Clear glass against a white background can nearly disappear. Against black, it can also vanish if no edge reflects light. The material becomes readable mainly through edges, reflections and changes to the background. You are therefore photographing not only the glass but also the surfaces it transmits and reflects.
Bright background with dark edges
Place the glass against an evenly bright surface and position dark cards beside it outside the frame. Their reflections can define the outline. Move them slowly nearer or farther and inspect the width of the dark line. Too broad a reflection makes the glass heavy and dark; too narrow a line may disappear in small output.
Avoid an uncontrolled bright lamp directly in front. It can appear as a large white patch that obscures transparency. Check the camera area as well: lens, hands and clothing may reflect. An uncluttered environment makes it easier to connect every visible patch with its cause.
Dark background with bright edges
Against a dark background, bright side surfaces can emphasize edges. Ensure they skim the glass rather than unintentionally light the background. Small positional changes can reveal stem, base and rim differently. Inspect all three instead of judging only the largest glass area.
Engraving or textured surfaces may need different lighting from perfectly clear glass. A side accent can reveal structure while a broad frontal reflection conceals it. Make one view for overall shape and another for detail if necessary instead of forcing every property into one angle.
Finish by checking cleanliness and focus
Fingerprints, small droplets and dust become obvious against smooth surfaces. Clean the glass appropriately and inspect it under the final light. Support the camera and focus on a recognizable edge. Several glasses at different distances may require more depth of field. Inspect both the front rim and the rear rim at full size before moving the setup. Check the final crop for cut-off reflection lines, which can unintentionally make the shape look open or damaged.
