  Variations in polarization efficiency (p/A) of interstellar grains as a
function of environment place vital constraints on models for the mechanism of
their alignment. We use polarimetry of background field stars to investigate
alignment in the Taurus dark cloud as a function of optical depth for
extinctions of A_K < 2.5 (A_V < 25). Results show a strong systematic trend in
polarization efficiency with extinction. We discuss the possibility of a
relationship between p/A and icy grain mantle growth, since mantles are
detected in Taurus only for extinctions where p/A is lowest. A simple density
model of alignment by paramagnetic alignment (Davis & Greenstein 1951) predicts
that the magnetic field strength (B) scales with density to the power k=0.28.
