We used a set of moderately-deep and high-resolution optical observations obtained with the Hubble Space Telescope to investigate the properties of the stellar population in the heavily obscured bulge globular cluster NGC 6256 .
The analysis of the color-magnitude diagram revealed a stellar population with an extended blue horizontal branch and severely affected by differential reddening , which was corrected taking into account color excess variations up to \delta E ( B - V ) \sim 0.51 .
We implemented a Monte Carlo Markov Chain technique to perform the isochrone fitting of the observed color-magnitude diagram in order to derive the stellar age , the cluster distance and the average color excess in the cluster direction .
Using different set of isochrones we found that NGC 6256 is characterised by a very old stellar age around 13.0 Gyr , with a typical uncertainty of \sim 0.5 Gyr .
We also found an average color excess E ( B - V ) = 1.19 and a distance from the Sun of 6.8 kpc .
We then derived the cluster gravitational center and measured its absolute proper motion using the Gaia-DR2 catalog .
All this was used to back-integrate the cluster orbit in a Galaxy-like potential and measure its integrals of motion .
It turned out that NGC 6256 is currently in a low-eccentricity orbit entirely confined within the bulge and its integrals of motion are fully compatible with a cluster purely belonging to the Galaxy native globular cluster population .
All these pieces of evidence suggest that NGC 6256 is an extremely old relic of the past history of the Galaxy , formed during the very first stages of its assembly .