A crucible is a cup-shaped piece of laboratory equipment used to contain chemical compounds when heating them to very high temperatures. The receptacle is usually made of porcelain or an inert metal.

Use in chemical analysis
Ash is the completely unburnable inorganic salts in a sample. A crucible can be similarly used to determine the percentage of ash contained in an otherwise burnable sample of material such as coal, wood, or oil. A crucible and its lid are pre-weighed at constant mass as described above. The sample is added to the completely dry crucible and lid and together they are weighed to determine the mass of the sample by difference. The crucible, lid, and sample are then fired to constant mass to completely burn up the sample, leaving behind only the completely unburnable ash. After cooling in dryness, the crucible, lid, and remaining ash are weighed to find the mass of the ash from the sample by difference. The fraction of ash (by mass) in the sample is determined by the dividing the mass of the ash by the mass of the sample before burning, which is done by subtracting the weight of the crucible and lid from the figure of the container, lid, and sample.