The walls of the coal shell were about 3/8 inch thick, creating a hollow space inside sufficient to hold 34 ounces of gunpowder.

In a closed spandrel stone arch bridge the hollow space can be filled with rubble and loose material.

The engulfed material eventually burns or decays away, but ends up leaving a hollow space with the original shape.

During the 19th and 20th centuries, more specific messages were attached to the clubs or inserted into a hollow space.

These caskets contain more or less complex, usually invisible opening mechanisms which reveal a small hollow space on opening.