enables differentiation among degrees of reflective light, from shadow to bright beams, and minute depictions of light effects through the use of transparent glazes. This new freedom in controlling light effects gave rise to more precise and realistic depictions of surface textures; van Eyck and van der Weyden typically show light falling on surfaces such as jewellery, wooden floors, textiles and household objects. The paintings were most often made on wood, but sometimes on the less expensive canvas. The wood was usually oak, often imported from the Baltic region, with the preference for radially cut boards which are less likely to warp. Typically the sap was removed and the board well-seasoned before use. Wood supports allow for dendrochronological dating, and the particular use of Baltic oak gives clues as to the artist's location. The panels generally show very high degrees of craftsmanship. Lorne Campbell notes that most are "beautifully made and finished objects. It can be extremely difficult to find the joins". Many paintings' frames were altered, repainted or gilded in the 18th and early 19th centuries when it was common practice to break apart hinged Netherlandish pieces so they could be sold as genre pieces. Many surviving panels are painted on both sides or with the reverse bearing family emblems, crests or ancillary outline sketches. In the case of single panels, the markings on the reverse are often wholly unrelated to the obverse and may be later additions, or as Campbell speculates, "done for the artist's amusement". Painting each side of a panel was practical since it prevented the wood from warping. Usually the frames of hinged works were constructed before the individual panels were worked on. Glue binder was often used as an inexpensive alternative to oil. Many works using this medium were produced but few survive today because of the delicateness of the linen cloth and the solubility of the hide glue from which the binder was derived. Well known and relatively well preserved – though substantially damaged – examples include Matsys' Virgin and Child with Saints Barbara and Catherine (c. 1415–25) and Bouts' Entombment (c. 1440–55). The paint was generally applied with brushes or sometimes with thin sticks or brush handles. The artists often softened the contours of shadows with their fingers, at times to blot or reduce the glaze. Guilds and workshops The most usual way in the 15th century for a patron to commission