Chrome yellow is a yellow pigment found in paints containing monoclinic lead(II) chromate (PbCrO4). It occurs naturally as the mineral crocoite, but the mineral itself has never been used as a paint pigment. After the French chemist Louis Vauquelin discovered the new element chromium in 1797, lead chromate was synthesized in the laboratory and used as a pigment starting in the second decade of the 19th century. Chrome yellow was commonly produced by mixing solutions of lead nitrate and potassium chromate and filtering the lead chromate precipitate. The pigment tends to react with hydrogen sulfide and darkens over time in air to form lead sulfide and contains lead, a toxic heavy metal, and chromate, which is toxic and carcinogenic. For these reasons it was replaced with another pigment, cadmium yellow (mixed with enough cadmium orange to produce a color equivalent to chrome yellow). Darkening can also be achieved by reduction with sulfur dioxide. The high quality pigments have been coated to avoid contact with gases which can change colour. Cadmium pigments are increasingly being replaced by organic pigments such as arylides (Pigment Yellow 65) and isoindoles (PY 110). After working with the natural mineral crocoite, Nicolas Louis Vauqelin was the first artist to encourage the use of chrome yellow as a pigment in 1804. The first recorded use of chrome yellow as a color name in English was in 1818. In 1888, artists from avant-garde artists such as Vincent Van Gogh used chrome yellow together with the other two primary colors. The Piper J-3 Cub aircraft had Chrome Yellow as its standard overall color, usually referred to as “Cub Yellow” or “Lock Haven Yellow” in aviation circles, from the Piper’s Lock Haven, Pennsylvania factory where it was manufactured in the 1930s and during World War II.