Up until now, I have been using lead glass and Dumet wire to make light bulbs and other items with glass to metal seals. While lead glass and soda lime glass are cheap and make sense for large scale production, borosilicate glass is much simpler to use in lampworking due to its lower thermal expansion, allowing much more flexibility in what may be easily made.

The feed-through wires consist of 26 AWG (0.0159 in / 0.405 mm diameter) nickel 205 wires butt-welded to 0.016 inch diameter tungsten wire. Connecting the wires to a 25,500 μF capacitor bank charged to 40 volts and touching the wire ends together tended to make good welds. Nickel welded much more easily to the tungsten than copper, so nickel was used on both ends.

To prepare the tungsten wire for sealing to glass, the tungsten is first degassed by heated it as hot as possible without melting the nickel in a sharp flame. The tungsten is then quickly pushed into some potassium nitrite while still hot to dissolve the tungsten oxide. The wire tends to cool too quickly to allow the potassium nitrite to clean all parts of the wire, so the clumps of potassium nitrite that stick to the wire can be carefully melted to wet the entire length of tungsten. The wire is then rinsed in water to remove the potassium nitrite, leaving a clean silvery finish. Skipping the potassium nitrite cleaning tends to result in over-oxidized seals.

A glass sleeve is then placed over the tungsten wire, with enough length to cover part of the nickel wire on both ends. The sleeve is melted from one end to the other, moving the flame along as the sleeve collapses and seals to the wire to prevent trapping air bubbles. I found that the oxidation the tungsten wire received during this step was sufficient for a good seal. When finished, a good seal has a golden yellow to reddish-brown color, with few bubbles (seen on the lower seal in the image below). An over-oxidized seal will be black (upper seal in the image below), while an under-oxidized seal will appear silver.

Slightly over-oxidized seals may be corrected by reheating the seal to allow the tungsten oxide to dissolve into the glass. The seal will eventually show the appropriate color, as seen in the corrected seal below (during this reheat some more glass was also added to the right side of the bead to cover the weld joint). If the tungsten was too heavily oxidized or dirty, the seal will remain black, and should not be used.

The lead-in wires are twisted together on one end to help fix their position while they are being fused to the glass bulb, and a filament is crimped onto the other end of the wires.

The base of the bulb is then fused with the glass beads on the lead-in wires. In the finished seal, the remnants of the glass beads should not be visible (at least not if the same glass is used). If a faint image of the beads can still be seen, the seal should be reheated to a higher temperature to allow the glass to completely fuse.

After this step, a faint white haze was seen on one side of the bulb, possibly from dirt burning off the filament and depositing onto the bulb. The haze was readily removed by rinsing the bulb a few times with acetone through the evacuation stem.

Because the two lead-in wires are free to move during fusing to the bulb, it may be difficult to control the position of both of wires for favorable filament positioning. Before narrowing down the neck of the evacuation stem where the tip-off will occur, a wire may be brought down the evacuation stem to reposition the filament wires. Once the positions are good, the evacuation stem is narrowed, and the lamp is ready to be pumped.

This lamp was pumped for one hour on a Welch DuoSeal 1400, after which the filament was flashed to remove any contaminants. Nothing was deposited onto the walls of the bulb during flashing, so the bulb was tipped off. I was expecting a white haze might show up again, in which case I would have removed the lamp from the pump and rinsed the bulb with acetone again, then pumped the bulb down for the final evacuation (or repeat the acetone rinse as necessary).