Indium and indium alloys have the unique ability to wet glass, allowing it to be used as a solder for glass. Gallium and gallium alloys can also wet glass, however the melting point of gallium is low (it will melt in your hand), and the melting point of its alloys are even lower.

I had some spools of an unknown indium alloy (it may be indium/tin, although it is harder than 52/48 In/Sn), and wanted to see if using this as a solder could make a reliable vacuum tight glass to metal seal. Using a 30 watt soldering iron, the solder easily wetted the glass. Brushing the iron back and forth while applying solder, I coated a portion of the glass inside and outside a lead glass envelope and stem near the edge where they would be joined. After coating the surface of both, they were joined together. The device was pumped down with a small hand vacuum pump to ensure there were no large leaks. A small hole was found and patched, and the resulting seal is shown below.


After leak checking (see Leak Checking) it was found that this particular seal is leaky. It is difficult to see imperfections in these seals, as the surface of the solder is bumpy and shiny, making any small defects difficult to see. A second attempt was made (in Homemade Triode), this time coating both inner and outer surfaces of the glass further up from the joint to better the chances of getting a good seal. More liberal use of the solder was also made. This resulted in a leak-free seal, shown below.

Because the glass is not being heated much when soldering with indium, it doesn't matter what the expansion coefficient of the glass is. This means indium solder may be used to join different types of glass without having to resort to graded seals. Indium joints may also be easily undone. The possible drawbacks to using indium for seals are the low melting point, the softness of indium (alloys will be a bit harder), and the cost.