Abstract: A method of manufacturing a display device having an air-tight envelope, in which a glass face plate and at least one further glass part are joined to form the air-tight envelope that is to be heated and/or evacuated at a later stage of the method of manufacture, comprises inducing a thermoshock in the face plate and/or the at least one further glass part prior to joining the glass face plate and the at least one further glass part to form the air-tight envelope; examining the face plate and/or the at least one further glass part for growth of cracks indicative of defects which could render the air-tight envelope liable to implosion during heating and/or evacuation; choosing between selecting and non-selecting the face plate and/or the at least one further glass part for use as part of the air-tight envelope depending on whether the growth of cracks has been found; using the face plate and the at least one further glass part as part of the air-tight envelope if the growth of cracks was not found; and performi