After finishing, I discovered another artist's elegant take on the burnout concept here: http://www.castordesign.ca/
I also realized a number of shortcomings upon completion; namely, the tubes being buried permanently in concrete makes it difficult to replace a broken bulb or to move the lamp without threatening the tubes. The next iteration, perhaps, will have sockets which facilitate swapping out the tubes for transport or repair. In the process of making the lamp, one tube broke; I've included replacement instructions here.
Fluorescent tubes are filled with hazardous toxins, namely mercury. Work in a well-ventilated area, and wear a mask if you break a tube. Clean up the shards of glass with gloves.
Salvage the bulbs from your office, home, or school. Counties and towns with bulb-recycling programs might be inclined to spare a few if you ask nicely.
You will need these materials:
eight 1-1/2" dia.burned out fluorescent tubes
8" dia. sonotube or similar
approx. 2' square of 3/4" plywood
approx. 12" of 2" dia. PVC pipe
50 lbs. quick-set concrete mix
three-wire grounded power cord
approx 3" x 6" x 1/4" piece of plexiglass
You will need these tools:
1-1/2" drill bit
2-1/4" hole saw
Dremel tool or similar
Step 1: Formwork (1)
On two of the circles, lay out eight center points in a radial pattern. Start by pulling one line across the circle, dividing it in two; then measure the center of that line. Take a square and pull a line perpendicular to the first, dividing the circle into four equal pieces. Subdivide again, just like cutting a pie, to create eight even pieces. Measure up each line 2-1/2" from the center and make a mark. Drill each mark with a 1-1/2" bit. Make two identical pieces like this. On one of them, drill out the center with the 2-1/4" hole saw bit.
Cut the two drilled-out circles out of the piece of plywood they are in. Cut them large, so they will rest on top of the sonotube without falling in. Cut the third small, so it will fit inside the sonotube.
Cut the PVC pipe and the sonotube to 12" long. Sand and smooth the pipe's cut edge.
Set all these pieces aside for a minute.
Step 2: Wiring
Start by removing the fixture from its metal housing, so it is just a ballast with two sockets wired to it.
Take the plug created by hole-sawing the formwork and sand it down until it can fit inside the PVC pipe. Cut the wires leading from the ballast to one socket end of the fixture and screw that fixture to the plug of wood. Drill two holes in the wood to feed the wires through. Re-attach the socket to the ballast leads with about three or four inches of wire between them. Tape up the wires.
Next, take the power cord (it must be a three-prong grounded cord) and wire the two power leads, white and black, to the power leads coming from the ballast. Fluorescent fixtures must be grounded to work; typically, in hard-wired installation, the ground cord is just attached the metal bulb housing. In this case, since we've discarded the housing, the ground can just be wired directly to the ballast itself. Run the green ground from the power cord up and attach it to one of the mounting holes on the ballast.
Cut the other socket off and re-wire the two leads with about four feet of black speaker wire or similar. Feed those wires through the wire holes in the wood plug. Tape off and paint the plug white if so desired.
Wrap everything up tight and test the bulb to make sure the wiring is right. Tape the wires to the ballast to make a compact package. Stuff the ballast and wires into the pipe, then push the plug in after. It should fit really tightly; if it won't go in, put the handles of some pliers on the wood and hit the head of the pliers with a hammer. Secure it with a screw or two.
Cut a little slot in the base of the pipe with a Dremel for the power cord to exit. Tape around the gap so concrete won't leak up into the pipe.
Step 3: Formwork (2)
Glue the pipe to the center of the plywood baseplate with caulk. Seal around the edge.
Place the sonotube around the baseplate-pipe assembly, feeding the cord through a little slot. Screw the sonotube into the baseplate. Seal with caulk. This can be tricky, given the tight clearances inside the formwork, but try to be fairly neat about it.
Align the two other plywood pieces with an eighteen inch or so piece of 2" x 4" in between. Screw them together, creating a sort of barbell-looking structure.
Place the formwork between some cinderblocks or similar. Level everything out. Place the cinderblocks so they come above the formwork, so the plywood barbell can rest on them.
Step 4: Pourin'
Fill the sonotube to within about an inch of the top, taking care not to slop concrete on the the socket or exposed wires. Place the barbell on the cinderblocks, hovering over the formwork. Align the center of the barbell and the center of the formwork visually. Push the fluorescent tubes through the holes in the barbell, down into the concrete, one by one. Let cure.
After a few hours, strip the formwork with a putty knife and screw gun. This is where I broke a tube; be extremely careful in handling the structure at this point. To get the bottom off, you need to tip the lamp on its side. In doing so, it rolled, and one of the tubes that was slightly out of vertical got pinned against the floor and broke.
Because all the tubes were not perfectly in line -- the plywood pieces with the holes were slightly twisted relative to one another -- the plywood was very difficult to get off. Amazingly enough, I was able to saw it out without breaking any tubes.
Step 5: Switchin'
I used a 1/4" piece of plexiglass, scrap that the hardware store let me have for free. Cut a rectangle approx. 2-1/4" x 6"; hold it above the tops of the tubes and scribe two opposing tubes onto it. Drill those out (carefully!) with the 1-1/2" bit. Drill another small hole for mounting the socket with a machine bolt, and a fourth for the switch. Make sure the switch is far enough from the socket that it won't interfere with the bulb.
Sand down the plexiglass with ascending grits of sandpaper to frost it. This way, it nearly matches the tubes and visually disappears.
Mount the socket with some plastic cement and a #8 machine bolt. Trim any excess length off the bolt. Mount the switch. Wire the speaker wires to both. Put in a 3' working bulb (it's tricky to get the bulb in, but you can do it . . .), then plug it in and enjoy!
Note on broken tubes: if you should have a broken tube, crack out the glass, then clean out the resultant socket in the concrete with a Dremel (wearing a mask and eye protection), then glue a new tube in. The new tube will be too long, since it will be impossible to get the buried base of the bulb out of the socket. Use a Dremel to carefully cut off a little of the replacement tube to get the heights to line up.