What is the technique of applying paint thickly to canvas?

The technique of applying paint thickly to canvas is called impasto.

In impasto, paint is laid on heavily enough that brushstrokes, palette-knife marks, or ridges remain visible after drying. The raised surface changes how light reflects from the image and can make painted forms appear almost sculptural. The word comes from the Italian impastare, meaning “to knead” or “to paste,” and is related to pasta, meaning paste.

Oil paint has traditionally been especially suited to impasto because its dense consistency and slow drying time allow artists to build substantial texture. Acrylic paint can achieve a similar effect when mixed with heavy-body gels. Watercolour and tempera are generally too thin unless a thickening medium is added.

Impasto is not the same as alla prima, which means painting wet-on-wet in a single session; scumbling, which uses a thin, broken layer of lighter paint; or glazing, which applies transparent layers. Rembrandt, Titian, Vincent van Gogh, Jackson Pollock, and Willem de Kooning all used impasto for texture, emphasis, or expressive brushwork.