Step 1: stuff you'll need
You will 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
first, you need to make the formwork. this will make or break (literally) this project. the more precise you can make it, the better.
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.
Step 3: make the lamp base
You can do this many different ways. I used steel, infilled with rockite (concrete). This requires welding and grinding equipment. You can also just cast a hunk of concrete. Or, a block of wood. Just remember that you need to get the cord in there somehow.
I used 1/4" thick steel bar stock at 1 1/2" wide. To make a 4" square, you need 2 at 4" and 2 at 3 1/2." Weld these together, grind the welds. (pictures 1-4) Then, because i'm going to pour concrete into the volume, i welded two little pieces of 1/4" bar to keep the hunk of concrete from slipping out someday... (picture 5).
Then drill a 1/4" hole as far to the bottom as possible on one of the sides. This is for the cord.
We won't fill the bottom all the way with concrete so that the cord can come out of the post and out our little hole. So put in a piece of foamcore to the top level of the hole we just drilled. Seal the edges with the glue gun. We'll pour the concrete from the top.
Then glue with the glue gun the threaded rod post onto the foamcore. Make sure it's not off at an angle.
Ok the last two photos are what it looks like when it's dry, top and bottom. Get the idea?
I think the next time I do this, I'll probably forget about floating it above the base. I think a hunk of wood could look nice. This took too long.
Now for our first pour. This will be good practice for the big one. I recommend getting rockite. It's available at a lot of smaller hardware stores. The stuff is awesome. It's like plaster, just a lot stronger. and it has a beautiful smooth finish. Mix it as per instruction so that it's like heavy cream. Or melted ice cream. Not too runny! The more water, the weaker. It's easy to add too much water. Add it slowly.
Pour it in just short of the top of the steel base.
Now, this is the annoying part. I don't think i would do this again this way, but here goes. I wanted the thing to "float" off the base a bit. So i decided to cast in some 1/4" steel bar legs. I welded these little bits of wire on to the legs so that it would adhere well to the concrete. I won't get into the details, because I think it's a bit of a cockamamied way to do this. But you need to make sure the wire won't interfere with the glass for the main pour.
For this step, I'm just setting them into the base about 1/4 inch, so that the shade will seat securely into the base. These will be easy to remove once it's hard, and then I'll cast the legs into the shade. I know, ridiculous.
The trick is you have to somehow hold these little guys in position while the concrete dries. This can be accomplished with small clamps. Or maybe duct tape But have a game plan before you pour. I didn't have one, so this is my improvised half-assed on-the-spot effort. It worked
Rockite dries fast. It's pretty amazing stuff. It will start to set up in about 15 minutes. After an hour you can carve it with a blade. In 24 hours it's wicked hard but you can sand it and shape it a bit. After a week it's rock solid.
Step 4: preparing the mold
Ok, you'll need to build some formwork for the void in the middle. I used foamcore and melomine. I think it would be better to use all foamcore, or a solid piece of ridgid insulation foam (usually pink or blue, available at home depot.) The reason for this is because you have to get it out once the concrete dries. I thought the foamcore would be squishable enough. But it was a pain to get it out. So, use foam, and you can coat it in packing tape for a better finish and easier release.
Regardless, it need to be 3" square, exactly, and the same length as the outer mold, PLUS the amount you want it to "float" above the base, if you want to do that. I used 3/4". This way, the base can rest on this while we cast the feet into the shade, and it'll keep the right spacing (see photos).
Ok the glass. You can glue the glass directly to the inner formwork, as shown. This way, the glass will be exposed on the inside of the shade. This glue will come off easily when you release it. The thick piece of glass overhangs 1/2" on either side of the inner formwork. The other 2 pieces butt into it and overhang 1/2" on their respective sides. Obviously, you can modify this layout. There should be at least 3/4 on the top and bottom. This is critical so that the concrete has some structure, since we're cutting it all up with the glass.
Now slide it into the outer formwork. This might require loosening some screws. This is not an easy step. You'll have to futz with it. If your measurements were good, it will be easier. It's critical that the glass butts into the outer formwork so that the edge will be exposed, and that it is securely glued to the inner formwork, so it will be exposed in the inside, so the light can get out. This will take patience. Trial, error, adjust.
The last step, once the formwork is adjusted, tight, and SEALED (you can use the glue gun like caulk if need be), invert base over the top so that you can cast the legs into the shade. Make sure they're reasonably straight, and that everything looks good. Take it out. Next step: pour.
Step 5: the big pour
Mix up enough rockite. I'll leave this up to you to figure out. Pour it slowly into the mold, until it comes flush with the top edge. you can overfill it a bit and then scrape it flush after about 10 minutes. Invert the base over it to cast in the legs (you can glue these temporarily into the base with a glue gun, or use duct tape or something. You don't want to lose them to the concrete abyss.
Once it all looks good. put it away for at least 12 hours.
Step 6: break the mold
Time to break the mold. Take off the base, but leave the sides attached. It's tempting to look, but it'll be easier and safer to ream out the middle if you leave the sides attached. If you used solid foam, get a big, fat, long drill bit and ream it out as much as possible Stick screwdrivers in there, putty knives, whatever you need to do to get that foam out. Foamcore is the same deal. Needlenose plyers and yanking might help.
This step is a pain in the ass. Take your time and be gentle. It's possible to break this thing.
After you get the center pretty well cleaned out, you can take the outer mold off. Make sure it didn't crack anywhere (if it did, you can fix cracks with superglue). You might need to excavate a bit the glass edges so they are exposed. Be gentle. you can use a flat edge screwdriver to do this. You might also need to do this on the inside, if some of the rockite got in between the formwork and the glass. It should chip off. Be gentle.
Finally you can sand lightly the sharp corners, and rinse the whole thing off in the sink. Make sure all the glass is exposed cleanly inside and out.
If it came out first try, nice work. If not, hopefully you can re-use the glass and re-pour.
Step 7: assemble lamp
Put the lamp together. Thread the cord up through the base holes, screw the thing on top. You can put a switch on the cord too, if you want. I'm not getting into wiring here. If you don't know what you're doing, ask someone who does.
Use a bright bulb. I found this fluorescent (incandescents will get the thing really hot - better to use compact fluorescents (and less energy!). This bulb is a 150 watt equivalent, which uses 32 watts. The shade will block a lot of the light.
Notice that you can see through the shade on the thick piece of glass (even when switched off!)
Ok, you're done. Please, if anyone is crazy enough to try this, let me know how it goes.