Dry corks should be
driven into the bottle whenever possible, but many small hand corking
machines cannot adequately compress hard, dry corks. One solution to the hard
cork problem is to soak the corks in cold water until they become soft enough
to drive with the corker available. A pinch of sulfite can be added to the
water. Very wet corks are undesirable because excessive "cork
water" will be left when the corks are driven into the bottle.
Corks can also be softened (and probably sterilized) in a microwave oven. First the corks are rinsed in clean water and then sealed a plastic, zip-lock storage bag. The sealed bag is then placed in a microwave oven and heated for a short time. Corks burn easily, so some experimentation with the time and power settings of the microwave oven may be necessary.
Rehydration is perhaps the best method of softening old, hard corks. This is a very easy and simple method. However, rehydration may require several weeks, so some advanced planning is necessary. An ounce or so of water and a pinch of sulfite powder is placed in a small glass bowl, a cup, shot glass, etc. A large freezer-type baggy is placed on a flat surface, and the corks and the glass container are placed in the baggy. The baggy is then tightly sealed and left undisturbed for a couple of weeks until the corks become soft and pliable. When the corks are soft, the sulfite solution can be carefully removed and the baggy resealed, and the corks will remain pliable for several months if the seal is tight.
Rehydrated corks are soft enough to be driven dry by most hand operated corking machines. Driving dry corks is preferred because they don't leave a messy residue, and they don't contaminate the wine with cork water. Sometimes, dry corks are easier to drive if they are sprayed with food grade silicone (hard to find) or with a very small amount of "Pam."