# Research on the Organic Binders in Archaeological Wall Paintings
## 1 Introduction
The information that can be drawn from these documents does not provide a certain tool of knowledge, the technical terms cannot be interpreted in a single way, they are imprecise. It should also be borne in mind that in the past knowledge was traditionally transmitted orally, so many notions have been lost. [PERSON] in Historia Naturalis [6] and [PERSON] in De Architectura [7] talk about Roman painting. [PERSON] refers to the _affresco_: << _Colores autem, udo tectorio cum diligenter sunt inducti, ideo non remittunt sed sunt perpetuo permanentes_ >> (However, when the colors are laid out on damp plaster, they do not loosen but remain permanent). [PERSON] also talks about arid painting, that is, _a secco_ painting, but there is no chapter in which he specifically delves into the binders, it is logical to think that the wall paintings were executed affresco. The study of organic binders in Roman painting is an open question: the chemical-physical approach has produced vague and partial data, often discordant [8]. A _\"affresco_ theory\" has been formulated in this regard, carried out by [PERSON] and [PERSON] [1]. He claims that Roman wall painting was exclusively affrescoed. According to this theory, the final aesthetic effect is similar to that of dry technique thanks to the final polishing. However, pigments have been detected that do not tolerate the alkality of lime, also suggested by [PERSON] [6] and [PERSON] [7] in their writings, therefore not suitable for _affresco_ technique. The Mappae clavicula is a medieval Latin document containing recipes for materials of art and craftsmanship [9]. A literary source regarding the use of organic binders around 1000 D.C. is represented by the book De Diversis Artibus by [PERSON] [10]. The only preserved text that refers to the technique of wall painting in the Upper Middle Age is a passage from the 8 th century manuscript of Lucca, whose author seems to be a Greek settled in Italy: the pigments were laid out without organic binder, therefore with the _affresco_ or lime technique [1]. The 14 th century is a time of great novelty with regard to wall painting. The wall painting of 1300 finds in [PERSON] its greatest exponent: in the _affresco_ technique the use of sinopia is introduced. The preliminary drawing for affresco l, named for the reddish-brown pigment traditionally used to draw or transfer it. In tempera the egg is preferred as a binder, but also milk and skin glue. The oil painting technique on the wall is introduced, linseed oil is widely used [2]. [PERSON] tells us about the technique of wall painting used around 1300 in The Book of Art, written at the beginning of the 15 th century, but which reports the technique in use since the 14 th century [11]. It is the first Italian treatise to deal with oil painting on a wall to which he devotes ample space. Some chapters are also dedicated to the tempera on the wall. For egg-based tempera there are three types of preparation suggested by [PERSON]:  
* Whole egg mixed with fig milk. Dry plaster should be impregnated with water-tempered egg. * Egg yolk. * Egg yolk mixed with animal glue. To understand the pictorial technique, a valuable help is provided by the scientific approach of the diagnostics of pictorial works, which allows the identification of the organic binder. In some cases, chemical-physical analyses, performed on wall paintings, revealed the presence of organic material used as binder for the application of pigments on the surface of dried plaster, as reported by different studies, some of which are chosen to be described in this paper. [PERSON] et al. carried out an extensive review of organic materials in mural paintings, examining historical literature and scientific research [12]. In particular, they focused their attention on the paintings in the period between the Middle Ages and the Baroque in Europe, period of great changes, with the aim to collect references about the type of organic material present in paintings.