values obtained refer to silica samples. For comparison, the turnover pressures inferred from the Al₂O₃ samples are illustrated as well.