My daughter has an old victorian brass microscope mounted in a mahogany case. Most of the time this sits in my study and is only brought out to look at on occasions. New Year's Eve turned out to be one such occassion, when some of our party, rather full of seasonal spirit, decided to investigate some of out antique scientific instruments. The last time my daughter had put the microscope away, she had stuffed a polyethylene supermarket bag into the box, to hold the package of slides in place - that was probably a couple of years ago. And something bizarre had happened: the bag had become extremely brittle, locked into its scrunced-up shape, It had also become extremely fragile, dissolving into tiny flakes when touched, and even smaller flakes/powder when a handful of flakes were rubbed together. The flakes and powder had a very strong 'brassy' smell. The bag was least friable in the centre of the scrucnched-up 'ball', but even there it fell into the same flakes with a slightly heavier touch

What has caused this? Googling to find out what reaction might be happening, I couldn't find anything, but mentions of brass fittings on polyethylene pipes failing came up, which could be related?

More information: The bag was definitely not a biodegradable bag, I've checked that. I'll see if I can find out more about it.

The last time the microscope was cleaned will have been at least 25 years ago and then it would probably have been with duraglit wadding... but I'm not sure whether it was ever cleaned like that. Most of the time it sits in a closed mahohany box.

So: no sunlight, it would have been completely dark in the box, which is stored in a corner of my study. Ambiant temperatures would have ranged between about freezing point and about 27 degreex C in summer, I suppose. No solvents I can think of, and as I said, a very strong 'brassy' smell to the degraded plastic. I am experimenting with another bag in a screw-top brass jar which I know has only ever contained water, and that 100 years ago!