Vasari on technique

by Giorgio Vasari

[249]. This mosaic, called the ‘Navicella,’ represents the Gospel ship manned by Christ and the disciples, with Peter struggling in the waves. It has been so much restored that little if any of Giotto’s work remains in it. It was replaced in the seventeenth century, after some wanderings, in the porch of the present Basilica, but Vasari saw it of course in the porch of the old, or Constantinian, church, the entrance end of which was still standing in his day.

[250]. This mosaic was executed at the end of the fifteenth century by Domenico Ghirlandajo and his brother over the northern door of the nave of the cathedral of Florence. It is still in situ but has been greatly restored. The date 1490 is introduced in the composition.

[251]. This corresponds with modern practice. The following is from a paper by Mr James C. Powell, who, as practical worker in glass, has been engaged with Sir W. B. Richmond in the decoration in mosaic of the vaults of St Paul’s. ‘The glass which is rendered opaque by the addition of oxide of tin, is coloured as required by one of the metallic oxides; this is melted in crucibles placed in the furnace, and when sufficiently fused is ladled out in small quantities on to a metal table, and pressed into circular cakes about eight inches in diameter and from three-eighths to half an inch in thickness; these are then cooled gradually in a kiln, and when cold are ready for cracking up into tesserae, which can be further subdivided as the mosaicist requires. It is the fractured surface that is used in mosaic generally, as that has a pleasanter surface and a greater richness of colour; the thickness of the cake, therefore, regulates the limit of the size of the tesserae, and the fractured surface gives that roughness of texture which is so valuable from an artistic point of view.’ (Transactions, R.I.B.A., 1893–4, p. 249).

[252]. This is a point attended to by the best modern workers in mosaic. Where gold backgrounds are used it is advisable to carry the gold into the figures by using it as Vasari suggests for the lights on the draperies. If this were not done the figures would be liable to tell as dull masses against the more brilliant ground. The use of gold backgrounds is specially Byzantine. The earlier mosaics at Rome and at Ravenna have backgrounds of blue generally of a dark shade, which is particularly fine at Ss. Cosma e Damiano at Rome and in the tomb of Galla Placidia at Ravenna. The mosaics at S. Sophia at Constantinople of the sixth century had gold backgrounds, and this is the case also with all the later examples in Italy from the ninth and tenth centuries onwards. The finest displays of these varied fields of gold, now deep now lustrous of hue, are to be seen in S. Sophia, S. Marco at Venice, and the Cappella Palatina at Palermo.

Vasari’s account of the fabrication of the gilded tesserae required for this part of the work is quite clear and agrees with modern practice. The gold leaf is hermetically sealed between two sheets of glass by the fusion of a thin film over it. The technique of the ‘fondi d’ oro,’ or glass vessels adorned with designs in gold, found in the Roman catacombs, was of the same nature.

[253]. It has been noticed at some places, as at Torcello, that before the cubes were laid in the soft cement the whole design was washed in in colour on the surface of the cement. This facilitated correct setting and avoided any appearance of white cement squeezed up in the interstices between the cubes. On this particular feature of the mosaic technique Berger has founded an ingenious theory of the origin of painting in fresco. It is his thesis, in his Beiträge zur Entwicklungs-Geschichte der Maltechnik, I, München, 1904, that the ancients did not employ the fresco process, but that this was evolved in early mediaeval days out of the mosaic technique as seen, e.g., at Torcello. The stucco, that Vasari describes, must be put on portion by portion, for it only keeps soft two or three days, and can only be used for setting the cubes while in a moist state. Now, Berger contends, if the design for the mosaic be painted in colours on the wet stucco, and the whole allowed to dry, without any use of the mosaic cubes, we should have a painting in fresco, and he imagines that fresco painting began in this way. Unfortunately for the theory, (1), the testimony of Vitruvius and Pliny is absolutely decisive in favour of the knowledge in antiquity of the fresco technique, and, (2), the use of the coloured painting on the stucco as a guide for the setting of the cubes was not normal, and can never have been used so freely as to give rise to a new technique of painting. As a fact, this colouring of the stucco is objected to by the best modern workers on aesthetic grounds, for they point out that the lines of grey cement between the coloured cubes answer to the lead lines in the stained glass window, and should be reckoned with by the designer as part of his artistic effect. No doubt the older mosaicists, like the workers in stained glass, instinctively apprehended this, and had no desire for the coloured cement.

[254]. One would expect here ‘lime of travertine,’ for what Vasari must mean is lime prepared by burning this stone, which he recommends elsewhere, e.g. ‘Architettura,’ cap. iv, and ‘Scultura,’ cap. vi (calce di trevertino). The cement here given is a lime cement mixed with water. A sort of putty mixed with boiled oil is also employed, and is said to have been introduced by Girolamo Muziano of Brescia, a contemporary of Vasari. Each mosaic worker seems to have his own special recipe for this compound.

[255]. The process described by Vasari of building up the mosaic in situ, tessera by tessera, according to the design pounced portion by portion on the soft cement, is the most direct and by far the most artistic, and was employed for all the fine mosaics of olden time. In modern days labour-saving appliances have been tried, though it is satisfactory to know that they are all again discarded in the best work of to-day, such as that of Sir W. B. Richmond in St. Paul’s. One of the methods referred to, which can be carried out in the studio, is to take a reversed tracing of the design, covered with gum, and place the cubes face downwards upon it according to the colour scheme. When they are all in position, as far as can be judged when working from the back, a coating of cement is laid over them and they are thus fixed in their places. The whole sheet is then lifted up and cemented in its proper place on the wall, the drawing to which the faces of the cubes are gummed being afterwards removed by wetting. A better plan than this is called by the Italians ‘Mosaico a rivoltatura.’ For this process the tesserae are laid, face upwards, in a bed of pozzolana, slightly damp, which forms a temporary joint between the adjacent cubes. Coarse canvas is pasted over the face of the work; it is lifted up, and the pozzolana brushed out of the interstices. The whole is then applied to the wall surface and pressed into the cement with which this has been coated. When the cement has set the canvas is removed from the face.

[256]. The Duomo of Siena is a veritable museum of floor decorations in incised outlines and in black and white, in the various processes described by Vasari. There is a good notice of them in Labarte, Histoire des Arts Industriels. None of the work is as early as the time of Duccio, but Beccafumi executed a large amount of it. See the Life of that artist by Vasari.

It is worthy of notice that Dante had something of this kind in his thoughts, when in the 12th Canto of the Purgatorio he describes the figure designs on the ground of the first circle of Purgatory.