Liquid-feed flame spray pyrolysis provides easily dispersed, unaggregated nanopowders with average particle sizes of 20–70 nm depending on the processing conditions. Their chemical compositions can be controlled to ppm levels via control of the initial precursor solution. In this paper, Y3Al5O12 composition nanopowders are produced that are atomically mixed but offer a hexagonal crystal structure rather than a YAG structure. Y2O3 and δ-Al2O3 nanopowders are also produced and mixed to evaluate reactive sintering. It is shown that nanopowder/polymer mixtures permit the extrusion of tubes that retain their shape on debindering and sintering to ≥95% theoretical density. More importantly, the sintering behavior of hex-Y3Al5O12 is compared with that of tubes formed using 3:5 Y2O3:δ-Al2O3 mixtures to test the so-called bottom-up paradigm, which suggests that mixing on the finest length scales should provide optimal control of sintering rates, final densities, and grain sizes. Instead, it is found that reactive sintering is faster and offers better control of final grain sizes. Dense sintered tubes are translucent, and dimensional uniformity is maintained from extrusion through sintering.