thanks for all the extra info.

I thought carbon tetrachloride was banned from sale or use in the UK long ago? Can you even still buy it? Or am I thinking of trichlorethylene?

These aren't valuable enough to go to the trouble of removing them from the mounts and so on, for proper conservation. I'm happy to give 'em a careful wipe down with whatever's handiest so they're scannable and make the best of whatever images are still there. There are (sadly) no further generations to hand them on to ...