The next period at which we know from a contemporary writer what vehicles were used, is the end of the fourteenth century, or the beginning of the fifteenth, when Cennino wrote; but he professes to give the exact process of Giotto, a century earlier, being himself very old when he composed his work, and having been the apprentice of Taddeo Gaddi, the immediate pupil, assistant, and, in some particulars, the rival, of Giotto. The usual vehicle or _tempora_ appears to have been the whole egg beat up with a gill of pure water to each egg, and mixed with the milky juice of the fig-tree, where it was procurable. Several colours, however, could not be used with this ordinary vehicle, because of the yellow colour of the yolk, which turned the blues green, and injured some other pigments. In that case, the white of egg clarified was used, or fine size made of the clippings of parchment, or even flour paste well, but not too much, boiled. A vehicle of the yolk of egg alone, for such colours as were not injured by the yellow, was found to answer equally well in fresco, in distemper, and on panel. Though Cennino knew, and perhaps occasionally practised painting in oil, it is evident that the oil was used by him and his masters chiefly as a varnish. He directs it to be prepared nearly according to the recipe of the monk Theophilus; the difference being, that the oil is to be simmered till one half is evaporated, and the pure resin is to be added, in the proportion of an ounce to every pound of the raw linseed oil. Armenino, in A. D. 1600, repeats nearly all Cennino says of vehicles; he adds several compositions, one of which only I shall notice, because he tells us that he had heard from the scholars of Corregio, that it was used by that great man. A varnish composed of the purest turpentine[273], made hot, to which was added an equal measure of petroleum, was spread over the picture, previously warmed in the sun or otherwise. This is said to have been thin, lucid, and durable. As to modern vehicles, there is no new oil discovered by chemists that has not been tried, nor any combinations of gums and resins, with oils, whether fixed or essential. The desire of quickly drying substances has also produced a variety of vehicles and varnishes, all of which, in particular cases, and for certain purposes, seem to have answered. But their use has disappointed the artist in others. Perhaps so great a variety by tempting to injudicious mixtures, may have caused the partial failure. This is a question, however, for practical artists; my business is only to relate historically what has been done, not to comment on what is actually doing, or should be done.