Fresco painting Fresco painting is a very old technique. It reached the height of splendor with the Renaissance masters and was used to decorate buildings and church vaults. A fresco is painted on fresh (fresco) plaster made of slaked lime and sieved siliceous river sand plus pigments diluted with plain water. The painting must be done quickly and surely, before the base has time to dry. (Murals are instead painted over dry plaster). The chemical reaction between the calcium hydrate and carbonic acid in the atmosphere fixes the painting and makes it insoluble. This technique was used to decorate buildings and church vaults.
First you prepare the wall. You’re going to apply plaster to the wall and you want it to stick, so you rough up the surface with a small pick-axe.
Arriccio is a layer of rough plaster made of a mixture of lime paste and large granules of sand. You smear it onto the wall and let that dry overnight. 14th century artist would sketch out the major outlines of his painting with a reddish-brown paint directly onto the arriccio. This part is called the SINOPIA, an underlayer of the fresco that can sometimes be uncovered through restoration and is sometimes displayed in museums. Sometimes you may actually see sinopia on walls where the top layer of the fresco has been ruined. This preparatory drawing in sinopia is a handy guide for the artist and also a way to show patron what he’ll be getting. Later in the 15th century, the practise of direct sinopia painting was used less often. As drawing became more important in the practise of the visual arts, many artists made a series of preparatory designs culminating in a CARTOON, a life-size drawing on paper. They pricked the cartoon with a needle and held it up to the arriccio; this page was then “pounced” with a sack of carbon so that a black outline was made. There were other ways to transfer drawings and to keep them in mind during the process, though these are the two most popular.
The day to paint has arrived. Obviously it’s not all done at once – each day’s work is called a GIORNATA. This corresponds to a plaster patch that is the amount of work the artist could do in one day. You prepare your intonaco plaster, which is made of the same lime and sand as the first layer, but the sand is a finer grain and there is more lime. This mixture is then spread onto the space you intend to work on that day. Interestingly, this covers up the underpainting (or charcoal outline), which the artist had to keep in his mind! If you look closely, sometimes you can see the giornata divisions in a fresco; these were applied judiciously to try to hide the lines but are usually around major shapes.
The paint is applied directly to the plaster while it is wet, which is only a 2-4 hour window of opportunity, after which the plaster starts to dry and it gets very difficult to paint. You work from the top down (because the paint drips!) and try to do large areas like sky all at once because it’s very difficult to match colours the next day. This process is very difficult because once you apply the paint, it’s there and you can’t make mistakes. In fact, paint layers are thin to transparent, so the pigment was added in layers. Colours could also be mixed by doing this.
Only certain types of colours are good for fresco painting. These are chemically stable EARTH PIGMENTS like terraverde, yellow ochre, red, white, charcoal black. Other pigments would react with air and discolour – lead white turned black over time, azzurite blue turns green.
Finishing touches were applied after it all dried and tend to be less permanent and fall off with time. Blue pigment was applied often over red underpainting, while gold leaf was applied last, being stuck on with fish glue. Sometimes the finest details in faces and other sections were done a secco. This means that if you see a red sky or a face with no detail, chances are you are looking at a fresco that has lost its a secco treatment.