Rice hulls
From PyroGuide
Rice hulls (rice chaff), a side product in the processing of mature rice, popularly serve as a core or inert material in the manufacture of granular burst charges (for fireworks shells, bombettes etc.). They are also used as a packing material for special shells.
In rice-growing-areas hulls are readily available for practically nothing. Elsewhere pyro suppliers will present the only source for imported material. Note that only one pound of hulls is usually sufficient to produce the burst charge needed for two dozens of 4" spheres.
Given they are intented to be coated with burst charge compositions (gunpowder, H3, KP etc.) the hulls are sieved (to remove any dust) and a known amount is wetted (by spraying, or better, by soaking them in water) before the compositions (including binders) are added as a dry powder. Then the cores are coated by shaking the blend in a sealed container or by pan-coating them in rolling drums. The ratio cores : composition usually is well defined (according to the diameter and the intended appearance of the shell) but can be varied (within certain limits) to tune the strenght of the charge. Popular ratios for round shells (chrysanthemums) are published in Shimizu FAST p.214. Apart from round shells meal coated hulls are frequently added to bombettes in order to widen the bursts.