Paint in which drying oils are used as the medium; linseed oil is the best known, but others that have been used in painting include poppy oil and walnut oil. It was long believed that oil painting was invented by Jan van Eyck in the early 15th century, but it is now known that its origins are older and obscurer (the treatise of Theophilus, written probably in the 12th century, describes ‘grinding colours with oil’). There is no doubt, however, that van Eyck revolutionized the technique and brought it to a sudden peak of perfection. He showed the medium's flexibility, its rich and dense colour, its wide range from light to dark, and its ability to achieve both minute detail and subtle blending of tones. Other painters soon took up his innovations—first in northern Europe, then in Italy—and over the next century oil progressively superseded tempera as the standard medium for serious painting (other than for murals, in which fresco continued to be the norm). During the period of transition, oil was often combined with tempera. Giovanni Bellini, for example, began his long career using tempera exclusively and ended it using oil exclusively, but in much of his work he seems to have combined the two methods, typically beginning a picture in the older technique and completing it in the newer. His portrait of Doge Leonardo Loredan (c.1501–4, NG, London) is one of the earliest examples of the use of deliberately rough oil paint to convey texture: in the doge's ornate costume he suggests light catching the gold thread in a way that would be impossible in tempera. This kind of handling was taken much further by Bellini's pupil Titian, who was the first artist to fully exploit the rich textural qualities of oil paint, giving the surfaces of his pictures an expressive life of their own. The revolution he wrought in technique was bound up with his increasing preference for canvas in place of wooden panels: in his later work he often used fairly coarse types, in which the rough grain shows through the brushwork and is part of the surface texture of the picture. This ability to show an artist's personal ‘handwriting’ has been a major factor in the long dominance of oil paint: it can attain any variety of surface from porcelain smoothness to violent impasto. Its versatility was increased still further in the 19th century with the invention of the collapsible metal tube (devised in 1841), which made it convenient to work out of doors. In the 20th century, however, acrylic became a serious rival to oil paint.