  We present our results on the non-perturbative evaluation of the
renormalization constant for the quark field, $Z_q$, in Landau gauge within
RI-MOM scheme. Using three lattice spacing we are able to isolate lattice
artefacts of various origin, both perturbative and non-perturbative. In
particular, the existence of the dimension-two gluon-condensate is discussed,
and confirmed.
