  We have collected spectra of about 2000 red giant branch (RGB) stars in 19
Galactic globular clusters (GC) using FLAMES@VLT (about 100 star with GIRAFFE
and about 10 with UVES, respectively, in each GC). These observations provide
an unprecedented, precise, and homogeneous data-set of Fe abundances in GCs. We
use it to study the cosmic scatter of iron and find that, as far as Fe is
concerned, most GCs can still be considered mono-metallic, since the upper
limit to the scatter in iron is less than 0.05 dex, meaning that the degree of
homogeneity is better than 12%. The scatter in Fe we find seems to have a
dependence on luminosity, possibly due to the well-known inadequacies of
stellar atmospheres for upper-RGB stars and/or to intrinsic variability. It
also seems to be correlated with cluster properties, like the mass, indicating
a larger scatter in more massive GCs which is likely a (small) true intrinsic
scatter. The 19 GCs, covering the metallicity range of the bulk of Galactic
GCs, define an accurate and updated metallicity scale. We provide
transformation equations for a few existing scales. We also provide new values
of [Fe/H], on our scale, for all GCs in the Harris' catalogue.
