The phrase "collage pieces" refers to various materials or objects that are cut, torn, or arranged together to create a mixed-media artwork or composition. Full definition
He hand paints and then collages pieces of paper onto the surface of his established image... In the process he moves away from observation and works with the materials and the surface. (painters-table.com)
I had a similar process for the animation, which included cutting leftover collage pieces and scanning them to make them move. (bombmagazine.org)
A German artist who works primarily in the medium of drawing, Jorinde Voigt returns for her third solo show at the gallery with a series of recent mixed media works on paper paired with collage pieces from 2012. (davidnolangallery.com)