Impasto
Impasto is paint laid on so thick that brush and knife marks stand up from the canvas. Who used it, why it changes with the light, and why it's so slow to dry.
What is impasto?
Impasto is paint applied so thickly that it keeps the shape of the brush or palette knife and stands up from the surface. The ridges, peaks and grooves become part of the picture, along with the color.
The word is Italian, from impastare, to knead into a paste or dough. It's mostly used for oil and acrylic, the two paints that are stiff enough to hold a ridge. Watercolor and ink have no body, so they can't do it.
From Titian to Van Gogh
The Venetians Titian and Tintoretto were among the first to build up thick paint in the 1500s, mostly in highlights such as a gleam on armor or a fold of white cloth. Rubens did the same a generation later. Rembrandt pushed it further in his late work, where the gold sleeve of The Jewish Bride is paint so thick it looks laid on with a knife.
Van Gogh used impasto everywhere, not only in the lights. In The Starry Night every swirl of sky is a separate rope of paint, which is why reproductions look flatter and duller than the canvas in New York. In the mid-twentieth century Frank Auerbach and Leon Kossoff in London scraped and rebuilt their portraits so often that some of the paint stands well off the board.
Why does impasto change with the light?
A ridge of paint has a lit side and a shaded side, and it throws a small shadow of its own. When light comes from low on one side, as it does from a window, those tiny shadows add texture that isn't in the color at all. Turn the light around and the texture changes.
Conservators use this on purpose. Photographing a painting under raking light, from a low angle, shows every stroke and every crack in relief. It's also why museum photos are usually lit flat from the front, which hides most of the impasto you'd see in the room.
How long does thick paint take to dry?
Oil paint doesn't dry by evaporation. The oil reacts with oxygen from the air and hardens, starting at the surface. A thin layer is dry to the touch in days, but a thick ridge forms a skin while the paint underneath can stay soft for months, and full curing can take years.
That causes trouble if a painter puts lean, fast-drying paint over thick, oily paint. The top layer hardens first and cracks or wrinkles as the soft layer below keeps moving, which is where the studio rule of painting fat over lean comes from. Modern painters often thicken acrylic with gel mediums instead, which dry in hours.
Getting the impasto look from a photo
The Oil Painting Effect has a preset called Impasto that uses a bigger, sharper brush than the default, so the picture breaks into broad, flat slabs of color. Raising Stretch to around 2 drags those slabs along the edges of the picture, closer to a knife pulled through wet paint.
The filter paints color, so it can't raise real ridges or cast the small shadows described above. For the most convincing result, pick a photo with strong side light, where the lit and shaded sides of things already do some of that work.
