Titolo della tesi: Multi-component archaeological adhesives: an innovative analytical approach to study chemical interactions and natural aging degradation
The core of archaeology is the reconstruction of past through the study of material remains, and, in such way, to understand the customs and traditions of our ancestors. This focus on material evidence has created, during times, the need to study things more and more thoroughly, which leads to the requirement of scientific approaches to the study of the past, and for this reason, archaeology has increasingly interacted with physics, chemistry, biology, and the Earth sciences, paving the way for new interdisciplinary scientific branch: archaeometry [1], [2]. It is important to underline how many consider archaeology itself a science, as it is based on the systematic sampling and analysis of data with the aim of theorising about past events. Undeniably, it is one of the few disciplines that, thanks to its approach, it can bridge the gap between the pure sciences and the humanities. Indeed, over the last three centuries, archaeology has evolved from being considered a discipline for the “embellishment of the contemporary world”, often driven by a spirit of "treasure hunt", to becoming a field based on the scientific method.
Nowadays, every archaeological excavation is considered as an unrepeatable experiment, making it essential to extract as much information as possible. In this context, chemistry can play a crucial role, providing independent and objective data, thereby complementing traditional archaeological methods. Within this framework, analytical chemistry proves to be the most suitable [2], [3]. In fact, this is the branch of chemistry which concerns the development of theories, methodologies, and instrumental techniques for the compositional – qualitative and quantitative – and functional determination of chemical systems, that are, portions of space or matter subjected to investigation. In this context, archaeological artifacts can be considered highly complex and heterogeneous systems, typically being composed of mixture of multiple chemicals [4].
In addition to what has already been said, it is important to keep in mind that the limitations imposed by archaeological samples are significant: they are small, fragmented, and, particularly concerning organic ones, degraded and often contaminated. This contamination can occur not only during the period of deposition, but also once the sampe has been collected: it can be contaminated by the materials used for conservation or during the study and handling phases. These issues, combined with the fact that analytical chemistry in the recent past required excessively large samples, often made analyses unsuitable and archaeological samples remained a complex analytical problem. However, today and looking forward, analytical chemistry can study increasingly small samples and determining analytes at lower and lower concentrations and has developed a greater sensitivity to the problem of contaminations that can occur between the recovery of the archaeological find and its study. Nonetheless, the application of analytical chemistry to archaeology remains challenging, even today [1], [4].
At first, analytical chemistry focused on inorganic artifacts, simply because they preserved better over time, but later, the focus shifted toward residues from natural organic substances such as waxes, resins, proteins, lipids, and even DNA. Naturally, this new interest demanded the use of novel, and often more sophisticated, analytical techniques.
In the optic to obtain information on past uses and knowledge, the study of adhesive materials results particularly important. In fact, since ancient times adhesives, collected after the observation of natural phenomena, have been used to produce objects which are fragments of our ancestor’s daily life. In addition, the beginning of the employment of these materials represents a key change in the evolution of prehistoric men, since joining together different parts to obtain a composite tool represents a fundamental step forward in the technological evolution and opened the way for numerous technological possibilities [5]. The first evidence of adhesives used goes back to the transition to the Middle Palaeolithic, around 300-200 thousand years ago [6]. After that moment several substances were used for this purpose such as plant resins, tars, fossil organic substances like bitumen and beeswax, and, in addition their formulation were modified during time to fulfil each new technological challenge that arose.
The knowledge of adhesives technology, alongside with the information gained from the artifact in which it is employed allow us to answer questions that archaeology poses to analytical chemistry:
• What is it made of?
This represents the basic question. In general, the levels of analysis required to answer this question are relatively simple. However, the composition of some archaeological artifacts can be uncertain for various reasons: the artifact is small, dirty, or it is made of a material that cannot be identified with the naked eye or with simple tests. The answer to this question could generate other more deeper questions.
• Where did this material come from?
This is a second level question, since it requires that we first answer the initial one. It can be approached in several ways, for example, by comparing the materials identified in the artefact with the local availability of raw materials and their chemical composition. However, to answer this question it is also necessary to take in account also all the events that the artefact may have undergone over time and this aspect make this question one of the most difficult to address.
• How old is this material?
Dating an object is one of archaeology crucial themes. Knowing the historical period to which an artifact belongs allows us to display events in their reciprocal relations. The traditional archaeological approach to date is based on comparing the artifact of interest with others whose origin is known. However, this approach easily becomes impossible to adopt, and modern scientific criteria are required [7]. It must be kept in mind that, from the moment it is recovered by the archaeologist, the artifact undergoes modifications due to physical, chemical, and biological phenomena. Thus, at the end of this chain of events, an important part of the artifact is lost or significantly altered. This makes the dating of archaeological finds a real challenge, even when using analytical techniques [2].
• What was it used for?
This question is generally difficult to answer through traditional chemical approaches. An interesting example of the importance of this question is the case of organic residues found inside ceramic containers. An example is the work of D. Namdar et al., in which, thanks to the analysis of the contents of some cone-shaped containers dating back 4700-3700 years found in the Southern Levant (a geographical area that includes modern-day Palestine, Israel, and Jordan), it was possible to determine their use, by the analysis of molecular markers [8].
• How can it be conserved?
This question is not directly linked to archaeological interpretation but is certainly among the most important. In fact, the general objective is to conserve the archaeological find, a need that leads to studying some of the main aspects of chemical and biological degradation processes, particularly relevant in the case of organic materials, and in this context, adhesive. By understanding all the phenomena, which derive from the interaction between the object and the environment, it is possible to establish an appropriate conservation strategy.