If they are old they could also be fragile when used. I don't know of a way of rejuvenating the transparent part. however you could try applying one to a white surface, you may be in luck that the warm water brings it back to life. If not I would let it dry and then clearcoat with your prefreed medium (varnish/polyurethane/car spray/future) and see if they become transparent again once dried. What I would do though is make sure that your soaking water is very warm and has plenty of washing up liquid in it and don't be in a hurry to use them, give them a good couple of minutes, the warmth of the water and the soap could work wonders for them.