The 30 micron dust emission band has been detected in the envelopes of
carbon-rich Asymptotic Giant Branch (AGB) stars and planetary nebulae
(Forrest et al. 1981). This dust band is amongst the strongest band in
the infrared regime and carry up to 25 percent of the infrared luminosity
of the source. The carrier of this band is still a matter of debate although
Magnesium Sulfide (MgS) is a potential candidate. Intense 30 micron bands
have also been measured towards carbon-rich proto-planetary nebulae
known to have the 21 micron emission feature (Omont et al 1995) and recently
it has been detected towards the Galactic Center (Chan et al. 1996)
indicating that the material is probably widespread in the interstellar medium.
Clearly more observations are requested to better constrain the potential
carriers of this prominent dust band. We thus propose to obtain high
quality SWS spectra in the wavelength range from 23 to 43 microns towards
a variety of carbon-rich sources which were selected from their IRAS properties
(presence of a 21 micron emission feature and infrared colors). This work will
be complemented both with further laboratory measurements and theoretical
modelling.