The scrolls survived because they were subject to low humidity, fairly constant temperatures, and because they had been left undisturbed since the first century A.D.
Unlike large glass mirrors which are subject to the vagaries of humidity, thermal dynamics and constant deformation from gravity itself, liquid mercury which remains molten at room temperature can reflect as much as 75 percent of incoming starlight with little of the fuss associated with large conventional mirrors.
In conclusion, incoming energy throughout the year is constant, with no large variations, and humidity controls the balance of this energy between sublimation and melting.