Illustration of the silicate removal with the synthetic silicate method. Top: B1-b optical depth is shown by the black solid line, whereas the dashed and solid lines are the scaled GCS 3 silicate profile and the synthetic silicate feature, respectively. Bottom: residual optical depth after synthetic silicate removal (black line). The blue and red colours show the residual optical depth after local and template continuum methods, respectively (see text for details; Bottinelli et al. 2010).