  Window glass is a ternary mixture, while pyrex (after window glass, the most
common form of commercial glass) is a quaternary. Building on our previous
success in deriving the composition of window glass (sodium calcium silicate)
without adjustable parameters, and borrowing from known reconstructed
crystalline surfaces, we model pyrex as silica clusters with a specific ternary
interface. Our global model explains the thermal expansivity contours of
ternary sodium borosilicates, and it is consistent with the optimized
resistance of pyrex to mechanical and thermal shocks. It suggests new
directions for studying the nanoscopic structure of these remarkable materials.
