The reason for this in most cases is the transition of the tooth's nature to moisture, although sometimes it happens that the tooth dries out greatly. The difference here is in the fragility of the tooth or in the opposite phenomenon. If there are signs of discoloration and erosion of the tooth, this indicates a damp nature with the presence of matter.
Treatment. In the first case, this is the retention of matter and the strengthening of the tooth with the strong astringents mentioned above. Alum has a strong effect on this. And if the tooth is also hot, then nothing helps as much as black hellebore mixed with honey. When tooth decay occurs from dryness, it is treated with the mentioned methods of treating dryness.