The behavior of the Wilson loop average W(C) is considered in non-perturbative QCD. The idea is to start from small contours and then proceed to larger ones treating the effects of vacuum fields phenomenologically, through matrix elements like 〈g2GμνaGμνa〉. An expansion is derived which gives W(C) as an infinite series of local gluonic operators. A few terms in this series are calculated exactly revealing a highly non-trivial dependence on the contour variables. Under a certain approximation an infinite set of leading terms is isolated and summed up. The result depends on the area of the contour, but this dependence is not quite satisfactory. As a byproduct of the analysis it is shown that in a framework of multicolor chromodynamics the bag picture of Callan et al., cannot be realized.