Finally, the resulting phases were grouped into three fractions according to the thermal stability of the transition aluminas : δ-Al2O3 (the target phase), underreacted material (the amorphous fraction and any transition aluminas, boehmites, etc. with a thermal stability below that of the δ-phase) and overreacted material (θ-Al2O3 and α-Al2O3). Although glassy alumina could potentially form as a result of overreaction (melting and rapid quenching), all amorphous aggregates were grouped with the underreacted fraction. This was done based on reference experiments in which no signs of melting or glass formation could be found within the range of considered process parameters when injecting colloidal suspensions of commercial Al2O3 powder. The results of XRD tests are shown in Figure . Furthermore, residual nitrates were excluded through a simple pH test (Figure ). Instead, the amorphous fraction was identified as being composed of alumogels . Phase compositions of powders resulting from the 51 individual PAAD campaigns are summarized in Table .