Article by: Kelly Dobos
Iron oxides are the main pigments used for matching skin tones in foundations, powders, concealers, and other make up for the face. Iron oxides come in a variety of shades from yellow to red and black. Various shades of brown iron oxides are also available but these are merely mixtures of the three previously mentioned colors. Sometimes referred to as natural mineral pigments, all iron oxides used in cosmetics are synthetic per the United States FDA Code of Federal Regulations (U.S. FDA – Listed in CFR 21 §73.2250). Because the chemical inputs in the synthesis can standardized and controlled, the iron oxides have a predictable safety profile.
Yellow iron oxide is Fe2O3•H2O. The red iron oxide (Fe2O3) is produced by calcination, heat treatment in the absence or limited supply of air or oxygen to bring about decomposition, of yellow iron oxide at about 800°C, so it is very heat stable in the relative temperature range where cosmetic products are produced. Black iron oxide, Fe3O4, is a mixture of ferric and ferrous oxides.
Stability and Compatibility
Iron oxides are opaque and exhibit with excellent light stability. They stable in acidic and alkali environments but yellow and black iron oxides are sensitive to high temperatures. Exposure to temperatures as low as 125 – 150°C can cause the color to visibly shift as the yellow will lose some it water of hydration. They are very opaque and exhibit excellent light stability. The yellow, however, will shed some of its water of hydration at temperatures as low as 125 – 150°C, causing some shift in color that can be seen. Like the yellow, black iron oxide will also shift color, becoming redder, at temperatures of 125 – 150°C. It is also important to know that iron oxide is magnetic and will coat iron or mild steel containers so care should be taken in selecting the proper vessels for scale up and production.