ashing
Ashing is burning a sample until only its mineral ash is left — the same grey powder you find at the bottom of a fireplace after the wood is gone. The flammable, carbon-rich part goes up as gas and smoke, and what remains is the inorganic residue: the metals and minerals that don't burn.
In the lab, dry ashing heats a sample in a furnace, often around 500 to 600 degrees Celsius in a crucible, to oxidize and drive off all the organic material. The point is to remove the bulky organic matrix — the flesh of a food, the tissue of a plant — leaving behind the trace metals concentrated in a small, clean ash that is then dissolved in acid for measurement.
It is a simple, reagent-light way to clear away a heavy organic matrix before measuring metals. But it has a serious limitation: the high heat can volatilize and lose elements such as mercury, arsenic, lead, cadmium, and selenium, so dry ashing is unsuitable for those. For volatile elements, the lower-temperature acid digestions are preferred.
To find the iron in a breakfast cereal, a weighed portion is burned in a muffle furnace at 550°C until only a white-grey ash remains, which is then dissolved in dilute acid and measured.
Burn off the organic matrix; measure the metals left in the ash.
"Dry ashing" (burning in air) contrasts with "wet ashing," another name for acid digestion in solution. Both destroy the organic matrix to free the metals; dry ashing uses heat and air, wet ashing uses hot acid — and wet methods keep volatile elements that dry ashing would lose.