  In cylindrical volumes with C-periodic boundary conditions in the long
direction, static quarks are confined even in the gluon plasma phase due to the
presence of interfaces separating the three distinct high-temperature phases.
An effective "string tension" is computed analytically using a dilute gas of
interfaces. At T_c, the deconfined-deconfined interfaces are completely wet by
the confined phase and the high-temperature "string tension" turns into the
usual string tension below T_c. Finite size formulae are derived, which allow
to extract interface and string tensions from the expectation value of a single
Polyakov loop. A cluster algorithm is built for the 3-d three-state Potts model
and an improved estimator for the Polyakov loop is constructed, based on the
number of clusters wrapping around the C-periodic direction of the cluster.
