  We present the most detailed data-driven exploration of cloud opacity in a
substellar object to-date. We have tested over 60 combinations of cloud
composition and structure, particle size distribution, scattering model, and
gas phase composition assumptions against archival $1-15 {\rm \mu m}$
spectroscopy for the unusually red L4.5~dwarf 2MASSW~J2224438-015852 using the
Brewster retrieval framework. We find that, within our framework, a model that
includes enstatite and quartz cloud layers at shallow pressures, combined with
a deep iron cloud deck fits the data best. This models assumes a Hansen
distribution for particle sizes for each cloud, and Mie scattering. We
retrieved particle effective radii of $\log_{10} a {\rm (\mu m)} =
-1.41^{+0.18}_{-0.17}$ for enstatite, $-0.44^{+0.04}_{-0.20}$ for quartz, and
$-0.77^{+0.05}_{-0.06}$ for iron. Our inferred cloud column densities suggest
${\rm (Mg/Si)} = 0.69^{+0.06}_{-0.08}$ if there are no other sinks for
magnesium or silicon. Models that include forsterite alongside, or in place of,
these cloud species are strongly rejected in favour of the above combination.
We estimate a radius of $0.75 \pm 0.02$ Rjup, which is considerably smaller
than predicted by evolutionary models for a field age object with the
luminosity of 2M2224-0158. Models which assume vertically constant gas
fractions are consistently preferred over models that assume thermochemical
equilibrium. From our retrieved gas fractions we infer ${\rm [M/H]} =
+0.38^{+0.07}_{-0.06}$ and ${\rm C/O} = 0.83^{+0.06}_{-0.07}$. Both these
values are towards the upper end of the stellar distribution in the Solar
neighbourhood, and are mutually consistent in this context. A composition
toward the extremes of the local distribution is consistent with this target
being an outlier in the ultracool dwarf population.
