  We present a characterization of the dust in the Wolf-Rayet (WR) nebula RCW
58 around the WN8h star WR 40 using archival infrared (IR) observations from
WISE and Herschel and radio observations from ATCA. We selected two clumps,
free from contamination from material along the line of sight and located
towards southern regions in RCW 58, as representative of the general properties
of this WR nebula. Their optical, IR and radio properties are then modelled
using the photoionization code Cloudy, which calculates a self-consistent
spatial distribution of dust and gas properties. Two populations of dust grains
are required to model the IR SED: a population of small grains with sizes
0.002-0.01 $\mu$m, which is found throughout the clumps, and a population of
large grains, with sizes up to 0.9 $\mu$m, located further from the star.
Moreover, the clumps have very high dust-to-gas ratios, which present a
challenge for their origin. Our model supports the hypothesis that RCW 58 is
distributed in a ring-like structure rather than a shell, and we estimate a
mass of $\sim$2.5 M$_\odot$. This suggests that the mass of the progenitor of
WR 40 was about $\approx40^{+2}_{-3}$ M$_\odot$. The ring morphology, low
nebular mass, large dust grain size and high dust-to-gas ratio lead us to
propose that RCW 58 has formed through a common envelope channel, similar to
what has been proposed for M 1-67.
