Things You'll Need
Paper towel or sponge
Steam rack (optional)
Cork is an extremely versatile substance because of its elasticity and fire resistant properties. It is used to seal wine bottles, cover instrument joints and buttons and insulate houses, and it is also used for flooring. Because this material is quite elastic and contains air bubbles, it is very easy to expand. It can be temporarily expanded by soaking it in water and it can be permanently expanded by heating it in a humid environment.
Temporarily Expand the Cork
Clean the cork with soap and water. Use a sponge or paper towel if necessary. This is especially important if your cork is attached to a musical instrument and has been recently lubricated. Make sure that any coating is completely removed.
Fill a bowl with hot water. Place the cork in the bowl. Let it soak in the bowl overnight.
Examine the cork after about 12 hours. If it has expanded to the correct size, pat it dry with the paper towel. If you need it to expand more, replace the water with hot water and let the cork soak in the bowl for a little longer. Keep checking it and refilling the bowl with hot water until the cork has expanded to the desired size.
Permanently Expand the Cork
Clean the cork using soap and water. Remove any lubrication or coating using a sponge or paper towel. Rinse the cork thoroughly to remove the soap.
Heat the cork in a humid atmosphere. There are several different ways you can do this, depending on what equipment is available to you. The easiest method is to use a steamer because you can watch the cork expand more closely.
Set the steamer to the highest setting and hold the steamer over the cork. Check every 30 seconds to a minute to see if the cork has expanded enough.
Check the cork often during this process to achieve the right size.
Verify that the cork you are using is pure cork if possible by checking any labels that came with the cork. The permanent expansion process requires heating, and if the cork is mixed with unknown materials, it could cause toxins to leak into the air
Try alternate ways of expanding cork if you do not have a steamer. Place the cork into an oven or microwave oven and set a dish of water next to it. If you use a microwave oven, heat the cork in increments of 10 seconds or less to retain control over expansion. Alternatively, place the cork on a steam rack above a pot of boiling water on a stove for 10 to 30 minutes.
Do not allow the cork to sit in a mild, dry heat for very long. The heat sometimes removes moisture from the cork and causes it to condense rather than expand.