old russian capacitors

You may have one of the rare earth elements that readily oxidize when exposed to air.

Niobium (atomic number: 41) is a shiny, white, soft, and ductile metal, and takes on a bluish tinge when exposed to air at room temperatures for a long time. The metal starts to oxidize in air at high temperatures, and when handled hot must be done so under a protective atmosphere so as to minimize oxide production.

Lanthanum (atomic number: 57) is silvery white, malleable, ductile, and soft enough to be cut with a knife. It is one of the most reactive of the rare-earth metals. It oxidises rapidly when exposed to air. Cold water attacks lanthanum slowly, and hot water attacks it much more rapidly. The metal reacts directly with elemental carbon, nitrogen, boron, selenium, silicon, phosphorus, sulphur, and with halogens. It is a component of, misch metal (used for making lighter flints).


Cerium (atomic number: 58) is an iron-grey lustrous metal. It is malleable, and oxidises very readily at room temperature, especially in moist air. Except for europium, cerium is the most reactive of the rare-earth metals. It slowly decomposes in cold water, and rapidly in hot water. Alkali solutions and dilute and concentrated acids attack the metal rapidly. The pure metal may ignite when scratched with a knife. It is the most abundant of the rare earth metals and is found in minerals including allanite, monazite, cerite, and bastnaesite.


Praseodymium (atomic number: 59) is soft, silvery, malleable, and ductile. It was prepared in relatively pure form in 1931. It is somewhat more resistant to corrosion in air than europium, lanthanum, cerium, or neodymium, but it does develop a green oxide coating that "spalls" away when exposed to air. The metal should be stored under an inert atmosphere or under mineral oil or petroleum. The rare-earth oxides, including Pr2O3, are among the most refractory substances known. It is a component of misch metal, used for lighter flints, and of the glass in welders' goggles.


Neodymium (atomic number: 60) is present in misch metal to the extent of about 18%. The metal has a bright silvery metallic lustre. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes the metal to further oxid