## 3 Results and Discussion
### EPR Investigation  
Smitting slags, very common on archaeological sites, can be classified in three main types, each related to a kind of metallurgical activity [34]. The first type, dominated by fayalite, with a variable amount of iron oxide (mainly wustite) and a small amount of interstitial glass, is mainly produced by hot oxidation of the metal with a small input of silica from various sources (hearth lining, charcoal, dust, flux) during forging. The second type, richer in silica and minerals deriving from granitic rocks (granites, sandstones, clays) and with low iron content, is produced during fashioning of iron pieces and processing of steel objects. The third type of slag is richer in iron (as metal, oxide, or oxy-hydroxide) contains fayalite and inclusions of charcoal, and is produced during the work of a poorly compacted metal, or when the smith is working close to the melting point of the metal, for example during welding. Smitting slags are generally formed under reducing conditions controlled by the CO/CO\({}_{2}\) ratio defined by the fluctuating oxygen pressure in the hearth atmosphere. Since formation of the ancient slags is very sensitive to the oxygen content, at the same chemical composition of the melt, it is possible the crystallization of mineral phases with differing Fe\({}^{2+}\)/Fe\({}^{3+}\) ratios. This issue can be described with the quartz, fayalite and magnetite (QFM) buffer equilibrium:  
\[3\;\mathrm{Fe}^{2+}\mathrm{_{2}SiO_{4}}+\mathrm{O_{2}}\leftrightarrow 2\; \mathrm{Fe}^{2+}\mathrm{Fe}^{3+}\mathrm{_{2}O_{4}}+3\;\mathrm{SiO_{2}}\]  
If the oxygen concentration in the gas atmosphere is sufficiently high, magnetite and a silica-rich compound (like pyroxene) crystallize first. If oxygen is low, no magnetite will be formed, but fayalite (or even metallic iron) precipitates [20]. The Fe\({}^{3+}\)/Fe\({}^{2+}\) ratio essentially will be then defined by the raw materials used and the manufacturing technology (the redox conditions which occur during the metallurgical process of iron processing). The knowledge of the Fe\({}^{3+}\)/Fe\({}^{2+}\) ratio could enable the samples' identification as primary (bloom refining) or secondary (iron bar processing) smithing slags [26]. EPR spectroscopy is widely used in materials research, predominantly in structural investigations [35; 36], and also has applications in geological and archaeological dating [37]. As the quantitative analytical method performs only occasionally, mostly in studies carried out on Fe and Mn containing clays, glasses, and ceramics. As the reliable standardization assuring the avoidance of matrix related errors in this case is difficult, EPR spectroscopy only delivers semiquantitative results [38]. However, semiquantitative data permit the adequate determination of the Fe\({}^{3+}\)/Fe\({}^{2+}\) ratio; in addition, the method has the advantage of simple and easy sample preparation, without the need for expensive standards, costly, and/or hazardous chemicals. Experimental EPR spectra recorded for Fe\({}_{2}\)O\({}_{3}\) at room temperature (a broader line superposed on a narrow line) are typical superparamagnetic resonance spectra. The broad component of the EPR signal shifts left with increasing Fe\({}_{2}\)O\({}_{3}\) concentration; the narrow one is observed at the same field-3500 G (g = 2) regardless of concentration. With the increase in the Fe\({}_{2}\)O\({}_{3}\) concentration, the narrow component also broadens and becomes less visible until it is completely unobservable in the highly concentrated samples [35; 36; 37; 38; 39; 40]. The integrated intensity \(I\) of the EPR signal (area beneath the absorption curve) is proportional to the concentration of the paramagnetic centers in the sample. EPR spectral features (\(I\)) recorded at room temperature on the untreated sample specimens (e.g., before any thermal treatment) correspond to the presence of the Fe\({}^{3+}\) paramagnetic centers alone, since, above 77 K, the resonance assigned to Fe\({}^{2+}\) can't be detected.