The casting of the bronze drums of Dong Son is a complex process requiring high order of techniques and artistic skills.

It was first necessary to produce a hollow clay core to minimise weight and ease its manipulation.
Separate clay pattern moulds would then be prepared, circular for the tympanum and rectangular for sections of the mantle.
The surface of the clay received ornamentation in more than one way. It could be impressed with a patterned old to create the panels of geometric ornament, or for more individual motifs or decorative elements, it could be incised with a stylus.
This pattern old would then have been filled with molten wax, such that the wax filled and duplicated the chosen decor.
It would then have been necessary to transfer the sheets of cooled wax to the clay core, having first place the bronze spacers strategically into the wax until they reach the surface of the pattern old.
This procedure resulted, in effect, a wax drum over a clay core.
Investment of the wax in a layer of very fine clay followed before the assemblage was covered in a coarse clay coat. It was then necessary to melt out the wax, and preheat the clay old. The critical point was then reached for the pour, in the case of larger examples, of nearly 100 kg of molten bronze into the conduits to reproduce in metal the wax image of the drum.

References: The Bronze Age of Southeast Asia, Charles Higham, 1996