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Rh also by mingling with them chemical substances known as " dryers, " of which certain salts of lead and zinc are the most familiar. How far back in antiquity such oils and their properties were known is doubtful. Certain varnishes are used in Egypt on mummy cases of the New Empire and on other surfaces, and, though some of these are soluble in water, others resist it, and may be made with drying oils or essential oils, though the art of distilling these last cannot be traced back in Egypt earlier than the Roman imperial period. (See Berthelot, La Chimie au moyen dge, i. 138 (Paris, 1893). When Phny tells us (.iv. 123) that all resins are soluble in oil, we might think he was contemplating a varnish of the modern kind. Elsewhere, however (xxiv. 34), he prescribes such a solution as a sort of emoUient ointment for wounds, so it is clear that the oil he has in view is non-drying olive oil that would not make a varnish. In two passages of his Natural History (xv. 24-32, xxiii. 79-96) Pliny discourses at length on various oils, but does not refer to their drying properties. There is really no direct evidence of the use among the Greeks and Romans of drying oils and oil varnishes, though a recent writer (Cremer, U titer suchun gen tiber den Beginn der Oelmalerei, Diiss., 1899) has searched for it with desperate eagerness. The chief purpose of painting for which such materials would have been in demand is the painting of ships, but this we know was carried out in the equaUy waterproof medium of wax, with which resin or pitch was commingled by heat. The earliest mention of the use of a drying oil in a process connected with painting is in the medical writer Aetius. of the beginning of the 6th century a.d., who says that nut oil dries and forms a protective varnish over gilding or en caustic painting. From this time onwards the use of drying oils and varnishes in painting processes is well established. The Lucca MS. of the 8th or 9th century a.d. gives a receipe for a transparent varnish composed of linseed Oil and resin. In the Mount Athos Handbook " peseri, "or boiled linseed oil, appears in common use, and with resin is made into a varnish. In the same treatise also we find a clear description of oil painting in the modern sense; but since the dates of the various portions of the Handbook are uncertain, we may refer rather to Theophilus (about a.d. hoc), who indicates the same process with equal clearness. The passages in Theophilus (i. chs. XX. and xxvi.-xxviii.) are of the first importance for the history of oil painting. He directs the artificer to take the colours he wishes to apply, to grind them carefully without water in oil of linseed prepared as he describes in ch. xx., and to paint therewith flesh and drapery, beasts or birds or foliage, just as he pleases. All kinds of pigments can be ground in the oil and used on wooden panels, for the work must be put out in the sun to dry. It is noteworthy that Theophilus (ch. xxvii.) seems to confine this method of painting to movable works (on panel, in opere ligneo, in his lanltmi rebus quae sole siccari possunl) that can be carried out into the sun, but in ch. xxv. of the more or less contemporary third book of Heraclius (Vienna Quellenschriften, No. iv.) oil-paint may be dried either in the sun or by artificial heat. Heraclius, moreover, knows how to mix dryers (oxide of lead) with his oil, a device with which rheophOus is not acquainted. Hence to the latter the defect ii the medium was its slow drying, and Theophilus recommends as a quicker process the gum tempera already described. In any case, whether the painting be in oil or tempera, the finished panel must be varnished in the sun with " vernition " (ch. xxi.), a varnish compounded by heat of linseed oil and a gum, which is probably sandarac resin. The Mount Athos Handbook, § 53, describes practically the same technique, but indicates it as specially used for flesh, the inference being that the draperies were painted in tempera or with wax. It is worth noting that the well-known " black Madonnas, " common in Italy as well as in the lands of the Greek Church, may be thus explained. They are Byzantine icons in which the flesh has been painted in oil and the draperies in another technique. The oil has darkened with age, while the tempera parts have remained in contrast comparatively fresh. Some of them are probably the earliest oil paintings extant.

Oil painting accordingly, though in an unsatisfactory form.

is established at least as early as a.d. hoo. What had been its previous history? Here it is necessary to take note of the interesting suggestion of Berger, that it was gradually evolved in the early Christian centuries from the then dechning en caustic technique of classical times. We learn from Dioscorides, who dates rather later than the time of Augustus, that resin was mixed with wax for the painting of ships, and when drying oils came into use they would make with wax and resin a medium requiring less heat to make it fluid than wax alone, and one therefore more convenient for the brush-form of en caustic. Berger suspects the presence of such a medium in some of the mummy-case portraits, and points for confirmation to the chemical analysis of some pigments found in the grave of a painter at Heme St Hubert in Belgium of about the time of Constantine the Great (i. and ii. 230 seq.). One part wax with two to three parts drying (nut) oil he finds by experiment a serviceable medium. Out of this changing wax-technique he thinks there proceeded the use of drying oils and resins as media in independence of wax. If we hesitate in the meantime to regard this as more than a hypothesis, it is yet worthy of attention, for any hypothesis that suggests a plausible connexion between phenomena the origin and relations of which are so obscure deserves a friendly reception.

The Trattato of Cennino Cennini represents two or three centuries of advance on the Schedula of Theophilus, and about contemporary with it is the so-called Strassburg MS., which gives a view of German practice just as the Trattato does of Itahan. This MS., attention to which was first caUed by Eastlake {Materials, i. 126 seq.), contains a remarkable recipe for preparing " oil for the colours." Linseed or hempseed or old nut oil is to be boiled with certain dryers, of which white copperas (sulphate of zinc) is one. This, when bleached in the sun, " will acquire a thick consistence, and also become as transparent as a fine crystal. And this oil dries very fast, and makes all colours beautifully clear and glossy besides. All painters are not acquainted with it: from its excellence it is called oleum preciosuni, since half an ounce is well worth a shilling, and with this oil all colours are to be ground and tempered, " while as a final process a few drops of varnish are to be added. The MS. probably dates rather before than after 1400.

Cennino's treatise, written a little later, gives avowedly the recipes and processes traditional in the school of Giotto throughout the 14th century. He begins his account of oil painting with the remark that it was an art much practised by the " Germans, " thus bearing out what was said at the commencement of this section. He proceeds (chs. 90-94) to describe an oil technique for walls and for panels that sounds quite effective and modern. Linseed oil is to be bleached in the sun and mixed with liquid varnish in the proportion of an ounce of varnish to a pound of oil, and in this medium all colours are to be ground. " When you would paint a drapery with the three gradations, " Cennino proceeds, " divide the tints and place them each in its position with your' brush of squirrel hair, fusing one colour with another so that the pigments are thickly laid. Then wait certain days, come again and see how the paint covers, and repaint where needful. And in this way paint flesh or anything you please, and likewise mountains, trees and anything else." In other chapters Cennino recommends certain portions of a painting in tempera to be put in in oO, and nowhere does he give a hint that the work in oU gave any trouble through its unwillingness to dry. His medium appears, however, to have been thick, and perhaps somewhat viscous (ch. 92). This combination of oil paint and tempera on the same piece is a matter, as we shall presently see, of some significance.

In the De re aedificatoria of L. B. Alberti (written about 1450), vi. 9, there is a mention of " a new discovery of laying on colours with oil of linseed so that they resist for ever all injuries from weather and climate, " which may have some reference to so-called " German " practice.

The next Italian writer w:ho says anything to the purpose is