Absorption (a), scattering (b), extinction (c) efficiencies, and emissivity index (d) for the six grains of the pilot sample. We remind the reader that all of the grains have an effective radius of 1 μm for their silicate core and carbon mantle part, while the iced aggregate has a total of a_eff = 1.26 μm. Panel d: emissivity index, β, computed as the slope of the extinction efficiency, and shown over a restricted wavelength range, focusing on the sub-millimetre and millimetre.