Update: See Homemade Light Bulbs for working light bulbs.
Here are my first two attempts at making home made light bulbs (more to come in the future... I will make this work). My first light bulb seemed to work just fine while it was was connected to the vacuum pump, but after sealing it off there was a small leak somewhere which caused the filament to oxidize. In the second attempt, I was more careful to flame anneal and slowly cool parts by burying them in vermiculite for 15 minutes, and I was more careful to make sure that all seals were air tight. But the second bulb oxidized even while connected to the pump. I later suspected was due to a leak in connection to the hose for the vacuum pump, and after attempting the pump-down again a few days later, the bulb immediately cracked upon turning on the pump.
Light Bulb 1
This is how the bulb looked before pump-down. The bottom seal on the bulb is pretty ugly, and I suspected this seal may be a weak point where air could leak. Making this seal was the most difficult part in constructing the bulb.
While pumping down the bulb everything seemed fine. After several flashes of the filament at higher and higher power to help the bulb pump down, the filament showed no signs of oxidation. So I sealed off the bulb, thinking everything may have worked out fine.
Upon heating the filament after sealing the bulb off, the filament was immediately oxidized. This was not just due to some residual air left behind, as the oxidation became progressively worse when heating it at later times.
Light Bulb 2
In my second attempt at making a light bulb, I decided to take pictures during its construction to demonstrate how it is made (and possibly look for steps that may be problematic). Unlike my previous attempt, after flame annealing the glass is immediately placed in a metal can under vermiculite to slowly cool for at least 15 minutes. This is done after all steps that involve heating glass (previously I allowed the parts to cool in air after flame annealing). I also tried to heat the glass with as low a temperature as possible, while still being able to melt the glass enough to make good seals. It may be hard to see in these pictures, but if you look at the discoloration of the glass to metal seal in the stem of the first bulb and compare it with this one, the first bulb's stem is more discolored, and the discoloration extends further down the stem.
The first step in constructing the bulb is to make the stem for the filament. 15mm lead glass is used, and the bottom is flared out to be sealed to the outer envelope.
The opposite end of the stem is partially flattened.
0.35mm borated Dumet wire is inserted into the flattened end, and surrounded with glass wool inside the stem. I found that if the Dumet wire is not kept away from the walls of stem, it will promote cracking in the stem due to the higher thermal conductivity of the Dumet wire. The glass wool helps to hold the Dumet wire in place, as well as insulate it and keep it from touching the wall of the stem.
The exposed Dumet wire is covered with heavy aluminum foil to prevent the torch flame from melting the wire.
After sealing the glass to the Dumet wire, the aluminum foil is carefully removed, leaving a bit of aluminum behind which has melted to the wire.
The wire is cut and sanded to clean off the aluminum, and remove any oxide and other dirt to allow a good contact to the filament. A tungsten filament is held in place by bending the wire over the ends of the filament.
While the stem is cooling in the vermiculite during its construction, the outer envelope is simultaneously being made. First a round bottom is blown in 25mm lead glass tubing. A hole is made in the round bottom, and 7mm lead glass tubing is sealed on for evacuation of the bulb.
With both the stem and envelope completed, the stem is inserted into the envelope, and the edge of the flared out stem and envelope are both heated and pressed together to form a seal. I've found this step to be the most difficult. To ensure this seal was good in this attempt (I blamed this seal for leaks in the first bulb) I made extra care to ensure it was sealed all around. The result is a fairly ugly seal, but I was at least confident there were no leaks.
The entire tube was flame annealed, and allowed to cool in vermiculite for several hours before I pumped down the bulb (after 15 minutes I don't think there is any point in leaving it in the vermiculite any longer, but I was taking a break). I also keep a lid on top of the can, but it is removed here to take a picture after the bulb had cooled.
The resulting bulb looks OK. The bottom seal is ugly, but I'm at least sure all seals are good this time.
Confident everything will work, I pump down the bulb, and upon the first flash of the filament it is immediately oxidized. I removed the bulb from the pump to think about what went wrong.
I suspected that there may have been a leak in the connection to the vacuum pump and the bulb may have been fine. A few days later I replaced the rubber tubing in the vacuum line and tried to evacuate the bulb again, when disaster struck. Upon turning on the vacuum pump I heard a pop. The stem had broke off where the glass to metal seal starts. I was not expecting this, perhaps for lead glass flame annealing is not enough. Or maybe I need to work with the glass at even lower temperatures. I may also try my luck with using borosilicate glass and Kovar wire.