The Quark-Gluon Plasma: a Perfect Thermostat and a Perfect Particle   Reservoir
  A system H with a Hagedorn-like mass spectrum imparts its unique temperature T_H to any other system coupled to it. An H system radiates particles in preexisting physical and chemical equilibrium. These particles form a saturated vapor at temperature T_H. This coexistence describes a first order phase transition. An H system is nearly indifferent to fragmentation into smaller H systems. A lower mass cut-off in the spectrum does not significantly alter the general picture. These properties of the Hagedorn thermostats naturally explain a single value of hadronization temperature observed in elementary particle collisions at high energies and lead to some experimental predictions. 
L. G. Moretto, K. A. Bugaev, J. B. Elliott and L. Phair