What medium did Georgia O'Keeffe use for her 1916 series Blue Lines?

Georgia O'Keeffe used watercolor for her 1916 series Blue Lines.

Blue Lines belongs to the group of early abstract works that established O'Keeffe’s distinctive approach to simplified form and color. Watercolor allowed her to work quickly and preserve transparent washes, making it especially useful for experiments with flowing lines and layered blue shapes.

O'Keeffe had already made important charcoal abstractions in 1915, but her early watercolors expanded the possibilities of her nonrepresentational language. These works were made before the large flower paintings for which she later became famous.

Her medium is sometimes misidentified as oil because many of her best-known mature paintings are oils on canvas. O'Keeffe worked in several media across her career, including watercolor, charcoal, pastel, and oil. Blue Lines specifically refers to watercolor experiments from her early modernist period.