Optical depth (in gray) in the region of the 9.7 μm band of amorphous silicate. The calculation of the optical depth is described in Sect. 4.2 and de Vries et al. (2010). In black the local continuum used under the 11.3 μm band. The flux levels of IRAS 21554 and OH 21.5+0.5 go below zero in the region of 9.7 μm. Those points are not used to calculate the optical depth and are excluded from these plots.