  The large quantities of dust that have been found in a number of high
redshift galaxies have led to suggestions that core-collapse supernovae (CCSNe)
are the main sources of their dust and have motivated the measurement of the
dust masses formed by local CCSNe. For Cassiopeia~A, an oxygen-rich remnant of
a Type~IIb CCSN, a dust mass of 0.6-1.1~M$_\odot$ has already been determined
by two different methods, namely (a) from its far-infrared spectral energy
distribution and (b) from analysis of the red-blue emission line asymmetries in
its integrated optical spectrum. We present a third, independent, method for
determining the mass of dust contained within Cas~A. This compares the relative
fluxes measured in similar apertures from [O~{\sc iii}] far-infrared and
visual-region emission lines, taking into account foreground dust extinction,
in order to determine internal dust optical depths, from which corresponding
dust masses can be obtained. Using this method we determine a dust mass within
Cas~A of at least 0.99$^{+0.10}_{-0.09}$~M$_\odot$.
