I can't say that I am familar with Crookes tubes, but I can explain about the sealing of incandescent lamps and it could be similar to what you have. Dumet wire was first put into use around 1914, when it replaced platinum. A good type of metal is needed for the lead in wires through the stem press in order to mantain vaccum in the lamp. Right from the very early days of incandescent lighting, it was/is known that platinum "wets" the glass in the stem press, and this is a good sealant, so that the vaccum/gas won't leak out when the lamp cools and the wires contracts. Unfortunately platinum is an expensive material...so a reliable replacement had to be found. Several materials were used, but none proved to be good. Finally, it was found that conventional lead in wires coated with copper made a good sealant, and was cheaper to manfacture, and as I said, it was put into wide use in 1914 and is still in use today.
