The surface on which the artwork is applied. Some materials like paper are naturally ready for use, but many materials, like wood, cement, and canvas require preparation in various ways. That might be as simple as 2 or 3 coats of acrylic gesso, although rougher surfaces need more work before painting. The first cout may be applied just as a sealer for porous surfaces with subsequent coats meant to modify color and texture. Tooth may be provided by materials like calcite mixed into the gesso. The ground may need to be smooth for certain techniques, or textured for others. Many oil painting grounds are colored in ochre or other colors of a mid tone. More transparent paints including acrylics benefit from a pure white ground.