Koolen has rarely dealt with sculpture. He has made busts in clay, possibly as 3d studies for portrait paintings. In the attic - in the fifties - was a sculptor's stand and some unbaked clay busts that have been lost. A portrait of his daughter Miriam (1940) still exists.
"Madonna and Child" (ca. 1947) is a gracefully styled but also the only sculpture that has been preserved. This Madonna was always shown in a tabernacle cabinet in the living room of Terworm.
Madonna and Child, ca. 1947, terracotta, 48.5 cm
Bust of Miriam, 1940, terracotta