We have developed a new method to determine the physical properties and the local circumstances of dust shells surrounding Carbon - and Oxygen-rich stars for a given pulsation phase. The observed mid-IR dust emission feature(s), in conjunction with IRAS BB photometry and coeval optical and near-IR BB photometry, are modelled from radiative transport calculations through the dust shell using a grid of detailed synthetic model input spectra for M-S-C giants. From its application to the optical Carbon Mira R For we find that the temperature of the inner shell boundary exceeds 1000 K, ranging between 1200 K and 1400 K. The optical depth of the shell at 11.3 \mu m is determined at \tau_11 \mu m = 0.105 with T_eff = 3200 \pm 200 K for the central star in the considered phase of variability. By-products of the analysis are the shell composition of 90% amorphous carbon and only 10% SiC grains with rather small average radii of 0.05 \pm 0.02 \mu m. The dust density distribution assumes a power law of r^-2 for a steady-state wind with a geomerrical thickness ranging between 10^4 and 510^4 times the inner boundary shell radius and with a high gas mass-loss rate of 3-4 10^-6 M_\odot y^-1 derived by radiation pressure onto the dust. We show that the optical and near-IR light curves are strongly affected by small changes of T_eff and of the shell optical depth with pulsation. A comparison of high resolution optical spectra of R For and medium/low resolution spectra of other carbon stars with the selected model input spectrum is also provided.