Abstract: This invention has as its subject a panel, preferably produced by the float process, reinforced by surface ion exchange for a long period at a temperature such that the exchanged depth is greater than 250 microns for surface compressive stresses exceeding 400 MPa, and greater than 75 microns for surface compresive stresses exceeding 700 Mpa, and the matrix of which complies with one of the following compositions, expressed in percentages by weight:SiO.sub.2 :65 to 76%Al.sub.2 0.sub.3 :1.5 to 5%MgO:4 to 8%CaO:0 to 4.5%Na.sub.2 :10 to 18%K.sub.2 O:1 to 7.5%B.sub.2 O.sub.3 :0 to 4%these elements representing at least 96% of the weight of the glass and complying, in addition, with the percentages by weight 0<CaO/CaO+MgO<0.45 and 0.05<K.sub.2 O/Na.sub.2 O+K.sub.2 O<0.35.