Fluidised bed furnaces employ an alloy retort filled with sandlike alumina particles. When a controlled stream of air or gas is passed upward through a distributor below the retort, the particles float on a cushion of the air/gas stream and move around turbulently without elutriation. The now "fluidised" bed looks and behaves remarkably like a boiling liquid bath.

When heated (externally or internally) and when the fluidising gas mixture is also the required heat treatment atmosphere (neutral or reactive), the fluidised bed becomes an excellent heat treatment furnace for components that are immersed into it.