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Using \emph{Spitzer} Infrared Spectrograph observations of G0--M4 III starsbehind 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 interstellarmedium extinction curve, but with a greater degree of extinction. For $A_K>1$,the curve exhibits lower contrast between the silicate and absorptioncontinuum, 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 similarconclusion, that the shape of the extinction curve changes as a function ofincreasing $A_K$. By calculating the optical depths of the $9.7\mu$m silicateand 6.0, 6.8, and 15.2 $\mu$m ice features, I determine that a processinvolving ice is responsible for the changing shape of the extinction curve andspeculate that this process is coagulation of ice-mantled grains rather thanice-mantled grains alone.

Observational $5-20\mu$m Interstellar Extinction Curves Toward Star-Forming Regions Derived from Spitzer IRS Spectra