  We interpret lattice data for the equation of state of pure gauge $SU(3)_c$
by an evaporation model. At low temperatures gluons are frozen inside the gluon
condensate, whose dynamics is described in terms of a dilaton lagrangian. Above
the critical temperature quasi-free gluons evaporate from the condensate: a
first order transition is obtained by minimizing the thermodynamical potential
of the system. Within the model it is possible to reproduce lattice QCD results
at finite temperature for thermodynamical quantities such as pressure and
energy. The gluonic longitudinal mass can also be evaluated; it vanishes below
the critical temperature, where it shows a discontinuity. At very large
temperatures we recover the perturbative scenario and gluons are the only
asymptotic degrees of freedom.
