  We set up an original apparatus to measure the grain grain friction stress
inside a granular medium composed of sodo-silicate-glass beads surrounded by a
water vapor atmosphere.We analyze here the influence of the physico chemistry
of water on our glass beads and its consequences on our shear experiment. We
found two scales in the analysis of the shear stress signal. On the microscopic
scale of one bead, the experimental results show a dependence on the size of
beads, on the shear rate and on humidity for the resulting stick slip signal.
On the macroscopic scale of the whole assembly of beads, the behavior of the
total amplitude of the shear stress depends on the size of the beads and is
humidity dependent only for relative humidity larger than 80%. For high degrees
of humidity, on the microscopic scale, water lubricates the surface of the
beads leading to a decrease in the microscopic resistance to shear while on the
macroscopic scale the resistance to shear is increased: the assembly of very
humid grains behaves as a non Newtonian fluid.
