The age calibration of the Washington \delta T _ { 1 } 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 T _ { 1 } versus C - T _ { 1 } colour magnitude diagrams aiming at exploring the intrinsic behaviour of the \delta T _ { 1 } index .
The analysis shows that \delta T _ { 1 } varies with age and metal content as well .
In general , the dependence on age weakens for ages greater than \sim 6 Gyr , and results even less sensitive to age as the metallicity decreases .
For ages younger than \sim 5 Gyr \delta T _ { 1 } shows a strong correlation with both age and metallicity .
The \delta C index -defined as \delta T _ { 1 } for the C passband- is also a combined measurement of age and metallicity .
We introduce a new age-metallicity diagnostic diagram , \delta T _ { 1 } versus \delta C - \delta T _ { 1 } , 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 \delta T _ { 1 } 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 .