I have noticed that sometimes when nursing a bottle over a long period, say, perhaps a year or two, some corks will seem to shrivel a little as time goes on and the bottle's contents dwindle. The cork remains spongy not hard and dry like what occurs when storing spare corks in a drawer. It looks sort of shriveled and prune like. With bottles where the cork never was a very tight fit (such as some WT bottlings) the cork is noticably loose.
I currently have a bottle of WT 12 open where this is the case. Additionally there seems to be a film of debris in the neck of the bottle where the cork sits. On one occasion I carefully wiped this debris out with a (damp with whiskey) paper towel only to have this film reappear. Is this a sign of a deteriorating cork? The whiskey tastes fine and the cork doesn't give off any odd odors. I thought of placing a small square of cellophane (Glad cling wrap) over the mouth of the bottle and then seating the cork as a way of preventing any further deterioration as well as a shim of sorts to prevent any evaporation from the poor fitting cork. Is this a good idea? would the alcohol have any negative effect on the cellophane?