A dry atmosphere is the enemy of the natural cork seal. A natural cork is compressed and forced into the bottle as a 100% natural seal. The resilient and elastic cork wants to expand and continuously pushes out against the glass to form a seal.

The air space between the bottom of the cork and the wine is called the ullage. The ullage will increase over time as the cork is far from a perfect seal. It will let some air in while also absorbing some wine, even when the bottles are stored on their sides. Low humidity combined with a defective cork results in the wine moving out of the bottle (increasing ullage) and air naturally moving into the bottle.

This is an excerpt from “How To Build Your Own Wine Cellar” available at http://www.winecellarsecrets.com