  A glass is a non-equilibrium thermodynamic state whose physical properties
depend on time. Glass formation from the melt, as well as the inverse process
of liquid structural recovery from the glass are non-equilibrium processes. A
positive amount of entropy is produced during such irreversible processes. In
this paper, we address the issue of the determination of entropy production
during glass transition. Firstly, we theoretically determine the entropy
production by means of the statistical model of a two-level system coupled to a
master equation driving the time dependency of the occupancy probability of
each state. Thermodynamic cycles of the type liquid-glass-liquid are considered
in order to test the validity of the Clausius theorem. Secondly, we determine
experimentally the production of entropy from differential scanning calorimetry
experiments on the PolyVinylAcetate glass-former. Aging experiments are also
considered. From the data treatments proposed here, we are able to determine
the rate of production of entropy in each part of the experiments. Although
being on the order of few % or less of the configurational entropy involved in
the glass formation, the positive production of entropy is clearly determined.
For all the thermodynamic cycles considered in these calorimetric experiments,
the Clausius theorem is fulfilled.
