Fresco is the art and technique of painting on fresh plaster. It is derived from an Italian word meaning fresh. There are two different types of fresco paintings: buon fresco (meaning good in Italian) and secco (meaning dry) or finto (fake) fresco. Buon fresco is created by applying the paint to fresh, wet plaster; secco fresco achieved by painting when the plaster has dried. However, the two styles are not mutually exclusive. A study of older buon frescos has led some to argue that many of them have had paint added to them, perhaps for renovation purposes, creating a meld of the two forms. Why differentiate between them? Buon frescos generally have a longer lifetime because the water color pigments used bind with the plaster during the curing process. In contrast, a painter creating a secco fresco has to add a binder (such as egg tempura or glue) or use oil paint to ensure that it will last.
Creating a Buon Fresco
The first step in creating a Buon fresco is preparing a surface on which to paint. To do this with a two coat system, apply a rough coat of lime or gypsum plaster to the substrate or lathed area and allow it to dry. You will need to compact and float this layer for best results. If desired, an outline of the picture can be etched onto this layer once it has dried in order to serve as a guide for later painting. Now the plaster finish can be applied to a small portion of the wall or ceiling. While the plaster is still wet, apply watercolor pigment (which has been mixed with small amounts of water) to produce the fresco. The color pigments used must be fine grained and alkaline resistant. Although even alkaline resistant colors will lighten as they dry and may change color due to the alkalinity of the plaster. In order for the paint to bind with the plaster, it is very important that the area unto which the finish is applied be small enough that one can finish painting while the material is wet. Once a single section has been completed, more lime plaster finish can be added and a new area painted.
Creating a Secco Fresco
As in the creation of a buon fresco, the first step is the preparation of a painting surface. In the case of a secco fresco, this means that both a rough coat and a finish coat of plaster (lime or gypsum) be applied to the substrate or lathed area and allowed to dry. Once dry, the painter can get to work using oil paints or other pigments provided they have a glue or binder added.
Making Lime Plaster
Lime mortar is made by mixing lime, sand, and water, usually in a 1 to 3 ratio. This means that the amount of sand used will be three times more than the amount of lime. The amount of water added is entirely dependent on how much it takes to bring the mixture to a spreadable consistency. Be sure to use only clean water.
What types of sand and lime should be used? For best results, use only washed sand (the kind you can buy at your local plaster supply store). Use a finer sand for the finish coat. In regard to the lime, hydrated lime is easier because it can be used without preparation while quicklime must first by slaked. By soaking the lime ahead of time and allowing it to set, a lime putty can be created to use in the mortar for increased plasticity and workability. There are several tricks for improving the quality of the lime plaster used in frescos. For instance, it used to be common practice to make and store mortar ahead of time for later use. In fact, lime mortar must be well aged to work properly. This was done because aged mortar spreads better and is more plastic. If you choose to use the mortar after it has begun to set, make sure to remix it and add enough water to replace the moisture lost though evaporation. Another very important trick is to add clean hair or fiber pieces (cut to about 1 inch) to the plaster in order to add strength to the scratch coat. Likewise, the finished surface will be less likely to crack due to shrinkage if the still-wet rough coat is compacted using a wooden float about a day after it is first applied. In addition, by scratching the first coat during the compacting process, a better bonding surface for the finish will be created. You do this by driving a nail into the forefront of a wooden float to a depth that permits only a small portion of the nail to scratch the surface. Finally, less traditionally methods can be used if you are not worried about being too historically correct. For example, the rough coat can also be strengthened and cracking prevented by adding Portland or Keenes cement to the lime mortar. However, be forewarned that adding cement will greatly reduce the setting time of the plaster. This means that you will have less time to paint before the finish dries. Most important, due to the fact that the material is dependent on environmental fluctuations, it is best to experiment with the mix and practice application on a similar area before beginning to work on the place you would actually like the fresco to appear.