Mirror, mirror on the wall

Really odd experience today. Finishing off a bathroom for a lady, one job was to hang a mirror on a wall.

Tightened screws up with a manual screwdriver and on doing up the second one a crack appeared radiating from the hole and around 20mm long. Highly visible.

I was certain I hadn't over tightened the screw.

'Oh bother' I said.

These things happen, all you can do is put your hands up. Called the lady into the room, explained what had happened, apologised & said if she got a new mirror I would pay for it & swap them at no charge.

Finished off some other jobs & about an hour later looked at the mirror again - the crack had shrunk from 20mm to about 3mm! I couldn't believe my eyes. Called the lady in & neither could she. Both of us gobsmacked!

We looked under the room light, natural light & a torch. The bugger had definitely shrunk!

Another hour & it had completely vanished! Not a 'kin trace!

I'm very glad it did, but what the hell is going on here? Its like something out of Harry Potter.

I'm fairly sure I have seen cracks "shorten" while unsuccessfully cutting glass. I don't think you need much "opening" to make a crack visible. I suspect the crack was being held open by friction in the clamp somewhere, or possibly by thermal distortion. Remember that if you are "hanging" the mirror on a screw which you are tightening, you are putting a frictional torque on the glass either from the screw or via the spacer before you have any tension in the screw.

I reckon either the temperature equalised or the friction relaxed, so the crack closed up. I don't know whether it might "heal" completely or whether condensation in the residual crack might act like windscreen fixing glue and make it near enough invisible.

You're the magician, you tell us. (Or is it forbiden in the magic circle?)