We study the mean absorption spectrum of the Damped Lyman alpha population at $zsim 2.6$ by stacking normalized, rest-frame shifted spectra of $sim 27,000$ DLAs from the DR12 of BOSS/SDSS-III. We measure the equivalent widths of 50 individual metal absorption lines in 5 intervals of DLA hydrogen column density, 5 intervals of DLA redshift, and overall mean equivalent widths for an additional 13 absorption features from groups of strongly blended lines. The mean equivalent width of low-ionization lines increases with $N_{rm HI}$, whereas for high-ionization lines the increase is much weaker. The mean metal line equivalent widths decrease by a factor $sim 1.1-1.5$ from $zsim2.1$ to $z sim 3.5$, with small or no differences between low- and high-ionization species. We develop a theoretical model, inspired by the presence of multiple absorption components observed in high-resolution spectra, to infer mean metal column densities from the equivalent widths of partially saturated metal lines. We apply this model to 14 low-ionization species and to AlIII, SIII, SiIII, CIV, SiIV, NV and OVI. We use an approximate derivation for separating the equivalent width contributions of several lines to blended absorption features, and infer mean equivalent widths and column densities from lines of the additional species NI, ZnII, CII${}^{*}$, FeIII, and SIV. Several of these mean column densities of metal lines in DLAs are obtained for the first time; their values generally agree with measurements of individual DLAs from high-resolution, high signal-to-noise ratio spectra when they are available.
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