Aleksander Archipenko (1887-1964) was born in Kiev, Ukraine. He attended the Kiev Art School as well as participating in an artist's colony in Paris, and eventually established his own art school in Kiev. He came to the US in 1923, and remained there for the rest of his life. Archipenko was a sculptor, grouped under the "cubist" movement. Many of his pieces consist of multiple media, and part of what makes his sculptures so interesting is his use of negative space and concavity.
One of his most famous pieces, "Woman Combing Her Hair" (1915) resides at MoMA. This piece is one of the best examples of his concavity utilized in full effect. The woman's face only exists as negative space, carved out of her hair. One of her breasts and both thighs are sculpted inward, as well as her shins. As MoMA notes, "protruding elements seems to recede and internal features to advance," resulting in a truly unique method of representation.
"Seated Female Nude" 1909-1911, The Hirshhorn
Archipenko began using mixed media in what eventually became known as "sculpto-painting." For the most part, he used clay and wood and then painted on top in acrylic. Below are some examples found at MoMA and The Guggenheim.