Manufacturing methods for making a substantially rectangular and flat glass preform for manufacturing a Li ion conducting glass separator can involve drawing the preform to a thin sheet and may involve one or more of slumping, rolling or casting the glass within a frame that defines a space filling region and therewith the shape and size of the preform. The thickness of the rectangular flat preform so formed may be about 2 mm or less. The frame may be slotted having a back surface and widthwise wall portion that define the height and width of the space filling region. The flat backing surface and surfaces of the widthwise wall portions are defined may be coated by a material that is inert in direct contact with the heated glass material, such as gold.