this is a project for futurecraft (blog here)
Step 1: Stuff you'll need
concrete (rockite works best). something to mix it in/with
something to make formwork. Plywood or melamine work well. Foam Core can also be used.
16" of 1 1/2" X 1/4" steel bar stock
3 pieces of glass: 1/4" thick: 12" X 4"; 1/8" thick: 12" X 3 1/2"; 1/16" thick: 12" X 3 1/2"
(you can cut these yourself, or easier yet have a glass store cut them. they need to be fairly precise.)
screws, drill, drill bits, driver
ceramic lamp socket
4" threaded lamp post
light bulb (bright)
space to make a mess
Step 2: Making the outer formwork
The inside volume needs to be exactly 4" square. You can use melamine, which will give a smooth surface. You can also use lumber or plywood, with can give the concrete texture.
If you use 3/4" material, you'll need 2 pieces 4" X 14", and 2 at 5 1/2" X 14". And 1 bottom (which is actually the top) at 5 1/2" square.
Again, make this as precise as possible. If you don't have access to a shop, you can use 1/2" foamcore, and a knife and ruler. Use a glue gun to glue it all together. If you coat the inside surface of the foamcore with clear packing tape, the concrete won't stick, and you'll get a smooth, glassy surface on the concrete. The foamcore method is probably easier and less forgiving, and works pretty much in the same way. Just make sure to seal the joints really well, with the glue gun or packing tape. Otherwise, it will leak. everywhere. The concrete will be very liquidy. Like heavy cream. Don't underestimate it's ability to find cracks and leak out of them.