Note 255 — Chapter XV · (XXIX.)

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.