The keystone is a small thick wooden or plastic disk inserted into the bunghole which completely seals the cask until it is needed .
When the cask is in service , it is laid on its side with the bunghole at the lowest point on the circumference .
The keystone is a small thick wooden or plastic disk inserted into the bunghole which completely seals the cask until cask is needed .
When the cask is in service , it is laid on its side with the bunghole at the lowest point on the circumference .