"Impasto" refers to the heavy application of paint that allows the making of it - ie the brush strokes, the trowel marks etc - to be seen. It makes no attempt to look smooth. It can also be achieved using mediums applied to the surface which are then painted over in Oil or Acrylic paint, or by adding foreign matter to the canvas.
Van Gogh used paint thickly with a quick brush or palette knife stroke, which resulted in an impasto effect. To my mind, this is the only reason to have impasto - as a necessary consequence of your painting process or the result of the energy applied to the canvas. This effect often results from painting in layers - ie through the hard work of applying, scraping, re-applying paint, blocking out, etc which forms the "art" in the work. When resulting from the struggle of painting,impasto looks "authentic" and intriguing and is usually noticed after the composition, colours, content of the painting - as an added bonus, if you like.
Alternatively, you can work like Jackson Pollock (and a number of the American Abstract expressionists and subsequent followers) who used dirt, sand, broken glass and other found matter to "beef up" the paint they used.
There is also readily available a medium (many different brands) which can be used with both oil and acrylic paint, called (funnily enough) "impasto". This is applied first, to build up the surface and then painted over. If you want the van Gogh effect, then you need to paint with a brush stroke over the impasto gel, after the thickness has been formed and in the direction you would like it to appear in the final painting, so that it looks authentic. Otherwise, it can be built up and/or moulded (usually with a palette knife) to form any surface pattern or shape you would like, and then painted over in multiple layers, if you prefer.
A wash of paint over this surface roughly formed can give an interesting effect - some of the paint settles in thicker pools than others and it can add an antique effect or look like buildings in the distance (as in an early painting of mine, above).
Do you have other ways to form impasto? Was this helpful? Post your comment here.