'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.

La tecnica dello strappo – Museo Benozzo Gozzoli di Castelfiorentino