A novel type of a furnace for Vertical-Gradient-Freeze growth VGF of semiconductors is introduced. The basic element - a silica aerogel crucible - allows us to detect the crystallisation front with a suitable IR-CCD-camera due to its transparency. The growth velocity and the temperature gradient ahead of the solid-liquid interface are directly obtained in an optical way from the experiment. This is demonstrated for the growth of InSb. The excellent thermal insulation properties of the aerogels lead to a nearly one-dimensional temperature field and a nearly planar crystallisation front.