Molybdenum has been used for many years in the lamp industry for mandrels and supports, usually in wire form. Today, several unique properties of molybdenum that satisfy more demanding industry requirements have increased the use of molybdenum as a material in applications requiring other mill forms.

Tungsten has a long history of use for filaments in the lamp industry. It offers exceptionally high strength at very high temperatures. In fact, it has the best high-temperature strength of the four common refractory metals. Its high-temperature strength, combined with its good electrical resistivity have made it a popular choice for other applications in addition to filaments.

Tantalum and its alloys are midway between tungsten and molybdenum in density and melting points. Tantalum can be worked easily at room temperature.

Its thermal conductivity is one-fourth that of molybdenum and its coefficient of expansion is one-third greater. Its elevated temperature strength is low compared with tungsten and molybdenum.

Rhenium is rare, expensive, and extremely difficult to machine. It is very limited in the sizes and shapes available. It is most commonly used as an alloying agent to improve the properties of other metals (mostly refractory metals).

Rhenium has several unique properties that are also imparted to its alloys. One of the most important of these is its high melting point of 3180 degrees C. This is second only to tungsten. The high melting point, coupled with its ability to maintain ductility even after recrystallization, offers many advantages. 

Rhenium also offers high electrical resistivity over a wide temperature range. The resistivity is higher than tungsten. This characteristic, plus having a low vapor pressure, make it a good choice for filaments. This is especially true when considering ductility and the fact that it is not affected by the oxidation/reduction cycle as is tungsten.