## Materials and Methods  
For this study, Duceram Kiss ceramics (DeguDent GmbH (a Dentsply Sirona Company), Hanau-Wolfgang, Germany) have been selected for evaluation. This choice has been based on the fact that the composition of each Duceram Kiss ceramic is adapted to meet the needs of its particular fabrication process and framework, with specific parameters to be observed, such as its coefficient of thermal expansion (CTE) [3]. Stress cracks and fractures are thus avoided and strength parameters such as bonding, adhesion and retention are assured. This ceramic material has the following characteristics: CTE Dentine 13.0 \(\upmu\)m/m\(\cdot\)K; dental ceramics, type 1, classes 2-8 according to DIN EN ISO 6872; metal-ceramic bond characterization, flexural strength and chemical solubility according to DIN EN ISO 9693. The optimization of the number of pigments is also a major part of the solution to the problem of metamerism, which refers to the display of a certain color shade of the dental prostheses depending on the source of light utilized, for example typical indoor lighting or outdoor daylight. In classical high-fusing veneering ceramics, the opalescent effect vanishes after a number of firing cycles [34]. By contrast, in Duceram Kiss, this effect will visibly persist. These superior characteristics simplify our study, as the temperature variations in the sintering oven are likely to be the only source of error/liability of these materials in the manufacturing process of the prostheses. On the other hand, a double check of the technician's work and his supervision during the procedure phases--to assure complying with the manufacturer's requirements--provide a minimization of human errors. A number of 50 Duceram Kiss metal ceramic crowns were used in the study, split into five groups, with 10 each. They were sintered at different temperatures and then investigated by using the MS/SS-OCT system. Fifty similar metal copings corresponding to the first maxillary incisor were obtained (Figure 1). The choice of these specific prostheses for the study is justified by how often they are needed by patients in dental practice. From these samples, five groups were considered: **Group N**, for which the ceramic layers were sintered according to the manufacturer's indications (i.e., at 930 \({}^{\circ}\)C); **Group L30**, for which the ceramic layers were sintered at 30 \({}^{\circ}\)C below the recommended temperature (i.e., at 900 \({}^{\circ}\)C); **Group L100**, for which the ceramic layers were sintered at 100 \({}^{\circ}\)C under the recommended temperature (i.e., at 830 \({}^{\circ}\)C); **Group H30**, with ceramic layers sintered at 30 \({}^{\circ}\)C above the recommended temperature (i.e., at 960 \({}^{\circ}\)C); **Group H50**, with ceramic layers sintered at 50 \({}^{\circ}\)C above the recommended temperature (i.e., at 980 \({}^{\circ}\)C). The samples were stored in special boxes, properly marked and prepared for imaging. Each of these temperature variations were meant to simulate a certain degree of loss of calibration in the ceramic oven in order to evaluate the possibility of detecting it by using MS/SS-OCT. Figure 2 presents a schematic diagram of the SS-OCT imaging system used in the study. As an optical source, an SS (Axsun Technologies, Billerica, MA, USA), central wavelength 1060 nm, sweeping range 106 nm (quoted at 10 dB), and 100 kHz line rate is used. The use of an optical source emitting at around 1060 nm cannot offer a depth penetration as good as at 1300 nm; however, it offers a better axial resolution, which, in our case, was 10 \(\upmu\)m (resolution measured in air by using a mirror as sample and measuring the Full Width Half Maximum (FWHM) of the A-scan peak). Using the procedure of measuring sensitivity described in [35], a sensitivity superior to 95 dB was obtained. This determines the minimum reflectivity measurable within any image. The interferometer configuration employs two single mode directional couplers, DC\({}_{1}\) and DC\({}_{2}\).