## 4 Discussion
Several studies suggest that dry or moist intracanal environment does not influence the setting of MTA up to 2 mm [12] or 4 mm [11; 13]. Microhardness tests have been used for the evaluation of the quality and progression of the hydration process and as an indicator of the setting process [41; 42]. Previous work marked on its capability to provide information on the effect of setting conditions and the strength of tested materials [43]. For accurate comparison with other materials, specimens should be polished and dimensions of at least 6 mm thick and 12 mm wide are required. These conditions are far from clinical endodontics. Thus, tests in this field are mainly comparative for the use within each study [44]. Unlike most of previous studies, we measured microhardness and modulus of elasticity of real size samples sectioned to longitudinal axis by depth sensing indentation. Depth sensing indentation (nanoindentation) has been adopted as one of the most common techniques developed for the assessment of local mechanical properties at nano- and microscale. It is a contact-based method where the well-defined probe is pressed into the investigated surface at defined conditions. Nanoindentation is mostly used for assessment of hardness and elastic modulus. Microhardness testing is based on the evaluation of resistance of material against plastic/elastic deformation [18]. Conventionally, the hardness test is based on the measurement of the diagonal's dimensions of the residual plastic impression created during the indentation process, where the hard indenter is pressed into the sample's surface. Vickers indenter, four-sided pyramid, is used in most studies. This approach relies on the visual observation of residual indentation and also on the assumption that only very limited elastic recovery of the residual indent takes place. Generally, the measurement of the actual size of the indent becomes intricate with the decrease of its dimensions and increase of surface roughness. On the contrary, the advanced approach of hardness testing, also called depth sensing indentation, is based on the continuous recording of the force-displacement data during the whole test. This reduces the possible errors of the visual observation of the residual impression. The obvious advantage of depth sensing indentation is the ability to determine the elastic modulus of the specimen [19]. In order to take advantage of this advanced experimental technique, the prepared specimens had to be cross-sectioned and polished. This allowed precise local mechanical testing of the MTA material all along the longitudinal axis and not only on the surface [16], so we were able to assess indentation hardness and elastic modulus in vertical and horizontal direction. However, it should be noted that the polishing procedure itself can affect the chemical and structural composition of MTA if excessive cooling water is used during polishing procedure. Especially, dissolution and washout of MTA components (especially on the boundaries of material), as well as change of pH, can be considered as the main processes. Different polishing procedures are used in nanoindentation application to study hardened cement pastes [25; 45]. Another important phenomenon altering properties of MTA is carbonation. It is a process based on a reaction between atmospheric CO\({}_{2}\), and both calcium hydroxide and calcium silicate hydrate in cement are carbonated. Increased hydration time enhances the carbon dioxide uptake, which indicates that the calcium in the hydration products reacts more easily than the calcium in the clinker phase. In a humid CO\({}_{2}\) atmosphere, the carbonation process is so pronounced that it decomposes calcium silicate hydrate into calcium carbonate and silica [26; 46]. Furthermore, the carbonation is linked to shrinkage of material [47]. Humidity during exposure to carbon dioxide appears to be a major factor influencing the shrinkage directly produced by carbonation [27].