  Glass has long been considered a nonequilibrium material. The primary reason
is its history-dependent properties: the obtained properties are not uniquely
determined by two state variables alone, namely, temperature and volume, but
are affected by the process parameters, such as cooling rates. However, closer
observations show that this history dependence is common in solid; in crystal
growth, the properties of an obtained crystal are affected by the preparation
conditions through defect structures and metallurgical structures. The problem
with the previous reasoning of history dependence lies in the lack of
appropriate specification of state variables. Without knowledge of the latter,
describing thermodynamic states is impossible. The guiding principle to find
state variables is provided by the first law of thermodynamics. The state
variables of solids have been searched by requiring that the internal energy
$U$ is a state function. Detailed information about the abovementioned
microstructures is needed to describe the state function $U$. This can be
accomplished by specifying the time-averaged positions R_{j} of all atoms
comprising the solids. Therefore, R_{j} is a state variable for solids. Defect
states, being metastable states, represent equilibrium states within a finite
time (relaxation time). However, eternal equilibrium is nonexistent: the
perfect crystal is thermodynamically unstable. Equilibrium states can only be
considered at the local level. Glass is thus in equilibrium as long as its
structure does not change. The relaxation time is controlled by the energy
barriers by which a structure is sustained, and this time restriction is
intimately related to the definition of state variables. The most important
property of state variables is their invariance to time averaging. The
time-averaged quantity R_{j} meets this invariance property.
