With the exception of some countries, especially eastern countries, which include among their murals those 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 more often artificial walls, 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 plasters can be both single and multiple, 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, Maya, etc.), painting with plant tempera was among the first to be realized. It is obtained by 'softening' pigments, usually of mineral nature, into binders of a polysaccaride nature (plant gums mostly). From painting to plant tempera, the art moved to that of protein tempera. Protein binders consist of animal glue, egg and casein (skim milk). Glue, however, as a soluble binder or otherwise sensitive to water, has always been used minimally. Much more stable and commonly used are the other two: egg 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, with some exceptions, was 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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• Wall paintings
• plant gums
• egg
• animal glue
• milk