# Research on the Organic Binders in Archaeological Wall Paintings
## 1 Introduction  
Wall painting realized using binders is the oldest form of parietal painting and precedes the birth of the _affresco_ by about 20,000 years. The first testmonies of parietal paintings made with organic binder date back, in fact, to the Upper Paleolithic: between 40,000 and 10,000 years ahead of Christ. These are cave paintings found in different geographical sites: in Africa, Europe and Asia. They were made using egg binders, blood, blood serum, urine, animal fat, animal marrow and milk. Throughout history the use of organic binders on the wall had a widespread diffusion, at all times, often combined with _affresco_ in a mixed technique [1]. Between 18,000 and 10,000 years ago the rock paintings of Altamira (discovered in 1879) and Lascaux (discovered in 1940) were produced. In this case, however, it is not a dry technique with organic binders: the pigments were applied pure on the wall, naturally rich in calcium hydroxide, the surface of which was impregnated by moisture condensation. Thus, a carbonation process took place that incorporated the pigment (lying in compact form) made the rock painting very resistant and durable. It is the same chemical reaction that occurs in the _affresco_ technique, but it was developed in later times. The early wall paintings were prepared by using simple procedures. As evidenced by studies carried out on ancient murals in different cultures paintings, plant tempera, using binders of a specifically vegetable nature, was among the first to be used [2]. It was obtained by \"softening\" pigments, usually of mineral nature, into binders of a polysaccharide nature (plant gums mostly). In Ancient Egypt dry paintings were made on the wall, with organic binders that were preserved well thanks to the very dry climate. The preparatory layer was based on gypsum, with additions of glue, or based on clay obtained from the Nile slusion (it naturally contains calcium carbonate and mixed with chopped straw to maintain moisture) or clay obtained from the base of the mountain reliefs. The organic binder in which the pigment was tempered could be tragacanth gum, acacia gum, egg, fish glue with additions of starch made from cereals [1, 3]. The investigation of the wall paintings in the tomb of [PERSON], the wife of [PERSON], recognized a gum Arabic, without rhamno which could have come from a local acacia species still growing in the Luxor region. Later, vegetable tempera painting was replaced by animal tempera, characterized mainly by the presence of proteins [4]. Another example of the use of organic binders on the wall are the African cave paintings in Algeria and Libya, dating back to the early Bronze Age. It is dry painting with the use of milk and casein as binders (probably since they were peoples of sheep breeders), application carried out not with hands, but with brush prodrome tools [1]. In Iran, between the 16 th and 18 th centuries B.C., dry wall paintings were made on cooked and raw brick walls covered with a clay-based preparatory layer and an additional layer of chalk. The pictorial layer is obtained by protein tempera, probably egg [1,3]. The study of documentary sources (recipe book, deeds of expenditure, contracts, inventories, epistolary collections, regulatory documents, treaties and other archival material) can be a good starting point to trace the type of organic binder used [3]. Among documentary sources, spending records are those that provide fairly accurate information about the materials used by the artist, however often the expenses of economic material, such as less valuable pigments or organic binders, are often not marked. However, the sources do not cover exhaustively and continuously every period and every place, for this reason reconstructing precisely the types of binders used in history is very complex. It should also be borne in mind that these documents offer an often-inaccurate knowledge of the materials used: the use of jargon from compilers not experts in the technique is wide, they can be lacking and lack some parts [5].