  We investigate the physical structure and dust reprocessing in the shocks
along the beam of a number of classical Herbig-Haro jets in the Orion and Lupus
molecular cloud. Spectral diagnostic techniques are applied to obtain the jet
physical conditions from the ratios between selected forbidden lines. The
presence of dust grains is investigated by estimating the gas-phase abundance
of calcium with respect to its Solar value.
  We find the electron density, ne, varies between 0.05-4 10^3 cm^-3, the
ionisation fraction, xe, is 0.01-0.7, the temperature, te, ranges between 0.6-3
10^4 K, and the hydrogen density between 0.01-6 10^4 cm^-3. Interestingly, in
the HH 111 jet, ne, xe, and te, peak in the High Velocity Interval (HVI) of the
strongest working surfaces, confirming the prediction from shocks models.
Calcium turns out to be depleted with respect to its Solar value, but its
gas-phase abundance is higher than that estimated in the interstellar medium in
Orion. The depletion is high (up to 80%) along the low-excited jets, while low
or no depletion is measured in the jets which show higher excitation
conditions. Moreover, in HH 111 the depletion is lower in the HVI of the faster
shock.
  Our results confirm the shock structure predicted by models and indicate that
the shocks occurring along the jets are partially destroying the dust grains
and that the efficiency of dust reprocessing strongly depend on shock velocity.
However, the high Ca gas-phase abundance estimated in some of the knots is not
well justified by existing models of dust reprocessing in shocks, and indicates
that the dust must have been partially reprocessed in the region where the flow
originates.
