  We present 2-45 micron spectra of a large sample of carbon-rich evolved stars
in order to study the ``30'' micron feature. We find the ``30'' micron feature
in sources in a wide range of sources: low mass loss carbon stars, extreme
carbon-stars, post-AGB objects and planetary nebulae. We extract the profiles
from the sources by using a simple systematic approach to model the continuum.
We find large variations in the wavelength and width of the extracted profiles
of the ``30'' micron feature. We modelled the whole range of profiles in a
simple way by using magnesium sulfide (MgS) dust grains with a MgS grain
temperature different from the continuum temperature. The systematic change in
peak positions can be explained by cooling of MgS grains as the star evolves
off the AGB. In several sources we find that a residual emission excess at ~26
micron can also be fitted using MgS grains but with a different grains shape
distribution. The profiles of the ``30'' micron feature in planetary nebulae
are narrower than our simple MgS model predicts. We discuss the possible
reasons for this difference. We find a sample of warm carbon-stars with very
cold MgS grains. We discuss possible causes for this phenomenon. We find no
evidence for rapid destruction of MgS during the planetary nebula phase and
conclude that the MgS may survive to be incorporated in the ISM.
