Keystone (cask)

For some years wooden casks have been effectively obsolete, with the majority now being aluminium or stainless steel, and a few sturdy plastic ones beginning to appear.

The keystone is a small thick wooden or plastic disk inserted into the bunghole which completely seals the cask until it is needed.

In the plastic variety it is much thinner than the surrounding material; if made of wood it is partially pre-cut so that it is easier to punch out.

Some time before the cask is to be served, the end of the tap is placed against this weakened section and, with the aid of a mallet, driven through it.

A small amount of beer may leak out during this process, but with sufficient skill and firm action with the mallet, no spillage at all can occur.