Abstract: A tempered glass dielectric material for use as an electrical insulator is thermally tempered to produce surface compression between 200 MPa and 400 MPa. At least part of the material is then chemically tempered under conditions of temperature and duration such that the surface compression due to the thermal tempering is reduced by between 50% and 80%. The chemical tempering is effected by applying to the relevant part of the material, at ambient temperature, a solution containing alkaline salts. This solution is dried and the material then heated for several hours at a temperature between 300.degree. C. and 500.degree. C. The solution has the following composition (in percentage by weight):______________________________________ H.sub.2 O 70% to 90% Starch 2% to 10% KNO.sub.3 4% to 12% K.sub.2 HPO.sub.4 + KCl + K.sub.2 SO.sub.