An opacifier is a substance added to a material in order to make the ensuing system opaque. An example of a chemical opacifier is tin dioxide (SnO2), which is used to opacify ceramic glazes[1] and milk glass; bone ash is also used.
Opacifiers must have a refractive index (RI) substantially different from the system. Conversely, clarity may be achieved in a system by choosing components with very similar refractive indices.[2]
Opacifiers must also form small particles in the system. Opacifiers are generally inert.
Sometimes opacifiers are added to medical implants to make them visible under X-ray imaging. This is especially true in the case of most polymers which are often unrecognizable in the body when viewed using X-rays.
In solid rocket propellants and some translucent smokeless powders, the primary method of heat transfer into the propellant grain from the combustion process is by radiation, and opacifiers such as "lamp black" may be added to the propellant mixture to ensure the heat does not penetrate far below the surface of the grain, which could cause detonation. The opacifiers also prevent sub-surface overheating and localized premature ignition in the grains where imperfections absorbing the thermal radiation are present. Carbon black is commonly used for this purpose; other possible additives are nigrosin, Prussian blue, methylene blue, etc. in amounts ranging commonly between 0.1-0.5%.[3]