If you've seen an artwork by Vincent van Gogh, you might have noticed one thing: impasto. Here's how you can identify it. Look closely for thick brushstrokes of layered paint. They often stick out from the surface of the painting, giving it a tactile quality. In honor of Vincent van Gogh's birthday, we declare it #IrisesWeek. We'll be celebrating with all things Irises. Image: Irises, 1889, Vincent van Gogh. Getty Museum