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We aim to constrain the structure of the circumstellar material around thepost-AGB binary and RV Tauri pulsator AC Her. We want to constrain the spatialdistribution of the amorphous as well as of the crystalline dust. We presentvery high-quality mid-IR interferometric data that were obtained withMIDI/VLTI. We analyse the MIDI data and the full SED, using the MCMax radiativetransfer code, to find a good structure model of AC Her's circumbinary disk. Weinclude a grain size distribution and midplane settling of dustself-consistently. The spatial distribution of crystalline forsterite in thedisk is investigated with the mid-IR features, the 69~$\mu$m band and the11.3~$\mu$m signatures in the interferometric data. All the data are wellfitted. The inclination and position angle of the disk are well determined ati=50+-8 and PA=305+-10. We firmly establish that the inner disk radius is aboutan order of magnitude larger than the dust sublimation radius. Significantgrain growth has occurred, with mm-sized grains being settled to the midplaneof the disk. A large dust mass is needed to fit the sub-mm fluxes. By assuming{\alpha}=0.01, a good fit is obtained with a small grain size power law indexof 3.25, combined with a small gas/dust ratio <10. The resulting gas mass iscompatible with recent estimates employing direct gas diagnostics. The spatialdistribution of the forsterite is different from the amorphous dust, as morewarm forsterite is needed in the surface layers of the inner disk. The disk inAC Her is very evolved, with its small gas/dust ratio and large inner hole.Mid-IR interferometry offers unique constraints, complementary to mid-IRfeatures, for studying the mineralogy in disks. A better uv coverage is neededto constrain in detail the distribution of the crystalline forsterite in ACHer, but we find strong similarities with the protoplanetary disk HD100546.

The evolved circumbinary disk of AC Her: a radiative transfer, interferometric and mineralogical study