  The occurrence of non-equilibrium transitions between arrested states has
recently emerged as an intriguing issue in the field of soft glassy materials.
The existence of one such transition has been suggested for aging colloidal
clays (Laponite$^{\circledR}$ suspensions) at weight concentration 3.0%,
although further experimental evidences are necessary to validate this
scenario. Here, we test the occurrence of this transition for spontaneously
aged (non-rejuvenated) samples, by exploiting the rheological tools of
Dynamical Mechanical Analysis. On imposing consecutive compression cycles to
differently aged clay suspensions, we find that a quite abrupt change of
rheological parameters occurs for ages around three days. For the Young and
elastic moduli, the change with the waiting time is essentially independent
from the deformation rate, whereas other "fluid-like" properties, such as the
loss modulus, do clearly display some rate dependence. We also show that the
crossover identified by rheology coincides with deviations of the relaxation
time (obtained through X-Ray Photon Correlation Spectroscopy) from its expected
monotonic increase with aging. Thus, our results robustly support the existence
of a glass-glass transition in aging colloidal clays, highlighting
characteristic features of of their viscoelastic behaviour.
