Organic Materials in the Wall Paintings II

Dear Colleagues,

Wall paintings created using binders are the oldest form of parietal painting and precede the advent of the fresco by about 20,000 years.

When considering wall paintings, the characterization of organic compounds is a very valuable source of information for elucidating the materials used, which allows one to know the paint technique of an artist. Understanding the composition of paint materials allows us to distinguish between painting techniques that have been used over the centuries. The knowledge of organic substances in wall painting materials is of great importance for the preservation of works of art, because the organic components of the paint layers are particularly subject to alterations and degradation. From this knowledge it is therefore possible to propose adapted storage conditions and better conservation/restoration methodologies.

With the exception of some countries, especially eastern countries—which include murals made on wooden walls—generally murals are only painted on walls of a mineral nature, both natural (such as cave paintings painted on walls of caves) and artificial walls, such as stone or bricks. In most cases, the walls are further prepared with plasters as a sort of preparatory layer, the composition of which varies from simple clay (in the most primitive types) to 'calce and sand,' which constitute the most recurrent case study. The thickness of the plasters can vary from very thin to a few centimeters and the plaster layers can be both single and multiple or overlapping. 

As for the techniques, it was obviously started by simple procedures. In order to obtain stable pictorial drafting, it was still necessary to have pigments and binders. As evidenced by the analytical studies carried out on ancient murals in different cultures (Egyptian, Mayan, etc.), painting with plant tempera was among the first to be used. It is obtained by 'softening' pigments, usually of a mineral nature, into binders of a polysaccaride nature (plant gums mostly). From painting with plant tempera, the art then moved to using protein tempera. Protein binders consist of animal glue, eggs, and casein (skim milk). Glue, however, as a soluble binder or otherwise sensitive to water, has always been used minimally. More stable and commonly used are the other two: eggs and milk. Local traditions, probably linked to the first experiences, have made one or the other the preference; for example, egg tempera was preferred in Italy, while in northern Europe, the casein was usually, but not always, preferred. The egg is mainly the 'red' or the mixture of 'white' and 'red', The 'white' alone was instead employed as a fixative 'drink' for the restoration of frescoes in times past. Some artists (especially at the end of the fifteenth century) have also made use of greasy tempers or egg tempers added to drying oils.

Dr. Antonella Casoli
Guest Editor

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