From PyroGuideEditSolid reaction by-products Dross is a solid by-product of a combustion reaction. For example the residue left behind after a composition has burnt. Dross can be a useful by-product in relation to prime as the red hot dross particles have better ignition properties than hot gases. In other instances it can cause slag in nozzles and cause fireworks to either explode as the gases become trapped or it can reduce the fireworks performance.
The example image to the right shows the solid dross particles remaining after a pyrotechinc composition has finished burning.