Dross is a mass of solid impurities floating on a molten metal. It appears usually on the melting of low melting point metals or alloys such as tin, lead, zinc or aluminium, or by oxidation of the metal(s). It can also consist of impurities such as paint leftovers. It can easily be skimmed off the surface before pouring the metal into a mold or casting flask.
Dross, as a solid, is distinguished from slag, which is a liquid. Dross product is not entirely waste material; aluminium dross, for example, is used in secondary steelmaking for slag deoxidation, reducing processing costs by minimizing use of pure aluminum.
Dross is a wasteful material. In the electronic wave soldering process, dross can account for over 50% of the metal required. With the advent of lead-free solders, the cost of replacing metal lost to dross has become unacceptably high. Dross also reduces the expected quality of the solder joint as measured in defects per million opportunities (DPMO). With the advent of the lead-free soldering process, the cost of the new alloys are so high that accepting dross in the process has become a significant cost issue.