# Research on the Organic Binders in Archaeological Wall Paintings
## 2 The Binders in the Wall Paintings  
For the realization of wall paintings and, in later times, for their preservation, different materials with functions of binders, adhesives, paints, protective and consolidating were and are still necessary. There is a very large class of products which, as far as I know, can have both constitutive functions but also a function of conservation and restoration. Until the mid-1900s, for one or the other destination, the main source of supply of these products was the natural one, without prejudice to a quantity of manipulations to extract, purify, slightly modify them, etc. Binding media are complex materials, employed to allow pigment grains to adhere to each other and to the surface of the support, through the formation of a coherent and homogeneous film. The function of the binder consists, therefore, in keeping the pigment particles firmly together and at the same time adhering them in the form of a coherent thin film to the surface of the support. The binder must obviously be in the fluid state, in order to form with the pigments a stable, homogeneous, stretchy, and viscous dough. The choice of the type of medium used to paint the wall depends on various factors, such as the type of pigment chosen, the historical period and the geographical location. Specific chemical, physical and optical properties are required for the binder, in order to fully perform its function and they can be summarized as follows:  
* Cohesive and adhesive: to allow the cohesion of pigment grains and their adhesion to the support in the form of a thin film, adherent and well consistent to the surface. For this purpose, the painting medium must therefore possess filmogenic properties, in order to create with pigments a homogeneous fluid, with a right degree of viscosity, in such a way as to be laid easily; after application it should have the property of drying in a solid, resistant, non-sticky and elastic layer, so that it can last over time. The stability of this suspension should also be left to completed and dried paint. * Chemicals: the binder must not react chemically with the pigment, nor solubilize it; it must also have a high resistance to photochemical reactions, atmospheric chemicals and all those substances that could cause its degradation. * Protective: to isolate pigment particles from the atmosphere preventing their alteration. * Optics: transparency and absence of color are the main qualities that a good binder should possess, so as not to significantly alter the optical characteristics of the pigment. The principal materials used as binder in archeological wall paintings were natural, based on proteinaceous, lipidic, glucidic or waxy material. The proteinaceous materials most used in the artistic field as pictorial binders in tempera technique, as well as protective, adhesive and consolidating, are obtained from the following materials:  
egg, which can be used either whole, or separately yolk (red) or egg white (white);  
animal glues, extracted from scraps of animal skins and other cartilage parts (skin glue), mammalian bones (bone glue) and various parts of fish (fish glue);  
milk and casein. The most widely used polysaccharide materials in the artistic field refer to sugary substances deriving from vegetable gum exudates, honey, steels and starches contained in cereal seeds, tubers and other parts of green plants. The types of vegetal gums most used historically were: arabic gum, tragacanth gum and gum from fruit plants. In this regard, [PERSON] reports \"... it also indicates rubber glue and for Egypt's best tragacanth rubber; then the one extracted from almonds, cherries, plums\" [6]. Glycerolipids, oils and fats, are a heterogeneous class of natural organic compounds of both plant and animal origin. The term \"oils\" refers to liquid materials at room temperature; the term \"fat\" to solid or semi-solid ones.