in ancient days; probably the design was drawn on the wall, and
there were no cartoons. The master not only invented, but he
was the master-workman also, and that is how it should be.
The probability is that the custom of drawing the design upon
the wall practised by the early frescanti was the survival of a
method adopted by the mosaicists, just as their method repeated
that of Roman and Greek wall-painters. Of course this direct
method leads to a large style, a style harmonizing with environment,
scale, &c.; the tendency is to draw large in a large building,
to draw small in a small one. Anyhow, this is quite certain, that
all the fine Byzantine and 13th century mosaics, as well as wall
paintings, were executed in situ and not away, as was the usual
custom in England and elsewhere until recently.
Mr Harry Powell has permitted the writer to make use of some of his reflections upon the mosaicist’s art in the following notes. The mosaicist should not separate the artistic from the technical details of his craft. He must study not only the decorative effect, form, colour and spacing of his design, but the surface to be covered as well as the materials with which he builds.
Surface.—Good brick-work, the mortar joints slightly cut back, affords the best foundation for mosaic. The hollow and sharp-edged joints provide a key for the cement into which the cubes will be set, and they diminish the risk of sagging, a not uncommon event if the cement is not welded to the wall by being well pressed into the joints. If the mosaic is to be applied on stone, the stone must be notched and well roughened to provide support. Whether the surface is brick or stone, it must be well saturated with boiled oil to prevent suction, because if too much suction takes place the powder only of the cement will remain and the cubes will drop out.
Cement.—A cement suitable for mosaic is one which retains its tenacity, which can be applied in layers, which sets slowly, and which is not liable to change colour after long exposure. These conditions are best met by an oil cement. One consisting of equal weights of white oxide of zinc and carbonate of zinc, mixed with double boiled oil and containing small proportions of wax, gold size and slacked lime gives good results. This cement can either be white or red, white where greyness of tone is desirable, red where a richer effect is desirable. It is generally mixed with a small portion of oxide of iron or oxide of manganese, which prevents the whiteness of the joints from rendering adjacent tints grey from a distance.
Atmospheric Corrosion.—As the atmosphere of modern towns is more corrosive than that of medieval Venice or medieval Rome, it is important that, in choosing the cement and the materials to be imbedded in it, the mosaicist should be certain that they are impervious to atmospheric impurities.
Glass.—Although marble, mother of pearl, and other substances have been, and are still, occasionally used, the predominant material in ancient as well as modern mosaics is glass. When prepared with due regard to the continuing proportions of its ingredients, glass is impervious to the action of ordinary acids, and is practically indestructible. It can be made to assume almost every shade and tint of colour (see Glass). There are many kinds of glass, but for mosaic work either a potash-lead or a soda-lime glass is usually employed. Both of these glasses can be rendered opaque by mixing with the ingredients either oxide of tin or a mixture of felspar and fluorspar. Glass rendered opaque by the admixture of felspar and fluorspar has a bright, vitreous, easily cleaned surface, and readily develops brilliant colours.
Production of Colours.—Colours are obtained by mixing and melting with the ingredients of the opaque glass small proportions of certain metallic oxides. Oxide of chalk gives a purple blue; oxide of copper gives a peacock blue; oxide of copper with oxide of iron gives a green; oxide of copper mixed with oxide of iron and a strong reducing agent gives a red; oxide of chromium a green; oxide of nickel a purple; oxide of uranium a yellow; and oxide of manganese a violet—or a black if a larger quantity of oxide is used.
Manufacture of Glass Slabs.—The mixtures, in a state of powder, are shovelled into crucibles standing round the grate of a furnace, and when fusion is complete the viscous glass can be coiled upon the heated end of an iron rod and removed for use, very much in the way that thick treacle may be coiled round the bowl of a spoon. A mass of molten glass, thus collected, is allowed to fall upon a flat iron table, and is pressed into a slab about 6 in. square and 12 in. thick. The slabs are removed to an oven, where they are allowed to cool slowly, and when cool are removed and broken by a hammer or a miniature guillotine into tesserae or cubes The fractured edge of the tesserae is used for the surface of the mosaic.
Gold and Silver Slabs.—The tesserae containing gold or silver leaf are as impervious to surface corrosions from the effects of atmosphere as the solid colours. The process of manufacturing a gold or silver slab for mosaic work is to spread the metallic leaf on a very thin tray of transparent glass, about 5 in. in diameter, and after it has been heated to press upon the surface of the leaf a mass of molten glass, so as to create cohesion between the molten glass and the glass tray through the pores of the metallic leaf. he slabs thus formed contain gold, silver or platinum leaf hermetically imprisoned between two layers of glass. The slabs are cut up into tesserae or cubes by means of a diamond or glass-cutter’s wheel. Only one surface can be used for mosaic work.
Tinted Metals.—By using coloured glass for the thin glass trays which form the surface of the metallic slabs a variety of tinted metallic effects are obtained. Moreover, if the glass which is to form the background is coloured, and if the slab after it has been cooled is strongly reheated, the leaf becomes sufficiently disintegrated to allow the colour of the background to show through, with the result that the colour effect of the metallic leaf is modified.
Palette and Tools.—The palette of the mosaic worker is a shallow box with many partitions, each division containing different-coloured tesserae. The only tools required are clippers, for shaping the tesserae, and a pointed awl for pricking through the cartoon into the cement the outlines of the design. Although the process and tools are simple, it requires prolonged training of mind, hand, eye and fingers to enable a workman to create in mosaic a living representation as distinguished from a lifeless copy of the master craftsman’s design.
Drawing directly on the Wall. Curved Surfaces.—If the mosaicist desires to draw his cartoon directly upon the wall, a necessary procedure where curved surfaces are presented, he goes to work in the following manner. He causes a model to be made to scale of a dome, semi-dome or spandrel and upon it he draws his design with a brush in strong red pigment, having previously squared up the whole surface to scale. This done, he causes the dome, semi-dome or spandrel to be covered over with thick brown paper. This being attached to the wall with white lead sufficient only to give temporary adhesion, the brown paper is squared up to the scale of the small sketch; each square being relatively numbered. The master then sets his pupils to work to draw mechanically and copy accurately from the small design on to the full-sized dome, semi-dome or spandrel. This done, the master follows on, correcting with charcoal or brush until the whole design is developed in strong outline. Having made a slightly coloured sketch, the master with the aid of his pupils proceeds to mix all the tints in water-colour, adding colla di pesce or fish glue, and a little honey to prevent cracking. He then applies every tint separately, keeping each distinct, and above all minding that the local colours of all half-tints are different from the colour of all shadows. This done, he dips his brush in black and draws all the outlines, the thickness of which depends upon the distance which will intervene between his work and the spectator; in order that the black may not appear cold from a distance, he will add to one side of the line, a red line, thicker or thinner than the black, according to the effect he wishes to produce. It is sometimes effective to add upon the other side of the black line a green line, so that the purple effect of the black and red shall be modified.
Colour.—We now come to the great question of colour and how to obtain it simply, and so that from a distance a blurred and woolly effect is not obtained. There should be a marked and sharp definition between all tints; they should not be fused; they should look sharply defined, as the squares upon a chessboard, and appear crude and brutal. The work which looks least refined near at hand looks more finished at a distance. Red and blue lines alternately laid, either more red or more blue as the purple is intended to tend towards red or blue, make the best purple. Green is best made with yellow and blue lines, the masses being separated by red lines, and the shadows of green should be red or blue: if red, they should be outlined with blue; if blue, with red. Red should be treated flatly, shaded with a deeper red, which should be of a warmer tone than the lights. Blue should be shaded with blue or red; and it is well to mix green tesserae with the blue in the lights, and again green tesserae with the blue or red shades to modify crudity. Pure white should be very sparingly used: it expands greatly at a distance. The best white is that which is of the tone of Naples yellow. Whenever it is necessary to use pure white, either a yellow or pink line should be set on one side of it.
It is impossible to keep the flesh too simple. The local colour, i.e. a red orange, is the staple colour. Features should be drawn
