  In this paper we study the system of a scalar quantum field confined between
two plane, isotropic, and homogeneous parallel plates at thermal equilibrium.
We represent the plates by the most general lossless and frequency-independent
boundary conditions that satisfy the conditions of isotropy and homogeneity and
are compatible with the unitarity of the quantum field theory. Under these
conditions we compute the thermal correction to the quantum vacuum energy as a
function of the temperature and the parameters encoding the boundary condition.
The latter enables us to obtain similar results for the pressure between plates
and the quantum thermal correction to the entropy. We find out that our system
is thermodynamically stable for any boundary conditions, and we identify a
critical temperature below which certain boundary conditions yield attractive,
repulsive, and null Casimir forces.
