Reduced χ² between our model posteriors and the observed surface brightness (upper panel) or spectral index (bottom panel) radial profiles for different dust compositions, colour-coded by the outer-most disc radius the χ² was averaged over. Mixtures including amorphous carbonaceous grains (e.g. ‘Ricci (compact)’ and ‘Zubko’) can fit our data the best. No constraints on the relative abundance of organics and other materials (especially water ice and silicates) can be drawn.