What should I do about old bottles that have had their foil removed? Should I be concerned that the cork is exposed, or does that not matter so long as I take care not to expose the bottles to temperature fluctuation and/or solvent fumes?
You need not fret. That foil – or capsule – is mainly there for decoration.
Most wines sealed with a cork come wrapped with a capsule, the coloured sleeve at the top of the neck. Originally, the capsule was designed to keep rodents and bugs from gnawing away at the cork, but the threat is pretty minimal. And if you've got rats in your basement, you've got bigger things to worry about; they're probably gnawing away at more enticing food sources down there than your Château Lafite.
Capsules will not provide much protection against the more hazardous threat to your corks: mould. Carbohydrates such as cork or bottle labels provide a food source for mould. If your cellar is too humid (above 75 per cent relative humidity), mould becomes an issue. Eventually, it can chew through a substantial part of the cork and compromise the seal, exposing the wine to too much oxygen. Your first course of action should be to reduce the wine's exposure to humidity, ideally by storing the bottles at least half a metre away from an exposed-brick or concrete wall. (Investing in a dehumidifier is probably not wise because you need a good amount of humidity to ensure that the corks won't dry out entirely. Ideal conditions lie between 65 and 75 per cent.)
But don't worry if you find a bit of mould at the top of your corks, whether or not they had been covered with a capsule. This is normal in the case of older wines and doesn't necessarily mean the cork or wine has spoiled. Just wipe it off with a damp cloth before you pull the cork.
A clearer sign that your cellar is too humid lies in the labels. If the labels become dusted with white or black spots (that's mould), your corks are likely in danger, too. And, yes, you should take the same precautions with respect to solvent fumes; they're never good for long-term storage.
If you're curious about the condition of a cork, by all means remove the capsule and have a peek. But I wouldn't advise it if you're keeping the wine for investment purposes. Removing the capsule is like removing the pick guard from an old Fender Stratocaster guitar. It will cause no harm but it will reduce its price at auction.
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