  The properties of dust change during the transition from diffuse to dense
clouds as a result of ice formation and dust coagulation, but much is still
unclear about this transformation. We present 2-20 micron spectra of 49 field
stars behind the Perseus and Serpens Molecular Clouds and establish
relationships between the near-infrared continuum extinction (AK) and the
depths of the 9.7 micron silicate (tau97) and 3.0 micron H2O ice (tau30)
absorption bands. The tau97/AK ratio varies from large, diffuse interstellar
medium-like values (~0.55), to much lower ratios (~0.26). Above extinctions of
AK~1.2 (AV~10; Perseus, Lupus, dense cores) and ~2.0 (AV~17; Serpens), the
tau97/AK ratio is lowest. The tau97/AK reduction from diffuse to dense clouds
is consistent with a moderate degree of grain growth (sizes up to ~0.5 micron),
increasing the near-infrared color excess (and thus AK), but not affecting ice
and silicate band profiles. This grain growth process seems to be related to
the ice column densities and dense core formation thresholds, highlighting the
importance of density. After correction for Serpens foreground extinction, the
H2O ice formation threshold is in the range of AK=0.31-0.40 (AV=2.6-3.4) for
all clouds, and thus grain growth takes place after the ices are formed.
Finally, abundant CH3OH ice (~21% relative to H2O) is reported for
2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the other
targets.
