A thermally tempered aluminosilicate glass-ceramic composition includes a crystalline phase and a residual glass phase, wherein the two phases form a system wherein the thermal expansion curve of the system has two distinct sections diverging from an inflection point temperature in the range of about 450° C. to about 600° C., and wherein the difference between coefficient of thermal expansion of the glass-ceramic below and above the inflection point is greater than about 4 ppm/° C.

An apparatus for thermally strengthening a glass sheet includes a first heat sink surface, a second heat sink surface separated from said first heat sink surface by a gap between the heat sink surfaces of distance g, and a liquid feed structure positioned to be able to feed a liquid to the gap, wherein the distance g is sufficiently small relative to a thickness t of a glass sheet to be processed such that when a sheet of thickness t is positioned within the gap of distance g, thermal transfer from a first surface of the sheet facing the first heat sink surface is more than 20%, 30%, 40% or 50% or more by conduction from the first surface of the sheet through the liquid to the first heat sink surface.

A process of strengthening a glass sheet by cooling a sheet or portion of a sheet, the sheet comprising or consisting of a glass having a glass transition temperature, given in units of C., of T, wherein the cooling is performed starting with the sheet at a temperature above T, with more than 20%, 30%, 40% or 50% or more of said cooling, at some point during said cooling, being by thermal conduction through a liquid to a heat sink surface comprising a solid.