IRS spectrum of SSID 18 (black line) compared with synthetic SEDs, corresponding to the same luminosity and τ₁₀, and different percentages of SiC+MgS and of the relative width of the MgS mantle (a_man) on top of SiC core. The various lines correspond to the following: red (our best-fit model), with SiC+MgS = 9% and a_man = 0.05 μm; green, with SiC+MgS = 35% and a_man = 0.02 μm; blue, with SiC+MgS = 9% and a_man = 0.02 μm; and orange, with SiC+MgS = 6% and a_man = 0.05 μm.