The silica feedstock (Skyspring Nanomaterials, #6808NM) was stated to consist of spherical, non-porous NPs with nominal diameters of 20 nm and purity of ~99.5%. For reference, note that spheres with the bulk silica density and diameters from 15 to 25 nm would have masses ranging from ~3 MDa to ~13 MDa. The feedstock composition was given as SiO2-x, where x ≈ 0.5, and the impurities were listed as: Al ≤ 0.002%, Fe ≤ 0.001%, Ca ≤ 0.002%, Mg ≤ 0.001%, and Cl ≤ 0.001%.