Replacing the dark background with a white one results in what is referred to as black line lighting. In the silhouette image of the glass to the right, the dark lines are a result of the dark background areas to the right & left of the white center panel, as well as the thickness of the glass.

Once again, even tho the glass is in front of a brightly lit white background, which might diminish weaker reflections, we need to use a flag to prevent the point source of the Pro-light from hitting the glass and causing a strong reflective highlight.

Lighting from beneath a glass object can give dramatic results.

But every glass object is different and sometimes this approach may not show the object at its best. The bottom of the object is usually over lit, and there can be strange color refractions throughout the image as the light travels through the glass and its various surfaces.

However it can be a useful technique to be aware of.

Create a small stage by suspending a piece of glass or plastic between two supports, they can be blocks or books of identical thickness. Clear or translucent white glass / plastic is the most commonly used. The greater the open space you have under this platform, the better.

Cover the glass with your background material, an opaque paper is a good choice.

Put the object in place and trace around the bottom.

Carefully cut out the traced area, and place the object back over the cutout area.

Direct the light upward from underneath the glass, either straight up or by bouncing it off a white card / reflector propped at 45 degrees to the floor. Be prepared to block stray light from the floor spilling into the room. it could get reflected back into the glass.