Decrease of heat capacity on aging, long and linear extrapolations for fictive temperature, and the case of 20-million-year-old fossil-amber

A glass and its melt have the same value of a physical property at fictive temperature, T f , which is estimated by matching the area between two linearly extrapolated plots of the heat capacity, C p , against temperature T , one from C p of glass and the other from that of the melt. According to the DSC heating scans, the C p values of 20-million-year-old fossil-amber and its melt-cooled fresh-amber are the same at T < 273 K, and T f of fossil amber is 0.89 T g → l , where T g → l is the glass-liquid transition temperature of melt-cooled amber [Nat. Commun. 4 (2013) 1783]. A comparison of T f with the variation of stress relaxation time with T led to the conclusion that relaxation dynamics of amber deviates from super-Arrhenius variation. Here we show that use of the same C p s of the two ambers at T > 0 K violates basic thermodynamics, and argue that C p of (denser) fossil-amber would be less than that of fresh-amber at T < 273 K. The C p , c o n f of the melt, taken as the difference between the extrapolated C p s of fossil-amber and of its melt, increases by ∼13% on cooling from T = T g → l to T = 0.72 T g → l , and this increase and the corresponding increase of ( C p , c o n f / T ) = ( ∂ S c o n f / ∂ T ) p are too small to indicate an approach towards a second-order transition of the entropy theory. Hence we conclude that deviation from super-Arrhenius variation of relaxation time of amber is an artefact of a long and linear extrapolation of its melt`s C p used for determining T f .