  In polarimetry it is important to characterize the polarization properties of
the instrument itself to disentangle real astrophysical signals from
instrumental effects. This article deals with the accurate measurement and
modeling of the polarization properties of real aluminum mirrors, as used in
astronomical telescopes. Main goals are the characterization of the aluminum
oxide layer thickness at different times after evaporation and its influence on
the polarization properties of the mirror. The full polarization properties of
an aluminum mirror are measured with Mueller matrix ellipsometry at different
incidence angles and wavelengths. The best fit of theoretical Mueller matrices
to all measurements simultaneously is obtained by taking into account a model
of bulk aluminum with a thin aluminum oxide film on top of it. Full Mueller
matrix measurements of a mirror are obtained with an absolute accuracy of ~1%
after calibration. The determined layer thicknesses indicate logarithmic growth
in the first few hours after evaporation, but it remains stable at a value of
4.12+/-0.08 nm on the long term. Although the aluminum oxide layer is
established to be thin, it is necessary to consider it to accurately describe
the mirror's polarization properties.
