Loop calculations in confined QCD. Pt. 1

We present a method for calculating one loop corrections in QCD confined to a spherical cavity with bag boundary conditions. We use euclidean Greens functions separated into free space and boundary parts and use oldfashioned perturbation theory. As a first application we calculate that part of the quark self energy that gives rise to the confining mirror charge potential in the nonrelativistic approximation. We obtain a weaker potential for relativistic massive quarks. Some consequences are discussed. (orig.)