: We develop a new way to look at the high-temperature representation of the Ising model up to the critical temperature and obtain a number of interesting consequences. In the two-dimensional case, it is possible to use these tools to prove results on phaseseparation lines in the whole phase-coexistence regime, by way of a duality transformation. We illustrate the power of these techniques by studying an Ising model with a boundary magnetic field, in which a reentrant pinning transition takes place; more precisely we show that the typical configurations of the model can be described, at the macroscopic level, by interfaces which are solutions of the corresponding thermodynamical variational problem; this variational problem is solved explicitly. There exist values of the boundary magnetic field and temperatures 0 ! T 1 ! T 2 ! T c such that the interface is not pinned for T ! T 1 or T ? T 2 , but is pinned for T 1 ! T ! T 2 ; we can also find values of the boundary magnetic field and ..