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tokens: compared to the unambiguously identified [Si ii ] 34.81 and [S iii ] 33.48 μ m makes the [O iv ] identification much more plausible. If [Fe II] 25.98 μ m is not detected, the putative identifications of the intrinsically weaker [Fe II] 35.35 and [Fe II] 17.94 μ m (Smith et al. 2009 ) in the PR equilibrium ejecta are also doubtful. Isensee et al. ( 2012 ) clearly saw the 17.94 μ m line, and we interpret this as coming from the reverse shock. Gerardy Fesen ( 2001 ) saw even higher excitation [Fe II] lines with wavelengths in the range ∼1–2 μ m that are clearly from fast-moving knots or quasi-stationary flocculi and therefore must result from a shock–clump interaction. Koo et al. ( 2016 ) and Lee et al. ( 2017 ) presented mid- and near-IR observations of [Fe II] from ejecta and CSM knots in Cas A. Koo et al. ( 2016 ) demonstrated that in radiative shock models of this emission, most of the [Fe II] radiation comes from the postshock cooling zone, similar to our Figure 5 . Lee et al. ( 2017 ) found ejecta knots with abundances commensurate with O burning, with elements like Si and S present, as well as a sample of ejecta knots showing strong [Fe II] lines but no Si or S. They interpreted these knots as products of complete Si burning leaving almost “pure” Fe, as found in X-rays by Hwang Laming ( 2009 , 2012 ), or in some cases the products of α -rich freeze-out. It is unclear why such ejecta should be in knots, as opposed to diffuse clouds inflated by radioactivity, but compared to SN 1987A, Cas A was a smaller progenitor at explosion, with a presumably faster expansion and lower opacities leading to a reduced effect of this sort. Koo et al. ( 2018 ) estimated a mass of [Fe II]-emitting shocked dense ejecta of ∼3 × 10 −5 M ⊙ . Extrapolating this clumping to the whole inner ejecta is difficult to do with any accuracy, but assuming that the Fe clump density is the same throughout the inner ejecta as it is at the reverse shock, the total Fe mass should be 3 × 10 −5 M ⊙ multiplied by the ratio of the volumes represented by the inner ejecta (6 × 10 56 cm 3 ) and the reverse shock emission. This last piece is ambiguous, but taking it to be cm 3 , where R r = 1.7 pc = 5 × 10 18 cm is the reverse shock radius and the knot diameter 2 r knot ∼ 2.5 × 10 17 cm gives a ratio of volumes of 7.5 and an implied Fe mass in knots of 2 × 10 −4 M ⊙ . This could be uncertain by at least an order of magnitude either way but is likely to be smaller than the Fe mass in the diffuse Fe–Co–Ni bubbles, estimated below. Features coinciding with [Fe V] 20.85 and 36.34 μ m, which would imply Fe masses of order ∼10 −3 M ⊙ , are also visible in the inner ejecta region. These only appear at marginal significance and must wait for better data for confirmation. The [O iv ] 25.89 and [Si ii ] 34.81 μ m morphologies (Smith et al. 2009 ) and the mass estimate of Arias et al. ( 2018 ) suggest that most of the inner ejecta mass expected by Hwang Laming ( 2012 ) are clumped into a volume