  We recently presented evidence that stable umbra-penumbra boundaries are
characterised by a distinct canonical value of the vertical component of the
magnetic field, $B^{\rm stable}_{\rm ver}$. In order to trigger the formation
of a penumbra, large inclinations in the magnetic field are necessary. In
sunspots, the penumbra develops and establishes by colonising both umbral areas
and granulation, that is, penumbral magneto-convection takes over in umbral
regions with $B_{\rm ver} < B^{\rm stable}_{\rm ver}$, as well as in granular
convective areas. Eventually, a stable umbra-penumbra boundary settles at
$B^{\rm stable}_{\rm ver}$. Here, we aim to study the development of a penumbra
initiated at the boundary of a pore, where the penumbra colonises the entire
pore ultimately. We have used Hinode/SOT G-band images to study the evolution
of the penumbra. Hinode/SOT spectropolarimetric data were used to infer the
magnetic field properties in the studied region. The penumbra forms at the
boundary of a pore located close to the polarity inversion line of NOAA\,10960.
As the penumbral bright grains protrude into the pore, the magnetic flux in the
forming penumbra increases at the expense of the pore magnetic flux.
Consequently, the pore disappears completely giving rise to an orphan penumbra.
At all times, the vertical component of the magnetic field in the pore is
smaller than $B^{\rm stable}_{\rm ver} \approx 1.8$~kG. Our findings are in an
agreement with the need of $B^{\rm stable}_{\rm ver}$ for establishing a stable
umbra-penumbra boundary: while $B_{\rm ver}$ in the pore is smaller than
$B^{\rm stable}_{\rm ver}$, the protrusion of penumbral grains into the pore
area is not blocked, a stable pore-penumbra boundary does not establish, and
the pore is fully overtaken by the penumbral magneto-convective mode. This
scenario could also be one of the mechanisms giving rise to orphan penumbrae.
