So I've been sorting through a collection of old microscope slides once used for teaching. I came across 12 slides of cockroach mouthparts mounted in what I believe is Canadian balsam, but they've gone completely opaque. The slides look a bit sparkly, so maybe some kind of crystallisation has occurred, but under the microscope it looks like a mess of fibres. We have hundreds of slide mounted in Canada balsam but no others look like this. I have no other information about the slides but I know nothing new has been added to this collection in at least 10 years so they are likely much older. I'm more curious than anything, does anyone have any ideas what might have happened?