Now the fun starts. The plasterer thoroughly wets the underlayer of plaster and puts in laths or screeds in order to spread a uniform bedding layer. Into this surface the tesserae are set, once the design on the oiled or waxed paper has been pressed or traced onto the plaster.
The work is done from the bottom of the design upwards, a strong lath being inserted along the lower edge of the mosaic to support the first row of tesserae. If the design does not finish at the bottom in a horizontal line, the edge of the outline transferred from the oil paper drawing must be supported by a firm ridge of plaster.
In this way the first line of tesserae is held firm from underneath and provides the necessary support for the stones above. The stones are picked up one by one and pressed by hand into the soft plaster. The best procedure is to start with the two or three lowest lines of tesserae from the composition and to set them complete, and then to continue to build upwards the main motif of the design, such as a figure in the middle of the picture. This enables us to follow the traced outlines and to check the progress of the composition against the outline tracing.
The setting of the stones should go forward as fast as possible so that the plaster is not given time to alter its consistency. Now and again, when a fairly large piece of the composition has been completed, the stones should be lightly tapped. A board about 6 x 12 inches, covered with thin felt or rubber, is laid over the mosaic and tapped or pressed with the hand to ensure an even surface.
This should be done as little as possible, for there is a danger of shaking the tesserae from their bed while the cement is still binding. In a mosaic built up in this fashion we do not aim at a completely even surface; with practice and experience the use of the board for flattening the mosaic can be given up. The
irregularities which arise in the bedding of the stones by hand can be attractive in themselves.
Once we feel confident enough to undertake larger scale work, of the kind that is done on the front of a building from a scaffold, for example, it is no longer possible to transfer the pieces direct from a preliminary setting out. The method then is to divide the tesserae up in flat boxes of varied colors which are numbered conveniently so that the required color is easily found while working on the scaffold.
On large and complicated mosaics the plaster is likely to get too hard before the work is finished, and the stones will then not hold firmly. It is best in these cases to do only a section at a time, and where necessary to strip off the plaster and relay it, as has always been done with large frescoes.
If a mistake has been made and a few tesserae need replacing, it should be done at once. The tesserae must be carefully removed with the pincers, the space moistened slightly with a spray, the new stone given a touch of plaster on the bottom and pressed carefully into place. Corrections, however, should be avoided as far as possible.
Once the work is finished it is left until the plaster has hardened enough to give some hold to the stones. The ridge of plaster added to hold the bottom edge is cut away while it is soft enough to come off easily withollt shaking the whole composition. Once the stones are holding firmly, the whole mosaic is spread over with very fine watery plaster, which then is immediately wiped off the surface.
When this plaster is beginning to set in the joins the whole surface is brushed from top to bottom with a suitably hard nylon brush to clear the joins near the surface and let the separate stones stand out. Now the mosaic shows its characteristic texture for the first time. To finish off, each tessera is cleaned with a cloth moistened with an acid such as vinegar to remove the last traces of plaster from the surface.
Very old mosaics look now as though they had never been cemented from on top. The stones seem simply to have been fixed individually and to stand quite separate from each other. This relief texture is so attractive that we nowadays like to reproduce it, and it is possible to do so. In old mosaics the cracks are due to the decomposition of the lime plaster. This texture of empty cracks can be reproduced today by not placing the stones too close, and pressing them by hand into a very fine cement mortar without subsequently cementing them from above or pressing them smooth. It is the simplest process of all.
As a contrast we should mention how industrial mosaics are made. A mirror image of the design is drawn on sticky paper. The stones are arranged as close as possible onto this underlay with the top side down. The pieces are made to slope slightly in a cone shape towards the bottom edge, so that the plaster can run up higher and bind them firmer.
The paper with the stones stuck on it is cut into convenient pieces which are set clean and flat into the plaster; the paper is stripped off when the plaster has set hard enough to bind the tesserae. A very fine plaster is smeared over the top into the thin cracks. This method is very efficient and sure, and is fully justified for floors and purely decorative purposes.