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The technique of fresco painting

Sinopia painting

'To create a fresco, the painter began by coating the wall with a layer of plaster, which was made by mixing water with slaked lime, and large-grained river sand. This first layer, about one centimeter thick, is called the arriccio (rendering). The roughness of this surface promotes the adhesion of the second layer of plaster. The artist drew the preparatory drawing directly on this first layer. With a piece of charcoal - easily erasable - he first penciled in the design. Once satisfied with this drawing, he used earth-ochre to trace a second set of lines alongside the charcoal lines. A bunch of feathers was then used to brush away the charcoal lines, and a red earth pigment was used to draw over their faded yellow remnants and to flesh out the details of the representation (folds of drapery, faces, chiaroscuro, etc.). This technique, first introduced in the thirteenth century, was used during most of the fourteenth century for large preparatory drawings. Today sinopia painting is named after the reddish pigment (originally from the city of Sinope) used to execute the technique.
Once the sinopia painting was finished, the actual painting could begin. At this point a new layer of plaster, the ‘intonachino' (thin plaster layer) was spread over one area of the arriccio (rendering) to prepare a perfectly smooth surface to receive the coloring. Intonachino (thin plaster layer) is a thin, transparent layer composed of one part slaked lime and two parts finely-ground sand. It was only applied to the surface area the artist expected to be able to color during one day's work, because the surface must be humid during the coloring phase. Then the painter used a brush to trace over the sinopia painting outline, which remained visible through the thin layer of intonachino (thin plaster layer). At last, he began painting with grounded pigments mixed in water.

The technique of fresco painting consists of painting colour pigments onto a layer of plaster that is still wet or "fresh" as the Italian term fresco suggests.
Walls to be frescoed were normally constructed from a single material, generally stone or brick to prevent damage to the frescoes from any movement in the wall due to settlement.
The artist would begin by applying a layer of plaster, made from a mixture of water, slaked lime, and large-grained river sand, to the wall.
This first layer, known as the arriccio, was applied to a thickness of one centimetre. The surface had to be very rough to allow the second layer of plaster to adhere to it easily.
On top of the arriccio, the outline of the fresco was drawn in charcoal, which was then erased using feathers after a second outline had been applied in ochre. Over the ochre, another outline was added with a red pigment called sinopia, a term which came to be used to describe the preparatory drawings it was used for. Finally, a fine layer of plaster called intonachino or velo was applied. This was transparent and much smoother than the arriccio and was usually made from one part slaked lime and two parts finely-ground sand. The surface had to be perfectly smooth surface and remain wet throughout the entire painting process and was therefore only applied to an area that the artist could complete painting in one day. For this reason, each one of these sections came to be known as a giornata, or 'day's work'.

The painter began by copying the sinopia outline, which showed through from the layer underneath, onto the wet plaster, and then started to paint using ground pigments mixed with water. Faces were usually painted starting with light shades and progressing to darker ones. The opposite technique was used for clothing, in which dark shades followed lighter ones.

Not all pigments could be used. Indeed, when the plaster dries, the lime which it contained causes a chemical reaction called carbonation, which releases heat and can burn plant-based pigments. For this reason, some touching up was often required after the plaster had dried to perfect details. This technique is called a secco, or dry. Blues, too, had to be painted a secco. This was true for both the extremely costly lapis lazuli or ultramarine, and azurite or "German blue". By the mid 15th century, the sinopia technique began to be replaced with preparatory cardboard, or cartone. The drawings were done to a smaller scale on squared paper in the artist's workshop.
A larger-scale grid with the same number of squares was then drawn on a piece of paper the same size as the fresco. The smaller drawing was then copied onto the large one, enlarged square by square. Holes were poked in the large drawing using a big needle and a loosely-woven sack of coal dust was passed over it. This technique, known as spolvero or dusting, resulted in a continuous series of little dots which the artist would join together to create the drawing. During the Renaissance a third method was also developed in which the drawing, enlarged using the grid method, was transferred onto thin paper. This was laid over the fresh plaster and a long nail was used to trace the lines and transfer the composition onto the mortar.
Serena Nocentini | The technique of fresco painting | www.brunelleschi.imss.fi.it | Watch the movie

'Strappo' (Detachment)

During the 18th century, new techniques were perfected for the restoration and conservation of ancient works of art, including methods of detaching fresco paintings from walls.
Detachment involves the separating the layer of paint from its natural backing, generally stone or brick, and can be categorized according to the removal technique used.
The oldest method, known as the a massello technique, involves cutting the wall and removing a considerable part of it together with both layers of plaster and the fresco painting itself.
The stacco technique, on the other hand, involves removing only the preparatory layer of plaster, called the arriccio together with the painted surface.
Finally, the strappo technique, without doubt the least invasive, involves removing only the topmost layer of plaster, known as the intonachino, which has absorbed the pigments, without touching the underlying arriccio layer.

In this method, a protective covering made from strips of cotton and animal glue is applied to the painted surface. A second, much heavier cloth, larger than the painted area, is then laid on top and a deep incision is made in the wall around the edges of the fresco.
A rubber mallet is used to repeatedly strike the fresco so that it detaches from the wall. Using a removal tool, a sort of awl, the painting and the intonachino attached to the cloth and glue covering are then detached, from the bottom up.
The back of the fresco is thinned to remove excess lime and reconstructed with a permanent backing made from two thin cotton cloths, called velatini, and a heavier cloth with a layer of glue. Two layers of mortar are then applied; first a rough one and then a smoother, more compact layer.
The mortars make up the first real layer of the new backing. The velatini cloths and the heavier cloth serve only to facilitate future detachments, and are therefore known as the strato di sacrificio, or sacrificial layer.
Once the mortar is dry, a layer of adhesive is applied and the fresco is attached to a rigid support made from synthetic material which can be used to reconstruct the architecture that originally housed the fresco.
After the backing has completely dried, the cloth covering used to protect the front of the fresco during detachment is removed using a hot water spray and decoloured ethyl alcohol.'
Serena Nocentini | The technique of fresco painting | www.brunelleschi.imss.fi.it | Watch the movie

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