We describe the effective static quark-(anti)quark colour potential inside hadronic bags in terms of linear strings and semiclassical corrections of orderħ/r. According to (chromo-) electrostatics we generalize this potential to colour non-neutral configurations making use of the mirror charge method. A thermodynamical study of quark-antiquark pairs and many-quark systems shows a virtually higher order phase transition from the hadronic phase to colour deconfinement. The free plasma stage, furthermore, is achieved only at temperatures much higher thanT c≈200 MeV. Hadrons increase in size by a factor of 1.5 fromT=0 toT=T c.