Elastomer sealed vacuum flanges rely on especially sized elastomer O-rings and corresponding grooves to make an air-tight seal. Depending on the materials being sealed to one another, the O-ring/groove sealing mechanism may be designed for a specific percentage compression of the O-ring at the point of flange-to-flange contact. In the case of a glass viewport being sealed to a metal flange, however, direct contact between the glass and a mating metallic flange may produce stresses in the glass that can lead to failure during system evacuation or while the system is under vacuum. In this article, the authors first review fundamentals of safe O-ring seal construction that the reader will find useful when designing or inspecting viewports meant for vacuum systems. We then discuss an especially constructed adapter flange that allows the safe sealing of a 20 cm diameter glass viewport to an aluminum O-ring flange that was originally designed for a typical metal flange-to-metal flange seal. Factors affecting the strength of glass are discussed, and calculations for maximum deflection under load and safe glass window thickness are presented. [DOI: 10.1116/1.3442803]