use of drying oils as painting media was coming to be known,
and both on plaster and on wood these were to some extent
employed through the later medieval period, though without
seriously challenging the supremacy of tempera. From the
beginning of the 15th century, however, oil painting rose rapidly
in estimation, and from the end of that century to our own
time it has practically dominated the art. Wall-painting in
fresco continued to be practised till the last part of the i8th
century, and has been revived and supplemented by various
other monumental processes in the 19th, but even for mural
work the oO medium has proved itself a convenient substitute.
Water-colour painting in its present form is essentially an art
of the last hundred years. The old tempera processes have been
partly revived in our own time for picture-painting, but the
chief modern use of tempera is in scene-painting, where it is
more commonly called " distemper."
§ 34. Paintuig with Coloured Vitreous Pastes. — There is no single work that deals with the whole subject of this material and its different uses in transparent or opaque form in the arts, but details will be found in the special articles where these uses are described. (See Ceramics; Mosaic; Enamel; Glass, STAINED.) On the subject of the substances and processes employed in the colouring of the various vitreous pastes information will be found in H. H. Cunynghame's Art Enamelling on Metals (2nd ed., London, 1906, ch. vi.), but the subject is a large and highly technical one.
Coloured vitreous pastes are among the most valuable materials at the command of the decorative artists, and are employed in numerous techniques, as for example for the glazes of ceramic products including wall or iloor tiles; for painted glass windows; for glass mosaic, and for all kinds of work in enamels. The vitreous paste is tinged in the mass with various metallic oxides, one of the finest colours being a ruby red obtained from gold. Silver gives yellow, copper a blue green, cobalt blue, chromium green, nickel brown, manganese violet, and so on. Tin in any form has the curious property of making the vitreous paste opaque. It should be understood that though the vitreous substance and the metalHc o.xides are essentially the same in all these processes, yet the preparation of the coloured pastes has to be speciaUy conditioned in accordance with the particular technique in view. There are generally various ways of producing reds and blues and greens, &c., from oxides of different metals. The material is generally lustrous, and it admits of a great variety in colours, some of which are highly saturated and beautiful. It is on the lustre and colour of the substance, rather than on the pictorial designs that can be produced by its aid, that its artistic value depends; but though this imphes that it comes under the heading " Ornament " rather than " Painting, " yet in certain forms and at particular periods it has been the chief medium for the production of pictorial results, and must accordingly have here a brief notice.
The difference between opaque and transparent coloured glass is the basis of a division among the arts that employ the material. If it be kept transparent the finest possible effect is obtained in the stained-glass window, where the colours are seen by transmitted light. The stained-glass window came into general use in the early Gothic period, and was a substitute for the wall-paintings which had been common in the Romanesque churches of the nth and 12th centuries. Hence it is a form, and a very sumptuous and beautiful form, of the art of mural painting, representing that art in the later medieval buildings north of the Alps. In Italy, where the practice of wall-painting continued without a break from early medieval to Renaissance times, the stained-glass window was not a national form of art.
The most effective use of opaque coloured vitreous pastes is in ceramics (pottery) and in glass mosaic. The terra-cotta plaque, or tile-painted with designs in glazes of the kind was, as we have seen (§ 7), one of the chief forms of exterior mural decoration in ancient Mesopotamia. The best existing examples were found not long ago on the site of the ancient Susa ( Shushan the palace " of Scripture) and are now in the Louvre. Human
figures, animals, and ornaments, are represented not only in lively colours but also in relief; that is to say, each separate glaze brick had its surface, measuring about 12 in. by 9 in., modelled as well as painted for the exact place it had to occupy in the design. On these bricks there are formed small ridges in relief intended to keep the different liquid glazes apart before they were fixed by vitrifaction in the kiln. Chemical analysis has shown that the yellow colour is an antimoniat of lead, the white is oxide of tin, similar to the well-known opaque white glaze used by the Delia Robbia in Italy, the blues and greens are probably oxides of copper, the red a sub-oxide of copper (Semper, Der Stil, i. 332). This same region of the world has remained through all time a great centre for the production of coloured glazed tiles, but the use of " Persian, " " Moresque, " and other decorated plaques has been more ornamental than pictorial.
Glazed pottery only comes occasionally within the survey of the historian of painting. It does so in ancient Greece, because the earlier stages of the development of Greek painting can only be followed in this material; it does so, too, in a sense, in Italian faience and in some Oriental products, but these hardly fall within our view. The Greek vase was covered with a black glaze of extreme thinness and hardness, the composition of which is not known. Figure designs were painted in this on the natural clay of the vessel (see fig. 3, Plate IV.), or it was used for a background, the design being left the colour of the clay. Other colours, especially a red (oxide of iron) and white, were also employed to diversify the design and emphasize details, and these were also fixed by firing. A special kind of Greek vase was the so-called " polychrome lekuthos, " a small upright vessel, the clay of which was covered with a white " slip " on which figure designs were painted in lively tints. The technique is not quite understood, but the colours were certainly fired. There is an article on " The Technical History of White Lecythi " in the American Journal of Archaeology for 1907; the processes are not, however, analysed.
In glass mosaic thin sohd slabs of coloured vitreous pastes are broken up into little cubes of | in. to 5 in. in size and set in some suitable cement. The artist works from a coloured drawing and selects his cubes accordingly. Any number of shades of all hues can be obtained, and the modern mosaic workers of Italy boast that they dispose of some 25,000 different tints. As it is of the essence of the work to be simple and monumental in effect, a limited palette is all that is needed; and the mosaics recently executed in St Paul's in London are done in about thirty colours. The worker should have at hand appliances to cut to shape any particular cube wanted for a special detail.
The ancients used the art, and the finest existing ancient picture is in a mosaic, not indeed of glass pastes, but of coloured marbles. This is the famous " Battle of Issus " found at Pompeii. Glass mosaic came in under the early Roman Empire, but its chief use was in early Christian times, when it was the chief material for mural decoration of a pictorial kind. Ravenna is the place where this form of painting is most instructively represented, and the 5th and 6th centuries a.d. are the times of its greatest glory. At Rome and Constantinople there is fine early work, while that at Venice and Palermo is later. In the earliest and best examples the design is very simple, and a few monumental forms of epic dignity, against a flat background commonly of dark blue, represents the persons and scenes of the sacred narratives. The effect of colour is always sumptuous. Gold, especially for the backgrounds, is in later work freely employed.
The subject of enamel work forms the theme of a separate article. Here it need only be said that pictures can be produced by painting on a ground, generaOy of metal, with coloured vitreous pastes that are afterwards fixed by fusing. Limoges in France has been the great centre of the art, but enamelling loses in artistic value when a too exclusively pictorial result is aimed at.
§ 35. Fresco Painting. — Vitru'ius (De Architectura, bk. vii. chs. 2, 3; age of Augustus), Mount Athos Handbook (Hermeneia, chs. 54 seq.; date uncertain but based on early tradition); Cennino
