## 4 Discussion
The number of peaks in each plot (Figure 6), as well as their amplitude increase indicates an increase in both the number of the grains in each _en-face_ image and in their size, respectively. Figure 6: Average of 500 horizontal lines of the _en-face_ images, corresponding to sintering temperatures of (**a**) 830 \({}^{\circ}\)C (Group L100); (**b**) 930 \({}^{\circ}\)C (Group N) and (**c**) 960 \({}^{\circ}\)C (Group H50), i.e., the normal and the extreme temperature considered. The number of peaks in each plot indicates the number of “bright particles”, while their amplitude indicates their size. Rules-of-thumb can be extracted from these results, in order to allow for the monitoring of the temperature inside the ceramic oven by OCT imaging of the prostheses sintered:  
1. A relative uniform reflectivity characterizes the recommended temperature of the oven (Group N, Figure 4(c2)). 2. The normal temperature regime is also correlated with the good uniformity of the grains, which have been fully developed in this case (Figures 4(c1) and 6b). 3. A non-uniform reflectivity without a clear pattern (i.e., with ups and downs) characterizes an oven temperature lower than normal (Figure 4(a2,b2)). 4. The lower the oven temperature, the lower the values that the reflectivity reaches, according to Figure 4(a2) in comparison with Figure 4(b2); a similar trend can be seen as well in Figure 6a in comparison with Figure 6b. 5. The aspects in the two points above can also be seen directly from the _en-face_ image (Figure 4(a1,b1)), although such an evaluation is less certain, because higher temperatures of the oven may also produce such non-uniformity. Such non-uniformities of the grain distribution (and thus of the reflectivity) for lower temperatures, proportional to the deviation of temperature from normal is correlated with the incomplete development of the grains, as discussed above for the sintering process. 6. A global increase in the reflectivity characterizes samples sintered at oven temperatures higher than normal, as shown in Figures 5(b2) and 6c. 7. Too high temperatures can also be concluded by comparing an _en-face_ image as the one in Figure 5(b1) with an _en-face_ image as the one shown in Figure 5(a1). This aspect is correlated with the excessive growth of the grains in the sintering process, as it can be seen from the difference in peak amplitudes in Figure 6c in comparison with Figure 6b. 8. A deviation of the oven temperature to even higher values with regard to the prescribed one (as for Group H50) can also be characterized by a positive and constant slope of the reflectivity (Figure 5(c2)). 9. The previous aspect is also the easier and most direct to see on an _en-face_ image (Figure 5(c1)), as it produces significant defects (D) that lead to cracks in short periods of time. 10. One should not wait for such totally undesired effects to occur. Instead, a re-calibration should be performed immediately as the reflectivity extracted from the _en-face_ image increases. Such an increase, of approximately 40%, already corresponds to a 30 \({}^{\circ}\)C higher temperature in the oven with regard to normal, as it was concluded in Figure 5(b2) from the _en-face_ image in Figure 5(b1). It should be appreciated that oven temperatures higher than normal are more dangerous for the outcome of the prostheses than oven temperatures that are lower than the normal value. This can be clearly seen from the fact that samples sintered at 50 \({}^{\circ}\)C over the normal oven temperature exhibited defects (D, Figure 5(c1)), while samples sintered at oven temperatures with even 100 \({}^{\circ}\)C below normal displayed no such defects (Figure 4(a1)). However, this lack of defects at oven temperatures lower than normal must not be taken lightly or less serious than the loss of calibrations towards oven temperatures that are higher than normal.