  The age calibration of the Washington deltaT1 index is mainly used to
estimate ages of star clusters older than 1 Gyr, no age-metallicity degeneracy
effect is considered. We have profusely exploited synthetic T1 versus C-T1
colour magnitude diagrams aiming at exploring the intrinsic behaviour of the
deltaT1 index. The analysis shows that deltaT1 varies with age and metal
content as well. In general, the dependence on age weakens for ages greater
than ~ 6 Gyr, and results even less sensitive to age as the metallicity
decreases. For ages younger than ~ 5 Gyr deltaT1 shows a strong correlation
with both age and metallicity. The deltaC index -defined as deltaT1 for the C
passband- is also a combined measurement of age and metallicity. We introduce a
new age-metallicity diagnostic diagram, deltaT1 versus deltaC - deltaT1, which
has shown the ability of unambiguously providing age and metallicity estimates,
simultaneously. The new procedure allows to derive ages from 1 up to 13 Gyr and
metallicities [Fe/H] from -2.0 up to +0.5 dex, and is independent of the
cluster reddening and distance modulus. It does solve the constraints found in
the deltaT1 index and surpasses the performance of the standard giant branch
metallicity method. All these features make the diagnostic diagram a powerful
tool for estimating accurate ages as well as metallicities.
