The preparation of dense mullite ceramics by the transient viscous flow sintering (TVS) method was introduced by Sacks et al. some years ago [1]. In this method, “microcomposite” particles consisting of α-alumina particles coated with an amorphous silica layer are used as starting materials. Rapid and significant densification at temperatures as low as 1300 ◦C can be achieved by exploiting the viscous flow of the amorphous silica phase. In more recent developments, Bartsch et al. presented results on what they call a “novel” method to prepare mullite ceramics at temperatures below 1300 ◦C [2]. They developed a colloidal processing route employing “nanocomposite” particles consisting of amorphous SiO2-coated γ -Al2O3 core particles as starting materials. However, the idea of using “composite” nanoparticles to prepare mullite by the TVS route is not new. This approach was tried earlier by Miao [3] and Miao et al. [4], who used γ -alumina (and also αalumina) sol particles coated with amorphous silica sol (Syton W30TM). In fact, Miao went further and modelled the situation geometrically to determine the critical relative size ratio for achieving heterocoagulated “composite” particles of stoichiometric mullite composition [3]. A fundamental aspect of that work on the colloidal processing of mullite, which has prompted this letter, relates to the question of whether or not the creation of “composite” particles is really necessary when using nanosized precursors to fabricate dense mullite ceramics at relatively low temperatures (1250–1300 ◦C). In fact, diphasic gel precursors similar to those used by Bartsch et al. [2] have been used extensively for the preparation of dense mullite ceramics at relatively low temperatures, as shown in the literature [5–12], without the need to create heterocoagulated “composite” particles. It is, thus, likely that the results reported by Bartsch et al. [2] and attributed to the use of “composite” nanoparticles, are not a direct consequence of having used these “composite” particles but rather a consequence of the nanosized precursors employed. We will provide here a reasoned basis for this statement by considering our own work on the colloidal processing of mullite. For example, we have shown [6, 7] that dense mullite ceramics can be produced successfully at relatively low temperatures using amorphous silica and nanosized γ -Al2O3 particles as precursors. It was found that in aqueous suspension at pH 4, the silica and γ -Al2O3 particles were mixed intimately on the nanoscale due to their strong mutual electrostatic attraction; the silica and γ -Al2O3 particles bearing a net negative and positive surface charge, respectively. A summary of our previous results in terms of precursors employed, processing temperature, density achieved, and crystalline composition is shown in Table I. The results are nearly identical to those achieved by Bartsch et al. using “composite” nanoparticles (density= 2.96 g/cm3 after sintering at 1350 ◦C) [2]. The small difference in the densification temperature may be due to differences in the processing conditions used to obtain the dried powders, prepare the green bodies and/or sinter the materials. Local composition differences may also play a role. In our previous studies [6, 7], we had already postulated that the coating of the γ -Al2O3 particles by a silica layer is not strictly necessary for the development of dense mullite ceramics at relatively low temperatures by TVS, provided that the silica and alumina species can be mixed homogeneously on a nanometer scale. More important in terms of improving the densification behavior of the compacts is the achievement of green bodies with uniform and sufficiently high green densities, as remarked upon in the literature [5, 10–13]. Residual porosity in sintered compacts is usually the result of inhomogeneities in the green bodies; in particular the presence of hard agglomerates. These agglomerates lead to the creation of two types of pores: inter-agglomerate and intra-agglomerate pores, which have different sizes and lead to differential shrinkage.