After the X-ray seen in Figure 1 (see part 1), they put a little optical camera (pediatric size, for which I was grateful) in my mouth to obtain a 3-D digital model of the remaining tooth shape. Figure 2 shows the little CNC machining center that they had in the adjacent office. The inset shows the edge of the green ceramic block as it was being machined with a diamond burr (Fig. 3). The meaning of “green” here is “un-heat treated.” The actual color of the insert was purple at this time. More on the burr later.
Figure 4 shows the heat-treat oven. The technician was kind enough to let me watch and photograph all of this, and I gave her a little engineering lesson. We see that the furnace has started to heat up, but the chamber is open. I told her that might be to help reduce the chances of thermal shock as the little chunk of ceramic is gradually heated to bring it to full strength. Even metals are preheated during high-heat thermal processing to prevent distortion and cracks. Ceramics, being more brittle, are likely even more sensitive to cracking from extreme thermal stresses.
The inset at upper right of Figure 4 shows the now off-white chunk of ceramic that will soon be adhesive-bonded to the lightly ground surface of my tooth. The technician did not have any theories about how they control the color, or color change, of the blanks.
The dentist explained that my two rear teeth had sharp edges as a result of chewing. He rounded off the remaining sharp edges of the “subject tooth” and also of the mating upper tooth. While sound human teeth have enamel entirely enclosing them, I recall my heat-treating book explaining how rodents’ teeth always remain sharp because they have hard enamel only on the front surface.
This was given as a comparison to a carburized steel tooth for a piece of mining equipment. As the soft back is worn away, narrow chips break off, exposing a new strong, sharp biting edge. Rodents’ teeth, again unlike human teeth, keep growing as long as the animal is alive. In the case of a tooth on a bucket used for mining hard rock minerals, the tooth is designed as a consumable repair part. Getting back to my tooth, once the enamel on the inner surface of the cusp of the tooth was worn away, chewing forces could more easily bend the remaining thin shell of enamel and cause it to break away.