There are two techniques of boiling: in bou (煲), the stock is brought to a rolling boil over a high heat, before the flame is lowered and simmered for 3 hours.

Double boiling or dun (炖) is a much slower and more gentle process, characterised by submerging a ceramic pot within an outer pot of boiling water – similar in concept to the double layered metal pots the French refer to as a bain marie. The soup is not boiled twice, as its name suggests.

This technique is said to achieve the maximum extraction of flavour with little compromise to the flavour or texture of the ingredients used. It is therefore the method of choice for coaxing forth the delicate flavours of nourishing tonics. Full flavour extraction can take up to 5 hours.