When the part must be fabricated, must hold dimensions while hot, or must not be heavy. Molybdenum is roughly half the density, considerably more workable, and has very low thermal expansion, which suits furnace structure, hot zones, boats and supports. Tungsten is chosen where density, the very highest temperatures, or electron emission is the requirement.

Both oxidise rapidly once hot, and the oxides do not form a protective layer. Molybdenum trioxide volatilises from around 600 degrees Celsius, so the metal is consumed rather than passivated, and tungsten behaves similarly. High-temperature service therefore requires vacuum, hydrogen or an inert atmosphere, which is why they appear inside furnaces rather than outside them.

Density of about 19.3 grams per cubic centimetre, close to gold and roughly 70 per cent denser than lead, so a given mass occupies far less space. That matters wherever the volume available is fixed - balance masses in rotating assemblies, aerospace trim weights, and vibration-damping boring bars where mass in the tool holder suppresses chatter.

With difficulty and with specialist practice. Both are brittle at room temperature in many forms, so they chip rather than cut cleanly and are prone to cracking; tungsten in particular is usually ground or produced by powder metallurgy to near-final shape. Molybdenum is the more forgiving and can be turned and milled with sharp carbide tooling and light cuts.

It is the temperature below which the metal fails by brittle fracture instead of deforming, and for both metals it sits near or above room temperature depending on purity and product form. It means stock can crack during handling, bending or clamping operations that any steel would tolerate, and it is the usual cause of damage in service.

For molybdenum, pure Mo and TZM - a titanium-zirconium-molybdenum alloy with higher recrystallisation temperature and better strength when hot. For tungsten, pure W, lanthanated and thoriated grades for electrodes, and tungsten heavy alloys, which blend tungsten with nickel and iron to give nearly the density in a far more machinable form.

Tungsten is the non-consumable electrode in TIG welding, chosen because its melting point exceeds the arc temperature so the electrode carries the arc without being consumed. Electrode grades are distinguished by their oxide additions, which govern arc starting, stability and current capacity.