Calcined uranium yellowcake, as produced in many large mills, contains a distribution of uranium oxidation species in various forms ranging from most oxidized to least oxidized. Particles with short residence times in a calciner will generally be less oxidized than those with long retention times or particles recovered in the stack scrubber. Uranium content is usually referenced to U
3O
8, which dates to the days of the Manhattan project when U
3O
8 was used as an analytical chemistry reporting standard.

What will never be less oxidized than those with long retention times or particles recovered in the stack scrubber?