A technique is described for making a demountable vacuum seal of reasonably good quality between an all-metal evacuation cup and a flat glass surface. The seal is bakeable to temperatures close to the softening point of glass. The stainless steel evacuation cup has two concentric and coplanar sealing surfaces. The regions defined by these surfaces are differentially pumped. Pressures typically below 1 Torr are achieved in the annular space between the two sealing surfaces using a conventional rotary pump. The center region inside the inner sealing surface is typically evacuated to below 10(-4) Torr using conventional diffusion or turbomolecular pumps.