Overview of the Spitzer spectra (black solid lines) of the carbon-rich PNe with the 30 μm feature. The spectra are ordered according to the T_d from high to low temperature, top to bottom, and left to right. They are also normalised to the flux density at 18 μm (in case of SMP LMC 2 it is 18.4 μm) and offset for clarity. The names of objects and values of the T_d are shown above spectra.