This is the second part of my tutorial on lighting glass objects, in response to suggestions from Talk Photography, members who said that they would be interested in tutorials on
  1. Lighting glass
  2. Using an absolute minimum of equipment
  3. And doing it in a tiny space

The first part covers Brightfield lighting, where only the background is lit, and all that we can actually see of the subject is the edges, or other areas that distort the passage of light.

Darkfield lighting is the opposite, it  involves lighting the glass and nothing else, and I used the same nearly empty bottle of after shave. It has a fairly complex shape, with various bulges, convex and concave surfaces.   All of the branding on the front takes the form of labels. That's a pity, it would photograph better if the branding was integral to the moulding. It still has a drop of that green smelly stuff inside...
When people photograph glass without trying very hard they usually crop off the bottom of the subject. This makes life easier, but in these example shots I've shot 'full length'. Full length involves including whatever the object is standing on, and here I placed it on a piece of shiny black acrylic, which can create a very strong reflection - sometimes too strong, people really ought to experiment with all sorts of different surfaces such as acrylic, plain glass, ceramic tiles and various found materials. The setup shot above left is the basic arrangement I used for my brightfield lighting shots, with a softbox masked off with Cinefoil at the edges. Here though, although I used the identical softbox/table arrangement I masked it off the opposite way, with the centre blacked out but with light escaping at the sides. Here is the setup shot on the right. If you click on it to see a larger photo you'll see that the masked off area is providing a black background, the acrylic base is providing a black reflection and light from the sides is catching the edges of the subject and defining its shape. It's as simple as that!
But there's not a lot of detail showing (photo on left), so our next step is to use a couple of reflectors to push a bit of light onto the front and front edges. This setup shot, below right, shows how it's done, with pieces of A4 white paper supported by a book on each side.
You can see the result in the photo below left, and it's clear that the reflectors have lit the front pretty well in addition to putting more light on the edges.  You can adjust the amount of light reflected back onto the front of the edges and the front of the subject very easily, to make it brighter just move the reflectors closer, to make it darker, do the opposite.
I mentioned earlier that we don't actually need to use an acrylic base to get a reflection of the subject. Other surfaces will do just a well, and often better. In this next shot, I've removed the acrylic sheet and the aftershave bottle is now sitting directly on the top of a plain glass coffee table.
But I've made other changes too, because although I've made a point of using the simplest possible lighting and the smallest possible space for these tutorials, there are other ways of doing things too, which allow greater flexibility. In the shot on the right, I've taken away the softbox and am now using a plain white wall as my background. It photographs as black because absolutely no light is reaching it. If you click on the thumbnail on the left you'll see that I am now lighting the aftershave bottle with 2 standard reflectors, behind and from each side. Each of the standard reflectors is fitted with a honeycomb, this not only concentrates the light into a small area but also prevents lens flare, which is essential.
If required, we can also add in the white reflectors, these provide additional fill to the front, as in the shot on the left.  By this point, the shot is becoming less of a blackfield shot, and you may feel that the lighting is overdone, but all that I'm trying to show you here is what you can do, not what you should do - you have to make the decisions yourself...
One of my reasons for removing the softbox light source and using as plain white wall instead is that doing so also enabled me to light the background, which of course is also transmitted through the glass bottle. (If you don't want to see the light through the bottle you need to cut out a mask, the same shape and size of the bottle and bluetak it onto the unseen rear of the bottle). The technique here is simply to shine an additional light onto the background, again the light is fitted with a standard reflector to which a tight honeycomb is fitted. The role of the honeycomb is to limit the spread of the light. As you can see, I also fitted a blue gel over the front, and I used the normal professional method of simply draping the gel over the front and temporarily fixing it in place with a bit of sticky tape. It doesn't look pretty, but it works.
All of these lights are being used at very close distances and have been set to the lowest power possible. But the honeycombed and gelled light, even though it's on minimum power, is still too bright as you can see from the picture on the right. The answer to this problem is simple, just double up the lighting gel, this will reduce the power by at least one stop.
Having too much power, especially at short distances, is a problem that comes up all the time - so much for the statements that we see so often on photography forums, where people advise other people to buy powerful flash heads...
This is the result, above left. It might even have been a good idea to double it up yet again, to get a more subtle effect, but you can experiment with this yourself.
You'll see a distinct line between the lit background and the surface of the circular table, you can either leave it where it is or you can easily edit it in Photoshop.
One very important point...

Absolute cleanliness of the glass surface is critical, it will show every mark. The particular product photographed here had been lying around in a drawer for a while and was in far from perfect condition, but really you need to photograph new items, after cleaning them thoroughly. For these tutorials, I just use items that I happen to have lying around.