The ultrasonic has never made the hairline cracks larger. I would never try it on dials with larger than hairline cracks.
Here is a dial that I ran through both ultrasonic & polident soak. I cant find the before photo. so you will have to take my word.
Thanks for the advice guys. I'm surprised at the ultrasonic, I would have thought the vibration would have opened up the crack.
On this one the crack is not visible with the naked eye but under magnification there is a slightly darker line running from 8 minutes past to the centre hole.
So am I right in thinking this is a porcelain dial?