We now need a flat hair brush, or several if possible, of a width rather less than that of a tessera. With this we paint in the tesserae as square flecks of color. The brush has to be smaller than the tesserae, as it always spreads a bit. It is safest to experiment, before starting, to find the right thickness.
We then mix the basic colors, if possible again in waterproof paints, such as casein poster paint, and then fill in the tesserae, carefully but not too rigidly, fleck by fleck, as close as they will lie. In this way we have an approximate idea of what the design will look like. If necessary, corrections can be made by covering over a portion with gray and rebuilding the color structure.
This draft shows only a few uniform tones of color. It is only now time to contemplate transitional colors by laying in the mixed colored over the basic colored squares. There is no point in fussing with subtle mixtures of colors at the start. I't is much more important to indicate the transitions broadly. Gradually as we work over the design again and again we shall achieve the desired subtlety. If we happen to blur some of the join lines in this process, they can be drawn in again in gray with a pointed brush, although this is necessary only if it is considered important to have an exact picture of the final effect. Next we see to the stones.
It is best to find out beforehand from the supplier what colors are available and which of them show up most clearly. We can now furnish the supplier with the design and ask him to put together the assortment of tesserae required. It is probably better to be present when the material is selected, for we are then better able to judge the possibilities open to us, and no doubt pick up a few good tips. It is worth finding out how the
tesserae can be broken up and shaped, even if we intend to use them whole for the most part. One should always allow for a few more stones than the number that has been calculated, and a few of the lighter and darker shades than those scheduled. These extra pieces will certainly come in very handy.
In preparing the next stage of the work we draw the outlines of the original onto stout transparent foil, such as acetate, for checking later. The same drawing is copied again on soft oiled or waxed paper, which will later be laid on the fresh plaster for the outlines to be pressed through. While the bottom layer of rendering is laid on the wall in pure cement, or cement and sand, and left to set for a few days, we can again see to the stones. To ensure that no grease adheres to them they are washed in small lots in dishwashing detergent, rinsed with clean water, and left to soak.
This treatment is particularly important when the stones are unglazed, for their porous clay body absorbs a great deal of water, and were they left unsaturated, the stones would draw off from the plaster the water which is essential for its hardening. The same is true of natural stone; only glass or glazed material does not need to absorb more water.
The student is well advised to keep his first mosaic to a size which can be executed in a small space. The work will go something as follows: the first fullsize drawing on transparent paper is placed on a board of suitable proportions; it is not needed for other purposes. It is covered with a sheet of thick transparent foil, and the colored original is hung up for convenient reference. Then the whole mosaic is set out on the transparent foil with the tesserae, using the underlying outline as guide.
At a first attempt it will probably be necessary to make a trial setting to see whether the sizes of the stones correspond to those assumed in the plan, or whether the plan has to be modified to accommodate the actual sizes of the pieces. In this preliminary setting some individual tesserae will be shaped with pincers and whetstone where it is absolutely necessary.
The most important tool for setting is a pair of pincers, which can be conveniently used for changing over and moving the tesserae. One comes to realize that it is much more interesting to leave some accidentally set color as a means of enlivening the whole composition rather than to follow the model slavishly. Thus, there is no necessity at the start to root out from the assortment of stones the piece which fits exactly; it is better to get the forms approximately right in the first place and to revise them in detail afterwards. Of course, incorrect settings must not become too deliberate a habit.