Optical
colours are typical used to infer the metallicities of extragalactic
globular clusters. However the form of the colour-metallicity relation
has been a matter of debate. As part of the SLUGGS survey of nearby
early-type galaxies and their globular cluster systems we have used the
DEIMOS multi-object spectrograph on Keck to obtain spectra of thousands
of globular clusters. Previously we have used the strength of the near
infrared calcium triplet to derive metallicities showing that galaxies’
globular cluster metallicity distributions are bimodal like their
globular cluster colour distributions. However we see evidence that
different galaxies have different globular cluster colour-metallicity
relations. Different colour-metallicity relations would imply that
globular clusters in different galaxies have different ages, chemical
abundances or stellar mass functions. We have now stacked our spectra by
colour within each galaxy to allow us to measure the strength of weaker
metal lines allowing us to confirm our CaT metallicities. By
empirically comparing line ratios we can identify how globular clusters
differ between galaxies.