## 3 Applications  
### Mineral Reactions During Firing and Cooling  
The occurrence or non-occurrence of certain mineral phases and their relative proportions are often used, for instance, to (i) determine the firing temperature of ancient ceramics, (ii) to divide ancient ceramics into groups for provenance studies [63], (iii) to estimate the formation temperature of natural rocks [25], or (iv) to determine the firing temperature of brown coal ashes burnt in a power plant [64]. The difficulty here is that it is not always obvious whether a mineral formed during firing or during cooling. [PERSON] and coworkers [63], for example, could not solve the question whether large sparitic carbonate inclusions, observed in Cypriot Bronze-age plain white ceramics, represent incompletely decomposed or secondary calcites that were formed by a recarbonation process after firing. Such questions can principally be answered by _in situ_ experiments [24].  
Figure 11: A series of 16 hyperspectral Raman images of a fired ceramic that were recorded with a nominal depth \(z=0\) (focus at the surface) and \(z=-15\) μm (focus 15 μm below the surface). Note that despite the significant offset of the focus all images exhibit the same relative phase proportions and main morphological details.  
To study the decarbonation and possible recarbonation process, i.e., the influence of cooling on the final product, a sample composed of quartz and calcite (mass ratio 1:1.03) was fired with a heating rate of 10 \({}^{\circ}\)C/min and 50 \({}^{\circ}\)C temperature steps from 660 \(\pm\) 5 \({}^{\circ}\)C to 1106 \(\pm\) 5 \({}^{\circ}\)C. At each temperature step two hyperspectral Raman images (100 \(\times\) 100 \(\upmu\)m\({}^{2}\), 1 \(\upmu\)m step size, SWIFT 1.6 \(\upmu\)m/s) were successively recorded within a dwell time of four hours. After recording the last hyperspectral Raman image at the maximum temperature of 1106 \({}^{\circ}\)C, the sample was cooled to RT with a cooling rate of 10 \({}^{\circ}\)C/min.  
At 839 \(\pm\) 5 \({}^{\circ}\)C calcite gradually decomposes to CaO (images not shown here, but see Figure 1). Calcium oxide has a rock-salt structure and thus no first-order Raman spectrum [65], but exhibits second-order Raman features. Second-order Raman features are rarely used for Raman imaging so far [60]. However, CaO could be identified by Raman features near 540, 670, 740, and 900 cm\({}^{-1}\) and house-intron reference spectra of CaO were successfully used for data processing.  
With increasing temperature \(\upalpha\)'L-Ca\({}_{2}\)SiO\({}_{4}\) formed from a reaction between lime and quartz. During cooling \(\upalpha\)'L-Ca\({}_{2}\)SiO\({}_{4}\) transformed to \(\beta\)-Ca\({}_{2}\)SiO\({}_{4}\) that remained metastable at RT. Importantly, during cooling calcite recrystallized from a reaction of lime with atmospheric CO\({}_{2}\) which is released by the calcite decomposition during heating and partially trapped in the heating stage. This is noticed when comparing the _in situ_ Raman image recorded at 1106 \(\pm\) 5 \({}^{\circ}\)C with that recorded at RT, clearly showing that the calcite within the imaged area has fully decomposed to CaO and CO\({}_{2}\) at 1106 \({}^{\circ}\)C (Figure 12). However, without this _in situ_ information, one could come to the conclusion that the calcite grains observed in the burnt ceramic represent remnant precursor calcite grains. It follows that _ex situ_ quench experiments are not necessarily sufficient to fully investigate mineral reactions at high-temperatures during firing and subsequent cooling. However, it should be noted that only the sample surface is analyzed by Raman imaging. Therefore, based on the Raman data alone, it cannot be ruled out that only CaO from the first few micrometers recrystallizes to calcite because CO\({}_{2}\) cannot reach the interior of the ceramic body.