  Using \emph{Spitzer} Infrared Spectrograph observations of G0--M4 III stars
behind dark clouds, I construct $5-20\mu$m empirical extinction curves for
$0.3\leq A_K<7$, which is equivalent to $A_V$ between $\approx$ 3 and 50. For
$A_K<1$ the curve appears similar to the \citet{mathis90} diffuse interstellar
medium extinction curve, but with a greater degree of extinction. For $A_K>1$,
the curve exhibits lower contrast between the silicate and absorption
continuum, developes ice absorption, and lies closer to the \citet{wd01}
$R_V=5.5$ case B curve, a result which is consistent with that of
\citet{flaherty07} and \citet{chiar07}. Recently work using \emph{Spitzer}
Infrared Array Camera data by \citet{chapman08} independently reaches a similar
conclusion, that the shape of the extinction curve changes as a function of
increasing $A_K$. By calculating the optical depths of the $9.7\mu$m silicate
and 6.0, 6.8, and 15.2 $\mu$m ice features, I determine that a process
involving ice is responsible for the changing shape of the extinction curve and
speculate that this process is coagulation of ice-mantled grains rather than
ice-mantled grains alone.
