I too have seen this...

and was fortunate/unfortunate enough to watch it as it happened. I have never seen this is a watch before but when I was in my first watchmaking school we used to also service clocks. I had this happen on 2-3 mainsprings that has mild surface rust. These were treated with a mild acid treatment to remove the rust prior to lubricating and installing in the the barrels. In one of the instances it didn't even last long enough to finish preparing the barrel lid before I started hearing the familiar sound of a bowl of Rice Krispies. Upon closer inspection it was clear to see the individual cracks appearing as the pressure of the mainspring seemed to be shattering itself. It was like watching popcorn pop.

I had assumed it was due to the rust removal method causing the steel to become brittle, however further study was not undertaken.

TCP

Exploding mainspring?

By: Marcus Hanke : September 6th, 2013-09:55

Rainer Nienaber showed me these pictures of a mainspring barrel he had on his workdesk. He said that this seems to happen sometimes, but nobody has an idea why a mainspring would break instantanously not only in one spot, but practically into many fragmen...

I find it hard to believe/imagine

By: ei8htohms : September 6th, 2013-18:41

Hi Marcus, That is definitely a first. I've seen mainsprings messed up by watchmakers and reinstalled in barrels (some of the final services performed in a facility that was being shut down and the watchmakers were NOT happy about it) and I'd have to gues...

Let down too fast?

By: BDLJ : September 6th, 2013-19:18

..with the intial breakage flailing around and causing further breaks?

I learn new things all the time...

By: ei8htohms : September 6th, 2013-19:52

But I have access to information (directly and indirectly) about hundreds of thousands of repairs and I've never heard of such a thing. Just saying.

It was merely a guess.

By: BDLJ : September 6th, 2013-21:35

Unless someone really did snap the spring into many pieces and then carefully arranged it back into the barrel? Sounds like the infamous tales from the dark (were there any lightones?) days of British Leyland. ..

This is not a....

By: David Walter : September 7th, 2013-10:20

New thing. I have seen this happen a number of times over the years in both new white steel mainsprings and the old blue steel springs. Regarding the blue springs I feel it had more to do with slight surface surface corrision than anything else. I have fi...

WatchTech