  We describe a modified transfer matrix renormalization group (TMRG) algorithm
and apply it to calculate thermodynamic properties of the one-dimensional t-J
model. At the supersymmetric point we compare with Bethe ansatz results and
make direct connection to conformal field theory (CFT). In particular we study
the crossover from the non-universal high T lattice into the quantum critical
regime by calculating various correlation lengths and static correlation
functions. Finally, the existence of a spin-gap phase is confirmed.
