  The distances to which the optical flash destroys dust via sublimation, and
the burst and afterglow change the size distribution of the dust via
fragmentation, are functions of grain size. Furthermore, the sublimation
distance is a decreasing function of grain size, while the fragmentation
distance is a decreasing function of grain size for large grains and an
increasing function of grain size for small grains. We investigate how these
very different, but somewhat complementary, processes change the optical depth
of the circumburst medium. To this end, we adopt a canonical distribution of
graphite and silicate grain sizes, and a simple fragmentation model, and we
compute the post-burst/optical flash/afterglow optical depth of a circumburst
cloud of constant density n and size R as a function of burst and afterglow
isotropic-equivalent X-ray energy E and spectral index alpha, and optical flash
isotropic-equivalent peak luminosity L: This improves upon previous analyses
that consider circumburst dust of a uniform grain size. We find that
circumburst clouds do not significantly extinguish (tau < 0.3) the optical
afterglow if R < 10L_{49}^{1/2} pc, fairly independent of n, E, and alpha, or
if N_H < 5x10^{20} cm^{-2}. On the other hand, we find that circumburst clouds
do significantly extinguish (tau > 3) the optical afterglow if R >
10L_{49}^{1/2} pc and N_H > 5x10^{21} cm^{-2}, creating a so-called `dark
burst'. The majority of bursts are dark, and as circumburst extinction is
probably the primary cause of this, this implies that most dark bursts occur in
clouds of this size and mass M > 3x10^5L_{49} M_{sun}. Clouds of this size and
mass are typical of giant molecular clouds, and are active regions of star
formation.
