Spitzer colors I_ν(8   μm)/I_ν(24   μm) and I_ν(24   μm)/I_ν(70   μm) for carbon grains versus grain size. The solid line give the colors for radiative heating for the Eagle Nebula radiation field. The other lines show the impact of collisional heating for a range of plasma pressure and a fixed temperature T_e of 10⁶ K. A good fit of the shell SED is obtained for n_e × T_e ~ 3 × 10⁷   K   cm^-3 (see Fig. 18).