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We present a detailed study of the circumstellar envelope of thepost-asymptotic giant branch ``21 micron object'' HD 56126. We build a detaileddust radiative transfer model of the circumstellar envelope in order to derivethe dust composition and mass, and the mass-loss history of the star. To modelthe emission of the dust we use amorphous carbon, hydrogenated amorphouscarbon, magnesium sulfide and titanium carbide. We present a detailedparametrisation of the optical properties of hydrogenated amorphous carbon as afunction of H/C content. The mid-infrared imaging and spectroscopy is bestreproduced by a single dust shell from 1.2 to 2.6 arcsec radius around thecentral star. This shell originates from a short period during which themass-loss rate exceeded 10^(-4) M_sun/yr. We find that the strength of the``21'' micron feature poses a problem for the TiC identification. The lowabundance of Ti requires very high absorption cross-sections in the ultravioletand visible wavelength range to explain the strength of the feature. Othernano-crystalline metal carbides should be considered as well. We find thathydrogenated amorphous carbon in radiative equilibrium with the local radiationfield does not reach a high enough temperature to explain the strength of the3.3-3.4 and 6-9 micron hydrocarbon features relative to the 11-17 micronhydrocarbon features. We propose that the carriers of these hydrocarbonfeatures are not in radiative equilibrium but are transiently heated to hightemperature. We find that 2 per cent of the dust mass is required to explainthe strength of the ``30'' micron feature, which fits well within the measuredatmospheric abundance of Mg and S. This further strengthens the MgSidentification of the ``30'' micron feature.

The circumstellar envelope of the C-rich post-AGB star HD 56126