PW 21 – barium sulfate

Used as a safe alternative to lead white from the 19th century. Quite transparent, it is often used as a filler for other pigments.

Manufacturer/ product code:  minerals-water / CAS no: 7727-43-7
Chemical Description:  barium sulfate
Color: white
Color Index: PW 21
Date: from early 19th century

PW 21 – Barium Sulfate is an inorganic white pigment composed of the naturally occurring mineral barite (BaSO₄). It is valued primarily as an extender, filler, or matting agent rather than a high-opacity pigment, due to its low refractive index and relatively weak hiding power. PW 21 is chemically inert, non-toxic, and highly stable, making it suitable for a wide range of artistic, industrial, and commercial applications.

Chemically, barium sulfate is extremely resistant to light, heat, and most chemical agents. It is insoluble in water and organic solvents, which ensures excellent permanence in paints, coatings, plastics, and paper. Its fine particle size and white color allow it to be used to modify paint consistency, improve surface smoothness, and reduce the cost of high-performance pigments without significantly affecting the overall hue.

In practical applications, PW 21 is often mixed with other white pigments, such as titanium or zinc whites, to improve rheology, extend volume, and reduce chalkiness in the final film. While it contributes minimal opacity, it does enhance paint stability and texture. In artists’ materials, it has been used historically as a filler in oil paints, tempera, and pastels, as well as in industrial paints and coatings.

From an analytical and conservation standpoint, PW 21 is highly inert, leaving minimal chemical impact on adjacent pigments and media. Its presence in historical works often indicates cost-saving strategies or material modification rather than intentional chromatic effect.