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Context. Copper is an element whose interesting evolution with metallicity isnot fully understood. Observations of copper abundances rely on a very limitednumber of lines, the strongest are the Cu I lines of Mult. 1 at 324.7 nm and327.3 nm which can be measured even at extremely low metallicities. Aims. Weinvestigate the quality of these lines as abundance indicators. Method. Wemeasure these lines in two turn-off (TO) stars in the Globular Cluster NGC 6752and two TO stars in the Globular Cluster NGC 6397 and derive abundances with 3Dhydrodynamical model atmospheres computed with the CO5BOLD code. Theseabundances are compared to the Cu abundances measured in giant stars of thesame clusters, using the lines of Mult. 2 at 510.5 nm and 578.2 nm. Results.The abundances derived from the lines of Mult. 1 in TO stars differ from theabundances of giants of the same clusters. This is true both using CO5BOLDmodels and using traditional 1D model atmospheres. The LTE 3D corrections forTO stars are large, while they are small for giant stars. Conclusions. The Cu Iresonance lines of Mult. 1 are not reliable abundance indicators. It is likelythat departures from LTE should be taken into account to properly describethese lines, although it is not clear if these alone can account for theobservations. An investigation of these departures is indeed encouraged forboth dwarfs and giants. Our recommendation to those interested in the study ofthe evolution of copper abundances is to rely on the measurements in giants,based on the lines of Mult. 2. We caution, however, that NLTE studies may implya revision in all the Cu abundances, both in dwarfs and giants.

Cu I resonance lines in turn-off stars of NGC 6752 and NGC 6397. Effects of granulation from CO5BOLD models