Van Gogh has used a narrow brush and blue paint to indicate the outlines of the skull. Thin paint alternates with thick: for the light reflections on the skull he has used heavy splodges of light-coloured paint.
Drying cracks are visible in many places. Look at the area immediately below the skull, for example. The paint has cracked because Van Gogh painted the skull over an earlier picture. He covered up the earlier image with a layer of zinc white – a kind of paint that dries extremely slowly. The drying out process continued after the skull had been painted over it, causing cracks to form in the top layer of paint.
