At the same time his palette grew darker; he even produced a series of black pictures painted in oil and enamel paint on unprimed cotton duck canvases. (visual-arts-cork.com)
Rail: Another thing I noticed about this group of paintings in the show was that they were painted on beautiful primed linen, the opposite of the raw cotton duck canvas which allows for the natural absorption of the paint's liquidity as it often is associated with color field paintings by Helen Frankenthaler, Morris Lewis, and Kenneth Noland, among others. (brooklynrail.org)
Staining acrylic paint into the fabric of cotton duck canvas was more benign and less damaging to the fabric of the canvas than the use of oil paint. (en.wikipedia.org)