These additional media can aid the painter in adjusting the translucency of the paint, the sheen of the paint, the density or 'body' of the paint, and the ability of the paint to hold or conceal the brushstroke. These aspects of the paint are closely related to the expressive capacity of oil paint. Traditionally, paint was transferred to the painting surface using paintbrushes, but there are other methods, including using palette knives and rags. Oil paint remains wet longer than many other types of artists' materials, enabling the artist to change the color, texture or form of the figure. At times, the painter might even remove an entire layer of paint and begin anew. This can be done with a rag and some turpentine for a time while the paint is wet, but after a while the hardened layer must be scraped. Oil paint dries by oxidation, not evaporation, and is usually dry to the touch within a span of two weeks (some colors dry within days). It is generally dry enough to be varnished in six months to a year. The earliest discovered oil paintings date back to approx. 650AD in Afghanistan. These murals were presumably created by buddhist artists traveling along the silk road. These early oil works display a wide range of pigments and binders, and even included the use of a final varnish layer. This refinement of this painting technique and the survival of the paintings into the present day suggests that oil paints had been used in Asia even before the 7th century. It is possible, considering the history of tempera (pigment mixed with either egg whites or egg yolks, then painted on a plastered section) that oils were discovered in Europe independently around the 15th century. Outdoor surfaces and surfaces like shields—both those used in tournaments and those hung as decorations—were more durable when painted in oil-based media than when painted in the traditional tempera paints. Most European Renaissance sources, in particular Vasari, credited northern European painters of the 15th century, and Jan van Eyck in particular, with the "invention" of painting with oil media on wood panel supports ("support" is the technical term for the underlying backing of a painting). However, Theophilus (Roger of Helmarshausen?) clearly gives instructions for oil-based painting in his treatise, "On Various Arts", written in 1125. At this period, it was probably used for painting sculptures, carvings and wood fittings, perhaps especially for outdoor use.