## 3 Results
### Clay Analysis and Non-Invasive Analysis  
The ATR-FTIR was found to generate good-quality spectra from all samples. Muscovite has a very distinctive hydroxyl stretching band at 3624 cm\({}^{-1}\)[29].  
The calibration graphs for a representative set of the 12 muscovite clays in calcite are shown in Figures 11 and 12. The data sets for the two regression lines with the highest slope, two with the closest to mean slopes, and two with the lowest slopes are shown. The regression lines themselves are not included to improve clarity of the figures. The pre-viously determined calibration lines for sepiolite [25] are included for comparison.  
Figure 11: ER-FTIR muscovite clay calibration. High, med and low refer to the 2 highest, 2 lowest and 2 middle data sets of the 12 clay samples assessed.  
Figure 12: NIR muscovite clay calibration. High, med and low refer to the 2 highest, 2 lowest and 2 middle data sets of the 12 clay samples assessed.  
A decent set of calibrations for clay concentrations above 2 or 4% by mass is shown. This indicates the two non-invasive techniques can indeed be used to quantify clay pre-sent in calcite stones. There are small differences in the calibration for each clay sample, but using a general calibration is likely to provide results of sufficient accuracy to be use-ful. The NIR also showed a good calibration with the peak at 2217 nm [30]. The peak position is indicative of a lower Al(iv) content, with higher contents shifting the peak towards 2199 nm [31]. The external reflection FTIR was more sensitive, with a steeper slope than the NIR and lower detection limit.  
ER-FTIR is prone to generate anomalous spectra due to dispersion effects. If this was strong, the Kramers-Kronig (KK--Nicolet Omnic software) correction produced good peak shapes, but for some mixed examples, it was not successful. Over 60 stone objects were analysed with the muscovite clay present. With multiple attempts, less than 4% of these did not produce identifiable spectra with ER-FTIR, a much lower rate than stones with kaolinite, palygorskite, or sepiolite present. The comparisons with the conventional detection method of digestion followed by XRD are shown in Figure 13. Six of these required KK corrections of the ER-FTIR spectra (marked in black).  
Excluding the KK-corrected FTIR spectra, the NIR and FTIR results correlated well with those from digestion and XRD. The maximum deviation from the conventional method was 14.8% of the value. The KK values appear to be systematically low. Researchers have highlighted different results from KK corrections run on different software packages [32], although others have reported good results [33], and improvements to the method have been suggested [32].  
Both techniques could detect the muscovite clay to depths of 2 mm. The narrow-focus NIR fibre performed better, with clay at a 3 mm depth being detected. Previous examination of limestones and Amaravatti limestones with scanning electron microscopy (SEM) has indicated beds of parallel clay, no further from the surface than 1.5 mm. For the larger analytical area of the FTIR, this value drops to 1.3 mm due to the clay distribution in the stone. This appears to indicate that the information depth is sufficient, but a stone with deeper clay is possible. Deeper information depth can be achieved with the 2 mm NIR probe. However, this is more likely to miss clay bedded perpendicular to the surface, and several instances have been observed with 4 mm between clay beds. Both the larger probe (10 mm) and ER-FTIR (13 mm) would detect this. Figure 14 shows the measured muscovite clay contents across the stone fragment from Theftord.  
Figure 13: Comparison of NIR and FTIR with digestion and XRD analyses of clays.