The technique of painting on a dry plaster surface is called

Fresco (plural frescos or frescoes) is a technique of mural painting executed upon freshly laid, or wet lime plaster. Water is used as the vehicle for the dry-powder pigment to merge with the plaster, and with the setting of the plaster, the painting becomes an integral part of the wall.

What it's saying is that colored pieces of canvases is a piece of art

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