What is a painting technique where the lighter forms emerge from a very dark ground?

The painting technique you're referring to is known as "chiaroscuro." This approach involves using strong contrasts between light and dark to create a sense of volume and three-dimensionality in the artwork. By starting with a dark ground and layering lighter tones on top, artists can achieve dramatic effects and highlight specific forms or figures, enhancing the overall depth and mood of the painting. Chiaroscuro has been famously employed by artists such as Caravaggio and Rembrandt.