Fresco - Plastering the Wall

1. The first layer is called the rough cast, scratch coat, or trullisatio. It is usually made with a coarse aggregate.

1 part lime
3 parts filler [coarse sand, crushed brick, or tile]

2. The second coat is called the brown coat.

1 part lime
2 1/2 parts filler [coarse sand or coarse marble meal]

3. The third coat is called the sand finish.

1 part lime
2 parts filler [finer sand or marble dust]

4. The final coat, called the intonaco, is the coat on which the fresco painting is executed.

1 part lime
1 part filler [fine sand or marble dust]

Thus the mortar goes from coarse to fine and from lean to fat, producing a wall of maximum strength in which the layers cohere most effectively. As the moisture begins to evaporate, each layer sets initially. After a longer period, usually about eight hours, the lime crust of calcium carbonate forms on its surface. A new application of mortar cannot hold well to a smooth layer of mortar on which a lime crust has formed. For this reason the layers of mortar will adhere to each other best if each coat of mortar is laid "wet-in-wet" on the previous layer before the lime crust forms on the wall. If this is impossible, the lower layer of mortar ought to be laid with a very rough surface so that it will afford a good mechanical grip to the new layer of mortar. In all cases the wall must be thoroughly moistened before a new layer of mortar is applied. One must wait until the wall, although moist, has no water standing on its surface, before troweling on the mortar.