Perturbative and Non-Perturbative Quarks and Gluons
  BRST invariance supplies a sufficient condition for the observability of fields. We show that there is a global obstruction to the observability of quarks and gluons and argue that they will not become observables at finite temperature. We give expressions for quarks and gluons that are, however, {\it perturbatively\/} BRST invariant, and hence locally observable, up to order~$g^2$ and~$g$ respectively. 
Martin Lavelle and David McMullan