A process for preparing glass-ceramic body including the steps of providing a basic glass body and subjecting the basic glass body to a thermal treatment whereby a crystalline phase embedded in a glass matrix is formed. The basic glass body is made of a composition comprising 65 to 72 wt-% SiO.sub.2, at least 10.1 wt-% Li.sub.2O and at least 10.1 wt-% Al.sub.2O.sub.3 based on the total weight of the composition, the proportion of Li.sub.2O to Al.sub.2O.sub.3 being from 1:1 to 1.5:1. The thermal treatment involves a nucleation step followed by several crystallization steps at different temperatures, whereby at least two different crystalline phases are formed.

An apparatus for manufacturing a glass article includes a plurality of side portions spaced apart from each other; and a plurality of heat supply portions disposed on each of the side portions; where the side portions adjacent to each other are disposed to face each other, and a glass is allowed to be disposed between the adjacent side portions.